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A few questions about collinear coaxial antennas
Hello,
I am about to attempt to build a UHF collinear coaxial antenna and am trying to finalize a design. I have done a lot of reading and am a little confused on a few things. First off I have read contradicting statements whether to use odd or even number of 1/2 wave elements. 1, 3, 5... or 1,2,4... Also I don't understand what the 1/4 wave whip is doing on the top without a ground plane (found in most designs), is this necessary for a receive antenna?. Instead of using coaxial cable, I will be building the 1/2 wave and 1/4 wave transmission lines out of ridged copper pipe with air as it's dielectric in order to maximize the velocity of propagation and therefore making true 1/2 wave elements. Does anyone see anything wrong with this approach? Thomas |
A few questions about collinear coaxial antennas
"Thomas Magma" wrote in message ... Hello, I am about to attempt to build a UHF collinear coaxial antenna and am trying to finalize a design. I have done a lot of reading and am a little confused on a few things. First off I have read contradicting statements whether to use odd or even number of 1/2 wave elements. 1, 3, 5... or 1,2,4... Also I don't understand what the 1/4 wave whip is doing on the top without a ground plane (found in most designs), is this necessary for a receive antenna?. Instead of using coaxial cable, I will be building the 1/2 wave and 1/4 wave transmission lines out of ridged copper pipe with air as it's dielectric in order to maximize the velocity of propagation and therefore making true 1/2 wave elements. Does anyone see anything wrong with this approach? Thomas Hi Thomas I think you can design and develop a very good colinear coaxial array at UHF using copper pipe. Do you have any requirement for VSWR? Do you have need for operating the antenna at other than one frequency? It isnt easy to develop a good UHF colinear without good test equipment. How will you measure input impedance? Do you care about the angle of the radiation pattern maximum? End fed colinears will have beam squint with frequency change. Jerry KD6JDJ |
A few questions about collinear coaxial antennas
"Jerry" wrote in message ... "Thomas Magma" wrote in message ... Hello, I am about to attempt to build a UHF collinear coaxial antenna and am trying to finalize a design. I have done a lot of reading and am a little confused on a few things. First off I have read contradicting statements whether to use odd or even number of 1/2 wave elements. 1, 3, 5... or 1,2,4... Also I don't understand what the 1/4 wave whip is doing on the top without a ground plane (found in most designs), is this necessary for a receive antenna?. Instead of using coaxial cable, I will be building the 1/2 wave and 1/4 wave transmission lines out of ridged copper pipe with air as it's dielectric in order to maximize the velocity of propagation and therefore making true 1/2 wave elements. Does anyone see anything wrong with this approach? Thomas Hi Thomas I think you can design and develop a very good colinear coaxial array at UHF using copper pipe. Do you have any requirement for VSWR? Do you have need for operating the antenna at other than one frequency? It isnt easy to develop a good UHF colinear without good test equipment. How will you measure input impedance? Do you care about the angle of the radiation pattern maximum? End fed colinears will have beam squint with frequency change. Jerry KD6JDJ Hi Jerry, My application is at only one frequency so I intend to centre it on that frequency and the VSWR I get is the VSWR I get. I would hope to be 25 dB return loss anyways. I do have a HP8714C network analyzer in the lab I will be using so that is no problem. Due to the centre frequency (lower 400 MHz) I figure I can only realistically have about 4 of the half wave elements because of height, weight and wind loading. Oh wait was that 3 or 4 or 5 elements. I still haven't solved that fundamental issue yet. I don't suppose the radiation pattern is too much of a concern at this point, as long as it is omnidirectional. Thomas |
A few questions about collinear coaxial antennas
On Nov 25, 2:55*pm, "Thomas Magma"
wrote: Hello, I am about to attempt to build a UHF collinear coaxial antenna and am trying to finalize a design. I have done a lot of reading and am a little confused on a few things. First off I have read contradicting statements whether to use odd or even number of 1/2 wave elements. 1, 3, 5... or 1,2,4... Also I don't understand what the 1/4 wave whip is doing on the top without a ground plane (found in most designs), is this necessary for a receive antenna?. Instead of using coaxial cable, I will be building the 1/2 wave and 1/4 wave transmission lines out of ridged copper pipe with air as it's dielectric in order to maximize the velocity of propagation and therefore making true 1/2 wave elements. Does anyone see anything wrong with this approach? Thomas From modeling I did a long time ago: there is a slight advantage to using high velocity factor line, but it's very marginal. Just as a dipole doesn't need to be resonant to do a good job radiating (and receiving), so the elements in the coaxial collinear don't need to be resonant. The phasing among the elements is dictated by the coax between the feedpoints. Each gap between two elements is a feedpoint; across it is impressed the line voltage. Since each line segment is a half wave long and the conductors are reversed at each junction, the voltage across each feedpoint is the same and in phase, less a small amount for line loss. The element currents depend on mutual coupling among the elements, but my simulations for VF=0.66 to VF=1.00 indicated that the current phases were always very nearly the same. Generally, you'll want all the elements to look the same from the outside. The top element should be the same length as the rest. It's common to short the coax an electrical quarter wave up from the highest gap between elements; that reflects back an open circuit to the bottom of the top element, so really you could just as well make the top element a tube the same OD as the rest of the elements, connected to the inner conductor of the next lower section. The feedpoint impedance at the bottom of the antenna is just the parallel combination of all the feedpoints, which are generally each fairly high (since each one is feeding a full-wave doublet, essentially), but with ten or so sections, the net is modest, generally around 100 ohms. Whatever the feedpoint impedance is, you need to match to it properly-- to whatever degree of matching is "proper" in your book. I generally use a simple "L" network: a variable C across the feedpoint, and an inductor to the feed line center conductor. It matches the impedance and can tune out some reactance. Then you need to decouple the antenna from the feedline, and from other metal in the area where it's mounted. I generally use self-resonant coils in the small feedline, one immediately below the antenna and one another quarter wave lower. You could also try sleeves or radials... Summary: the coax provides proper feedpoint phasing (even if the elements are shorter than 1/2 wave because of the VF of the line used); a matching network lets you match to 50 ohms (or other impedance if you want); decoupling keeps "antenna" current off the feedline. Cheers, Tom |
A few questions about collinear coaxial antennas
"Thomas Magma" wrote in message ... "Jerry" wrote in message ... "Thomas Magma" wrote in message ... Hello, I am about to attempt to build a UHF collinear coaxial antenna and am trying to finalize a design. I have done a lot of reading and am a little confused on a few things. First off I have read contradicting statements whether to use odd or even number of 1/2 wave elements. 1, 3, 5... or 1,2,4... Also I don't understand what the 1/4 wave whip is doing on the top without a ground plane (found in most designs), is this necessary for a receive antenna?. Instead of using coaxial cable, I will be building the 1/2 wave and 1/4 wave transmission lines out of ridged copper pipe with air as it's dielectric in order to maximize the velocity of propagation and therefore making true 1/2 wave elements. Does anyone see anything wrong with this approach? Thomas Hi Thomas I think you can design and develop a very good colinear coaxial array at UHF using copper pipe. Do you have any requirement for VSWR? Do you have need for operating the antenna at other than one frequency? It isnt easy to develop a good UHF colinear without good test equipment. How will you measure input impedance? Do you care about the angle of the radiation pattern maximum? End fed colinears will have beam squint with frequency change. Jerry KD6JDJ Hi Jerry, My application is at only one frequency so I intend to centre it on that frequency and the VSWR I get is the VSWR I get. I would hope to be 25 dB return loss anyways. I do have a HP8714C network analyzer in the lab I will be using so that is no problem. Due to the centre frequency (lower 400 MHz) I figure I can only realistically have about 4 of the half wave elements because of height, weight and wind loading. Oh wait was that 3 or 4 or 5 elements. I still haven't solved that fundamental issue yet. I don't suppose the radiation pattern is too much of a concern at this point, as long as it is omnidirectional. Thomas Hi Thomas If you can use whatever frequency the antenna works best at, it may be practical to build one then use the frequency of best performance with that antenna. But, if you have some predetermined frequency that the antenna must perform well at, there is a problem building prototypes. It can get rather time consuming to build prototypes when using copper pipe. Aparently you are confident that you can evaluate the antenna's input impedance. I had figured that would be a fairly difficult task. I'll be very interested in this project. Please keep the group informed of your progress. Jerry KD6JDJ (who has designed similar antennas for commercial use) |
A few questions about collinear coaxial antennas
From modeling I did a long time ago: there is a slight advantage to using high velocity factor line, but it's very marginal. Just as a dipole doesn't need to be resonant to do a good job radiating (and receiving), so the elements in the coaxial collinear don't need to be resonant. The phasing among the elements is dictated by the coax between the feedpoints. Each gap between two elements is a feedpoint; across it is impressed the line voltage. Since each line segment is a half wave long and the conductors are reversed at each junction, the voltage across each feedpoint is the same and in phase, less a small amount for line loss. The element currents depend on mutual coupling among the elements, but my simulations for VF=0.66 to VF=1.00 indicated that the current phases were always very nearly the same. Generally, you'll want all the elements to look the same from the outside. The top element should be the same length as the rest. It's common to short the coax an electrical quarter wave up from the highest gap between elements; that reflects back an open circuit to the bottom of the top element, so really you could just as well make the top element a tube the same OD as the rest of the elements, connected to the inner conductor of the next lower section. The feedpoint impedance at the bottom of the antenna is just the parallel combination of all the feedpoints, which are generally each fairly high (since each one is feeding a full-wave doublet, essentially), but with ten or so sections, the net is modest, generally around 100 ohms. Whatever the feedpoint impedance is, you need to match to it properly-- to whatever degree of matching is "proper" in your book. I generally use a simple "L" network: a variable C across the feedpoint, and an inductor to the feed line center conductor. It matches the impedance and can tune out some reactance. Then you need to decouple the antenna from the feedline, and from other metal in the area where it's mounted. I generally use self-resonant coils in the small feedline, one immediately below the antenna and one another quarter wave lower. You could also try sleeves or radials... Summary: the coax provides proper feedpoint phasing (even if the elements are shorter than 1/2 wave because of the VF of the line used); a matching network lets you match to 50 ohms (or other impedance if you want); decoupling keeps "antenna" current off the feedline. Cheers, Tom Thanks Tom for the detailed explanation. My current sketch of my antenna design uses a quarter wave sleeve on the lower end of the antenna to stub the current off the feedline ground. I am hoping to see the antenna having a (somewhat) characteristic impedance of 50 ohms since the transmission elements although made out of copper pipe and copper rod are designed to be 50 ohm and the antenna is single end fed (unlike a dipole). I will make some provisions for a small tuning structure. If I do have to tune, the nice thing is that it will be a receive only antenna and I can get away with small RF components. Tom, since you seem to know quite a bit about collinear coaxial antenna design, do you know if I should be using an even or odd amount of half wave elements? I planned on four, do you see a problem with this. Thanks again, Thomas |
A few questions about collinear coaxial antennas
On Tue, 25 Nov 2008 14:55:52 -0800, "Thomas Magma"
wrote: I am about to attempt to build a UHF collinear coaxial antenna and am trying to finalize a design. What design? Drawing? Description? NEC model? Numbers? UHF is about 300 to 1000MHz. Any particular frequency? Incidentally, it's not a "coaxial antenna". It's an end fed vertical colinear using coaxial cable elements. First off I have read contradicting statements whether to use odd or even number of 1/2 wave elements. 1, 3, 5... or 1,2,4... Also I don't understand what the 1/4 wave whip is doing on the top without a ground plane (found in most designs), is this necessary for a receive antenna?. Instead of using coaxial cable, I will be building the 1/2 wave and 1/4 wave transmission lines out of ridged copper pipe with air as it's dielectric in order to maximize the velocity of propagation and therefore making true 1/2 wave elements. Does anyone see anything wrong with this approach? Yep, lots wrong. End fed colinear antennas are convenient but far from ideal. They're also deceptively simple where the problems only show up after the antenna is built. 1. Most of the RF comes out the bottom of the antenna. Very roughly, the first dipole belches 1/2 the RF, the next dipole belches 1/4 the RF, then 1/8th, and so on. This is NOT exact, but close enough to illustrate the problem. You can make it as long as you want, but if somehow manage to cover up the lower part of the antenna (a common problem on a rooftop or side mounted on a tower), most of the signal is history. 2. The alternating 1/2 wave coax cable type antenna is twice as long as necessary. Every other 1/2 wave coax section is essentially a non-radiationg phasing section. That's convenient for construction, but not very compact. A similar antenna, using a simple 1/2 wave hairping stub, with be half the length, with the same gain. 3. Coax is lossy. Coax phasing sections add un-necessary loss that is not present in an antenna that uses (for example) a hairpin stub or coil instead. Your copper pipe and air dielectric idea eliminates this problem, but I thought I would throw this in for those building them from coax cable scraps. 4. End fed antennas tend to have pattern uptilt. That may or may not be a problem depending on your unspecified application. The uptilt doesn't show up on free space models, but is certainly there if you include the effects of a rooftop, ground, or mast arm. If this is going on a mountain top, you might consider mounting it upside down. You can reduce the uptilt problem somewhat by cutting the antenna in half and feeding it in the middle (forming a dipole), rather than end feeding it. Several commercial antennas work this way. That also eliminates the need for ground plane radials at the base. 5. The effects of the radome can be critical. I built such a UHF antenna for 463MHz long ago. It worked well enough with exposed sections. However, when I potted it with urathane fence post foam in a PVC pipe enclosure, the center frequency drifted downward sufficiently to render the antenna useless. 6. Cutting the coax sections accurately is difficult. If you're not using a fixture for cutting, forget it. 7. Making it out of copper pipe is rather expensive but certainly possible. Making the insulators will be somewhat of a challenge. There's no velocity factor involved (Air=1) so the measurements will be simple. However, since there's an overlap between sections, I'm wondering from where to where you should measure. If you cut the outer copper tubing to exactly 1/2 wave, then you need a very thin insulator between sections to prevent shorts. Methinks there will need to be some cut-n-try along with some careful measurments (swept VSWR) along the way. Good luck. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
A few questions about collinear coaxial antennas
Hi Jerry, My application is at only one frequency so I intend to centre it on that frequency and the VSWR I get is the VSWR I get. I would hope to be 25 dB return loss anyways. I do have a HP8714C network analyzer in the lab I will be using so that is no problem. Due to the centre frequency (lower 400 MHz) I figure I can only realistically have about 4 of the half wave elements because of height, weight and wind loading. Oh wait was that 3 or 4 or 5 elements. I still haven't solved that fundamental issue yet. I don't suppose the radiation pattern is too much of a concern at this point, as long as it is omnidirectional. Thomas Hi Thomas If you can use whatever frequency the antenna works best at, it may be practical to build one then use the frequency of best performance with that antenna. But, if you have some predetermined frequency that the antenna must perform well at, there is a problem building prototypes. It can get rather time consuming to build prototypes when using copper pipe. Aparently you are confident that you can evaluate the antenna's input impedance. I had figured that would be a fairly difficult task. I'll be very interested in this project. Please keep the group informed of your progress. Jerry KD6JDJ (who has designed similar antennas for commercial use) Hi Jerry, It is a predetermined frequency that I am building the antenna for. It is not determined if it will become a commercial product yet but I am trying to design it as such. I can see that it might be a little time consuming working with copper pipe, but once I get the formula right I should be good to go. I'll start buy calculating the half wave elements based on theory knowing my dielectric constant will be dry air or Argon. The design I have sketch is pretty neat and clean (on paper anyways). It has all the elements stacked directly on top of each other, unlike the traditional staggered approach you see in other designs. Also the dielectric chamber of the transmission elements are sealed and can be filled with a noble gas such as Argon to prevent corrosion and detuning from humidity. My background is in receiver and transmitter design, so I'm quite familiar with impedance matching and I understand how a Smith chart works on a network analyzer. I'm looking forward to working with copper pipe instead of 0201 capacitors and a microscope! Thomas |
A few questions about collinear coaxial antennas
"Thomas Magma" wrote in message ... Hi Jerry, My application is at only one frequency so I intend to centre it on that frequency and the VSWR I get is the VSWR I get. I would hope to be 25 dB return loss anyways. I do have a HP8714C network analyzer in the lab I will be using so that is no problem. Due to the centre frequency (lower 400 MHz) I figure I can only realistically have about 4 of the half wave elements because of height, weight and wind loading. Oh wait was that 3 or 4 or 5 elements. I still haven't solved that fundamental issue yet. I don't suppose the radiation pattern is too much of a concern at this point, as long as it is omnidirectional. Thomas Hi Thomas If you can use whatever frequency the antenna works best at, it may be practical to build one then use the frequency of best performance with that antenna. But, if you have some predetermined frequency that the antenna must perform well at, there is a problem building prototypes. It can get rather time consuming to build prototypes when using copper pipe. Aparently you are confident that you can evaluate the antenna's input impedance. I had figured that would be a fairly difficult task. I'll be very interested in this project. Please keep the group informed of your progress. Jerry KD6JDJ (who has designed similar antennas for commercial use) Hi Jerry, It is a predetermined frequency that I am building the antenna for. It is not determined if it will become a commercial product yet but I am trying to design it as such. I can see that it might be a little time consuming working with copper pipe, but once I get the formula right I should be good to go. I'll start buy calculating the half wave elements based on theory knowing my dielectric constant will be dry air or Argon. The design I have sketch is pretty neat and clean (on paper anyways). It has all the elements stacked directly on top of each other, unlike the traditional staggered approach you see in other designs. Also the dielectric chamber of the transmission elements are sealed and can be filled with a noble gas such as Argon to prevent corrosion and detuning from humidity. My background is in receiver and transmitter design, so I'm quite familiar with impedance matching and I understand how a Smith chart works on a network analyzer. I'm looking forward to working with copper pipe instead of 0201 capacitors and a microscope! Thomas Hi Thomas Your plan for this colinear antenna appears to be identical to the one I designed for ACI in Van Nuys Calif.. It was stack of lingths of copper tubes with no stagger. I dont remember what I finally did nor how it was assembled. I do remember that it worked and that my supervisor was impressed. Also, I remember that alot of impedance measurements were performed. I am sure you will get your antenna to work. I suspect you will get more familiar with that Smith Chart in the process. Jerry KD6JDJ |
A few questions about collinear coaxial antennas
"Jeff Liebermann" wrote in message ... On Tue, 25 Nov 2008 14:55:52 -0800, "Thomas Magma" wrote: I am about to attempt to build a UHF collinear coaxial antenna and am trying to finalize a design. What design? Drawing? Description? NEC model? Numbers? UHF is about 300 to 1000MHz. Any particular frequency? Incidentally, it's not a "coaxial antenna". It's an end fed vertical colinear using coaxial cable elements. First off I have read contradicting statements whether to use odd or even number of 1/2 wave elements. 1, 3, 5... or 1,2,4... Also I don't understand what the 1/4 wave whip is doing on the top without a ground plane (found in most designs), is this necessary for a receive antenna?. Instead of using coaxial cable, I will be building the 1/2 wave and 1/4 wave transmission lines out of ridged copper pipe with air as it's dielectric in order to maximize the velocity of propagation and therefore making true 1/2 wave elements. Does anyone see anything wrong with this approach? Yep, lots wrong. End fed colinear antennas are convenient but far from ideal. They're also deceptively simple where the problems only show up after the antenna is built. 1. Most of the RF comes out the bottom of the antenna. Very roughly, the first dipole belches 1/2 the RF, the next dipole belches 1/4 the RF, then 1/8th, and so on. This is NOT exact, but close enough to illustrate the problem. You can make it as long as you want, but if somehow manage to cover up the lower part of the antenna (a common problem on a rooftop or side mounted on a tower), most of the signal is history. 2. The alternating 1/2 wave coax cable type antenna is twice as long as necessary. Every other 1/2 wave coax section is essentially a non-radiationg phasing section. That's convenient for construction, but not very compact. A similar antenna, using a simple 1/2 wave hairping stub, with be half the length, with the same gain. 3. Coax is lossy. Coax phasing sections add un-necessary loss that is not present in an antenna that uses (for example) a hairpin stub or coil instead. Your copper pipe and air dielectric idea eliminates this problem, but I thought I would throw this in for those building them from coax cable scraps. 4. End fed antennas tend to have pattern uptilt. That may or may not be a problem depending on your unspecified application. The uptilt doesn't show up on free space models, but is certainly there if you include the effects of a rooftop, ground, or mast arm. If this is going on a mountain top, you might consider mounting it upside down. You can reduce the uptilt problem somewhat by cutting the antenna in half and feeding it in the middle (forming a dipole), rather than end feeding it. Several commercial antennas work this way. That also eliminates the need for ground plane radials at the base. 5. The effects of the radome can be critical. I built such a UHF antenna for 463MHz long ago. It worked well enough with exposed sections. However, when I potted it with urathane fence post foam in a PVC pipe enclosure, the center frequency drifted downward sufficiently to render the antenna useless. 6. Cutting the coax sections accurately is difficult. If you're not using a fixture for cutting, forget it. 7. Making it out of copper pipe is rather expensive but certainly possible. Making the insulators will be somewhat of a challenge. There's no velocity factor involved (Air=1) so the measurements will be simple. However, since there's an overlap between sections, I'm wondering from where to where you should measure. If you cut the outer copper tubing to exactly 1/2 wave, then you need a very thin insulator between sections to prevent shorts. Methinks there will need to be some cut-n-try along with some careful measurments (swept VSWR) along the way. Good luck. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 Hi Jeff, Thanks for all the good points, but you haven't scared me away yet:) My target frequency is around lets say 418MHz (that's not really it, I like to remain anonymous). It was interesting what you said about the radome and how it detuned the antenna. Do you think it was mainly the PVC or the urethane foam that caused the issue. I plan to use a fibreglass tubing and spacers so hopefully I don't see as much near field effects as you did. I have learned that some PVC pipes have certain conductive additives and are not so good for antenna use, plus it might be tough trying to sell a 'poop pipe' antenna commercially if it ever became a product of ours. Do you happen to know if I should be using a odd or even number of half wave elements in my design? I'm beginning to think it doesn't really matter. Thomas |
A few questions about collinear coaxial antennas
On Nov 26, 9:32*am, "Thomas Magma"
wrote: From modeling I did a long time ago: *there is a slight advantage to using high velocity factor line, but it's very marginal. *Just as a dipole doesn't need to be resonant to do a good job radiating (and receiving), so the elements in the coaxial collinear don't need to be resonant. *The phasing among the elements is dictated by the coax between the feedpoints. *Each gap between two elements is a feedpoint; across it is impressed the line voltage. *Since each line segment is a half wave long and the conductors are reversed at each junction, the voltage across each feedpoint is the same and in phase, less a small amount for line loss. *The element currents depend on mutual coupling among the elements, but my simulations for VF=0.66 to VF=1.00 indicated that the current phases were always very nearly the same. Generally, you'll want all the elements to look the same from the outside. *The top element should be the same length as the rest. *It's common to short the coax an electrical quarter wave up from the highest gap between elements; that reflects back an open circuit to the bottom of the top element, so really you could just as well make the top element a tube the same OD as the rest of the elements, connected to the inner conductor of the next lower section. *The feedpoint impedance at the bottom of the antenna is just the parallel combination of all the feedpoints, which are generally each fairly high (since each one is feeding a full-wave doublet, essentially), but with ten or so sections, the net is modest, generally around 100 ohms. Whatever the feedpoint impedance is, you need to match to it properly-- to whatever degree of matching is "proper" in your book. *I generally use a simple "L" network: *a variable C across the feedpoint, and an inductor to the feed line center conductor. *It matches the impedance and can tune out some reactance. *Then you need to decouple the antenna from the feedline, and from other metal in the area where it's mounted. *I generally use *self-resonant coils in the small feedline, one immediately below the antenna and one another quarter wave lower. You could also try sleeves or radials... Summary: *the coax provides proper feedpoint phasing (even if the elements are shorter than 1/2 wave because of the VF of the line used); a matching network lets you match to 50 ohms (or other impedance if you want); decoupling keeps "antenna" current off the feedline. Cheers, Tom Thanks Tom for the detailed explanation. My current sketch of my antenna design uses a quarter wave sleeve on the lower end of the antenna to stub the current off the feedline ground. I am hoping to see the antenna having a (somewhat) characteristic impedance of 50 ohms since the transmission elements although made out of copper pipe and copper rod are designed to be 50 ohm and the antenna is single end fed (unlike a dipole). I will make some provisions for a small tuning structure. If I do have to tune, the nice thing is that it will be a receive only antenna and I can get away with small RF components. Tom, since you seem to know quite a bit about collinear coaxial antenna design, do you know if I should be using an even or odd amount of half wave elements? I planned on four, do you see a problem with this. Thanks again, Thomas A couple comments: First, about the feedpoint impedance. With four elements (a rather short antenna at UHF), there are three gaps between elements, that represent three feedpoints. Each feedpoint directly couples to the two attached elements, and through mutual coupling to the other elements. Consider driving just two such elements: it's just a (nominally) full wave doublet, and the feedpoint impedance is pretty high. Large diameter elements drop the feedpoint impedance, but even with large (not "huge") elements, the impedance of such a doublet will be several hundred ohms. To a rough approximation, in the coaxial collinear, you are simply putting three of those in parallel, since you're connecting them through 1/2 wave (electrical length) coax. If the feedpoint is just a half wave (electrical length) of coax away from the bottom gap between elements, you'll see the same impedance echoed there. In ten element designs, I typically see around 100 ohms at that bottom antenna feed spot. I fully expect to see quite a bit higher than that with only four elements. You can simulate this pretty easily with a program like EZNEC: just make a structure with four elements, with equal voltage sources between each pair of elements; figure the parallel combination of the reported impedances at each feedpoint, and that will be a good estimate of the impedance you'll see any even number of half-waves of feedline removed from the antenna, assuming negligible loss in the feedline. Second, about number of elements: it really doesn't matter a whole lot. It will affect the impedance you see at the feedpoint, but the antenna gain should go up monotonically with the number of elements, if you get the phasing right and decouple the antenna from its surroundings appropriately. (Beware sleeve decoupling: it may not be as effective as you think...) I learned about these antennas when I got ****ed that I could never seem to make one work right when I followed the instructions given in the old ARRL pubs. I finally sat down with pen and paper and EZNEC and worked out what was really going on. One thing I learned was that the "magic" of the wax fill in the Stationmaster commercial antennas wasn't really magic; neither the line velocity factor nor the dielectric environment just outside the radiating elements is super- critical. It IS important to get the element lengths right so the phasing of the feedpoints is right. Once I understood, building one that worked right became pretty easy. Three steps: (1) use the coax VF and length to get the phasing right. Accept whatever feedpoint impedance that gives you. (2) Use a matching network (lumped or distributed, your choice) to match to that impedance. (3) Decouple the antenna from its surroundings (including but not limited to the feedline), so it can do its work properly. And I suppose (4) mount it up high and in the clear for best coverage. The rest is things like proper choices for good mechanical strength and reliability. Cheers, Tom |
A few questions about collinear coaxial antennas [radomes and dielectrics]
"Thomas Magma" wrote in message
... remain anonymous). It was interesting what you said about the radome and how it detuned the antenna. Do you think it was mainly the PVC or the urethane foam that caused the issue. I plan to use a fibreglass tubing and spacers so hopefully I don't see as much near field effects as you did. I have learned that some PVC pipes have certain conductive additives and are not so good ^^^^^^^^^^ for antenna use, plus it might be tough trying to sell a 'poop pipe' antenna commercially if it ever became a product of ours. There is a correction that should be made here. Polyvinyl chloride has high radio frequency losses, and the addition of plasticizers usually increases these losses. But these dielectric losses are NOT due to conduction. Rather, they are the result of hindered rotational movement in the chemical dipoles within the polymer structure itself. In an insulator, when an AC voltage is applied, most of the current through the capacitor formed by the insulator leads the applied voltage. In a perfect capacitor, the current leads the voltage by 90 degrees. But in a real capacitor, the insulator has dielectric losses which means that the current leads the applied voltage by less than 90 degrees; i.e. a portion of the current is now in phase with the applied voltage. This current produces heating of the insulator. AT A GIVEN FREQUENCY, the capacitor acts as if is a pure capacitance in series with a resistance (or in parallel with a conductance). This model of a real capacitor is only valid at that ONE frequency. At DC, for example, most capacitors show extremely little conduction. Their insulation resistance can be over 10^10 ohm-cm. At high RF frequencies, the dielectric loss increases. In the case of polyvinyl chloride, which is a hard, very brittle material, additives known as plasticizers are compounded into the PVC to produce the desired mechanical properties. A little plasticizer makes PVC tougher and easier to process. A lot of plasticizer makes PVC soft and pliable. Clear vinyl tubing can be as much as 40% plasticizer. Plasticizers are not chemically attached to the PVC polymer. This means that over time, the plasticizer can leach out or evaporate from the soft vinyl, leaving it hard and brittle again. Everyone is probably familiar with vinyl automobile seat covers. When your car is parked in the hot sun, a portion of the plasticizer evaporates out. Eventually the vinyl cracks and tears, and you wind up with a greasy, difficult to remove, oily film on the inside glass of the car. The plasticizer has left the vinyl, causing the cracking, and condensed on the glass making a greasy mess. The sticky, gooey mess seen on old vinyl power cords is also due to the plasticizer leaving the PVC and accumulating on the surface. Did you ever wonder what was meant when coaxial cable was described as having a non-contaminating vinyl jacket? This means that the plasticizer in the cable jacket leaches out, but very slowly compared to the service life of the cable. In older, and cheaper coax cable, conventional plasticizers are used which leach out or evaporate fairly quickly. This makes the cable stiffer and more prone to cracking. But long before this happens, the plasticizer has migrated into the polyethylene insulation surrounding the inner conductor, greatly increasing its RF losses. This can take just a few years. In some of the newer cables, a foil or metalized polyester layer surrounds the polyethylene under the shield. This effectively prevents the migration of the plasticizer. To go back to the antenna issue, polyurethane foams of low density (lots of void space) have a low dielectric constant and small loss tangent (small dissipation factor). "The Handbook of Antenna Design" By A. W. Rudge, K. Milne, A. David Oliver, and P. Knight, has a discussion of high strength polyurethane foams as radome materials. However these foams are different from the "Great Stuff" foams in a can that you buy at the local hardware store. These foams are moisture cured so their dielectric losses will be somewhat higher. Do not confuse these with latex foams which have much greater dielectric losses. Also remember that these uncured urethane foams have 4,4-methylene bisphenyl isocyanate as one component. This is a nasty material from a safety viewpoint (a skin and lung allergic sensitizer), so follow the instructions carefully about gloves and eye protection. To conclude, I would avoid the PVC material as a protective cover. Most common fiberglass tubes are either fiberglass/polyester or fiberglass/epoxy composites. Both materials have some dielectric loss but far less than PVC. Urethane foam will be a fairly good material to hold the antenna rigid within the tube. However the tube and the foam WILL detune the antenna meaning you will need to do some experimentation before you can produce the desired results. 73, Dr. Barry L. Ornitz WA4VZQ |
A few questions about collinear coaxial antennas
On Wed, 26 Nov 2008 10:50:37 -0800, "Thomas Magma"
wrote: Thanks for all the good points, but you haven't scared me away yet:) Good. I don't mind spending the time if you're willing to build it. None of my objections are particularly fatal. However, I would suggest you at least investigate alternatives, which in my opinion, work better (and are easier to build). For example, the 4 bay stacked vertical folded dipoles, with coaxial power dividers, is far less complexicated, and methinks works better. I was building these for 463/468MHz in about 1968(?) out of strips of 1/2" wide aluminum and pop rivets. If you're interested, I'll see if I can find some photos or scribblings. There are a few in this photo: http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn02.html but I can't distinguish mine from the stock dB Products antennas. Incidentally, that's a great example of how *NOT* to install antennas. Those are all transmit antennas with no ferrite isolators. The intermod generated was monumental. Some more examples of commercial versions: http://www.radiowrench.com/sonic/so02202.html http://www.radiowrench.com/sonic/ (look for dB Products PDF's) My target frequency is around lets say 418MHz (that's not really it, I like to remain anonymous). Y'er no fun. It was interesting what you said about the radome and how it detuned the antenna. Do you think it was mainly the PVC or the urethane foam that caused the issue. Both. I suspect you have a sweeper and some means of measuing reflection coefficient or VSWR in real time (on a scope). If not, the reflection coefficient bridge is easy to build. Take any antenna you look at the VSWR curve on the scope. Then, shove the pipe over the antenna and watch what happens. If the tubing were fiberglass, some thin plastics, or glass, nothing will change on the scope. PVC and ABC will detune the antenna. So will common fence post compound (urathane foam) but to a lesser degree. Packing the empty space with styrofoam or styroam peanuts seems to work well enough and result in a repairable antenna. Real fiberglass tubing (masts or marine hardware) is easy enough to obtain, that I wouldn't bother with PVC. Besides, fiberglass is nice and stiff, while PVC flops around in the wind. I plan to use a fibreglass tubing and spacers so hopefully I don't see as much near field effects as you did. Yep. There's always hope. I have learned that some PVC pipes have certain conductive additives and are not so good for antenna use, plus it might be tough trying to sell a 'poop pipe' antenna commercially if it ever became a product of ours. If you want to try a real disaster, try black drainage PVC pipe. Carbon filled. For a good acid test, try putting a pipe section in a microwave oven. If it stays cold, you win. If it gets hot, thing again. If it melts and catches fire, forget it. It's also fun to take an ordinary 440 yackie talkie or scanner, shove a piece of PVC pipe over the antenna, and listen to the signal change. I like to do this demo at radio club meetings. The best of the bunch is fiberglass. A close 2nd is white ABS (acrylo-nitrile butadene styrene) which is a bit difficult to find. It's commonly used in vacuum forming and commonly found on GPS antennas and such. Do you happen to know if I should be using a odd or even number of half wave elements in my design? I'm beginning to think it doesn't really matter. It matters quite a bit. However, I can't offer an answer. Some designes use the bottom section as a matching transformer or counterpoise. That mangles the count. I would have to see what you're doing to make the determination. Also, I could probably figure it out, but it's midnight and I'm beat. I spent 5 days last week fighting a kidney stone and am still kinda wiped from that. If you can't figure it it, bug me and I'll do the dirty work. A good clue is that the center conductor of the input coax connector must connect to the center wire which goes to the 1/4 vertical whip section at the top. Also, there are patents worth reading: http://www.google.com/patents?id=JMweAAAAEBAJ&dq=6947006 http://www.google.com/patents?id=qDYWAAAAEBAJ&dq=6947006 http://www.google.com/patents?id=XpgfAAAAEBAJ&dq=6947006 etc. The accompanying explanations are usually sufficient to figure out how it works. You might notice that one of the construction methods is applicable to your copper tubing idea. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
A few questions about collinear coaxial antennas
"Jeff Liebermann" wrote in message ... On Wed, 26 Nov 2008 10:50:37 -0800, "Thomas Magma" wrote: Thanks for all the good points, but you haven't scared me away yet:) Good. I don't mind spending the time if you're willing to build it. None of my objections are particularly fatal. However, I would suggest you at least investigate alternatives, which in my opinion, work better (and are easier to build). For example, the 4 bay stacked vertical folded dipoles, with coaxial power dividers, is far less complexicated, and methinks works better. I was building these for 463/468MHz in about 1968(?) out of strips of 1/2" wide aluminum and pop rivets. If you're interested, I'll see if I can find some photos or scribblings. There are a few in this photo: http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn02.html but I can't distinguish mine from the stock dB Products antennas. Incidentally, that's a great example of how *NOT* to install antennas. Those are all transmit antennas with no ferrite isolators. The intermod generated was monumental. Some more examples of commercial versions: http://www.radiowrench.com/sonic/so02202.html http://www.radiowrench.com/sonic/ (look for dB Products PDF's) My target frequency is around lets say 418MHz (that's not really it, I like to remain anonymous). Y'er no fun. It was interesting what you said about the radome and how it detuned the antenna. Do you think it was mainly the PVC or the urethane foam that caused the issue. Both. I suspect you have a sweeper and some means of measuing reflection coefficient or VSWR in real time (on a scope). If not, the reflection coefficient bridge is easy to build. Take any antenna you look at the VSWR curve on the scope. Then, shove the pipe over the antenna and watch what happens. If the tubing were fiberglass, some thin plastics, or glass, nothing will change on the scope. PVC and ABC will detune the antenna. So will common fence post compound (urathane foam) but to a lesser degree. Packing the empty space with styrofoam or styroam peanuts seems to work well enough and result in a repairable antenna. Real fiberglass tubing (masts or marine hardware) is easy enough to obtain, that I wouldn't bother with PVC. Besides, fiberglass is nice and stiff, while PVC flops around in the wind. I plan to use a fibreglass tubing and spacers so hopefully I don't see as much near field effects as you did. Yep. There's always hope. I have learned that some PVC pipes have certain conductive additives and are not so good for antenna use, plus it might be tough trying to sell a 'poop pipe' antenna commercially if it ever became a product of ours. If you want to try a real disaster, try black drainage PVC pipe. Carbon filled. For a good acid test, try putting a pipe section in a microwave oven. If it stays cold, you win. If it gets hot, thing again. If it melts and catches fire, forget it. It's also fun to take an ordinary 440 yackie talkie or scanner, shove a piece of PVC pipe over the antenna, and listen to the signal change. I like to do this demo at radio club meetings. The best of the bunch is fiberglass. A close 2nd is white ABS (acrylo-nitrile butadene styrene) which is a bit difficult to find. It's commonly used in vacuum forming and commonly found on GPS antennas and such. Do you happen to know if I should be using a odd or even number of half wave elements in my design? I'm beginning to think it doesn't really matter. It matters quite a bit. However, I can't offer an answer. Some designes use the bottom section as a matching transformer or counterpoise. That mangles the count. I would have to see what you're doing to make the determination. Also, I could probably figure it out, but it's midnight and I'm beat. I spent 5 days last week fighting a kidney stone and am still kinda wiped from that. If you can't figure it it, bug me and I'll do the dirty work. A good clue is that the center conductor of the input coax connector must connect to the center wire which goes to the 1/4 vertical whip section at the top. Also, there are patents worth reading: http://www.google.com/patents?id=JMweAAAAEBAJ&dq=6947006 http://www.google.com/patents?id=qDYWAAAAEBAJ&dq=6947006 http://www.google.com/patents?id=XpgfAAAAEBAJ&dq=6947006 etc. The accompanying explanations are usually sufficient to figure out how it works. You might notice that one of the construction methods is applicable to your copper tubing idea. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 I'm anxious to get started so I've put my copper pipe design on hold well I wait for parts and decided to start with a coax approach. So I hit the hardware store and got some PVC pipe and mounting bits. I understand that the PVC is not as good as fiberglass because of it's near field effects, BTW if you can tune those effects out, what is the end result in loss? I plan on using LMR-200 because of it's slight rigidity and it's high velocity factor (83%). I bought 1-1/2 inch rubber washers with a 3/16 hole in the center that will slide over the coax and then be pulled into the 1-1/4 inch PVC this will center and support the coax up the length of the pipe. I will try using some clamp-on ferrites that we have laying around to stub the currents on the feed line and slide them around and see if I can tune the antenna using the network analyzer. I still don't understand what that quarter wave whip is suppose to do that sits on top of the array and I think I will try to omit that in my first design (unless someone convinces me otherwise). Anyways, time to get my hands dirty and build me an antenna! Thomas Magma |
A few questions about collinear coaxial antennas [radomes and dielectrics]
"NoSPAM" wrote in message ... "Thomas Magma" wrote in message ... remain anonymous). It was interesting what you said about the radome and how it detuned the antenna. Do you think it was mainly the PVC or the urethane foam that caused the issue. I plan to use a fibreglass tubing and spacers so hopefully I don't see as much near field effects as you did. I have learned that some PVC pipes have certain conductive additives and are not so good ^^^^^^^^^^ for antenna use, plus it might be tough trying to sell a 'poop pipe' antenna commercially if it ever became a product of ours. There is a correction that should be made here. Polyvinyl chloride has high radio frequency losses, and the addition of plasticizers usually increases these losses. But these dielectric losses are NOT due to conduction. Rather, they are the result of hindered rotational movement in the chemical dipoles within the polymer structure itself. In an insulator, when an AC voltage is applied, most of the current through the capacitor formed by the insulator leads the applied voltage. In a perfect capacitor, the current leads the voltage by 90 degrees. But in a real capacitor, the insulator has dielectric losses which means that the current leads the applied voltage by less than 90 degrees; i.e. a portion of the current is now in phase with the applied voltage. This current produces heating of the insulator. AT A GIVEN FREQUENCY, the capacitor acts as if is a pure capacitance in series with a resistance (or in parallel with a conductance). This model of a real capacitor is only valid at that ONE frequency. At DC, for example, most capacitors show extremely little conduction. Their insulation resistance can be over 10^10 ohm-cm. At high RF frequencies, the dielectric loss increases. In the case of polyvinyl chloride, which is a hard, very brittle material, additives known as plasticizers are compounded into the PVC to produce the desired mechanical properties. A little plasticizer makes PVC tougher and easier to process. A lot of plasticizer makes PVC soft and pliable. Clear vinyl tubing can be as much as 40% plasticizer. Plasticizers are not chemically attached to the PVC polymer. This means that over time, the plasticizer can leach out or evaporate from the soft vinyl, leaving it hard and brittle again. Everyone is probably familiar with vinyl automobile seat covers. When your car is parked in the hot sun, a portion of the plasticizer evaporates out. Eventually the vinyl cracks and tears, and you wind up with a greasy, difficult to remove, oily film on the inside glass of the car. The plasticizer has left the vinyl, causing the cracking, and condensed on the glass making a greasy mess. The sticky, gooey mess seen on old vinyl power cords is also due to the plasticizer leaving the PVC and accumulating on the surface. Did you ever wonder what was meant when coaxial cable was described as having a non-contaminating vinyl jacket? This means that the plasticizer in the cable jacket leaches out, but very slowly compared to the service life of the cable. In older, and cheaper coax cable, conventional plasticizers are used which leach out or evaporate fairly quickly. This makes the cable stiffer and more prone to cracking. But long before this happens, the plasticizer has migrated into the polyethylene insulation surrounding the inner conductor, greatly increasing its RF losses. This can take just a few years. In some of the newer cables, a foil or metalized polyester layer surrounds the polyethylene under the shield. This effectively prevents the migration of the plasticizer. To go back to the antenna issue, polyurethane foams of low density (lots of void space) have a low dielectric constant and small loss tangent (small dissipation factor). "The Handbook of Antenna Design" By A. W. Rudge, K. Milne, A. David Oliver, and P. Knight, has a discussion of high strength polyurethane foams as radome materials. However these foams are different from the "Great Stuff" foams in a can that you buy at the local hardware store. These foams are moisture cured so their dielectric losses will be somewhat higher. Do not confuse these with latex foams which have much greater dielectric losses. Also remember that these uncured urethane foams have 4,4-methylene bisphenyl isocyanate as one component. This is a nasty material from a safety viewpoint (a skin and lung allergic sensitizer), so follow the instructions carefully about gloves and eye protection. To conclude, I would avoid the PVC material as a protective cover. Most common fiberglass tubes are either fiberglass/polyester or fiberglass/epoxy composites. Both materials have some dielectric loss but far less than PVC. Urethane foam will be a fairly good material to hold the antenna rigid within the tube. However the tube and the foam WILL detune the antenna meaning you will need to do some experimentation before you can produce the desired results. 73, Dr. Barry L. Ornitz WA4VZQ Thanks Barry for the in-depth enlightenment of radome material :) Do you happen to know the answer to this question: When you tune the near field effects of PVC out, what is the end result in loss? and how is this compared to fiberglass? Thomas |
A few questions about collinear coaxial antennas
On Thu, 27 Nov 2008 10:10:25 -0800, "Thomas Magma"
wrote: I'm anxious to get started so I've put my copper pipe design on hold well I wait for parts and decided to start with a coax approach. Sigh. So I hit the hardware store and got some PVC pipe and mounting bits. Schedule 40, schedule 80, water, or electrical? They're all different. Did you at least do the microwave oven test on a small piece to see if you're headed for a problem? I've had a few surprises with different vendors and styles. I understand that the PVC is not as good as fiberglass because of it's near field effects, BTW if you can tune those effects out, what is the end result in loss? No. You can't make the radome (pipe) big enough to get out of the near field. Minimum is a few wavelengths. Try a chunk of PVC over your 440 HT or scanner whip antenna and see if you want to continue blundering along this path. I plan on using LMR-200 because of it's slight rigidity and it's high velocity factor (83%). The added rigidity doesn't buy you much if you're going to shove it down a pipe. I bought 1-1/2 inch rubber washers with a 3/16 hole in the center that will slide over the coax and then be pulled into the 1-1/4 inch PVC this will center and support the coax up the length of the pipe. Why such a large diameter pipe? There's no difference in loss. I will try using some clamp-on ferrites that we have laying around to stub the currents on the feed line and slide them around and see if I can tune the antenna using the network analyzer. Got a ferrite that works at 418MHz? Even if the ferrite does work, the RF its blocking is converted to heat. Wouldn't it be better if you built a proper matching contrivance to that RF is radiated instead of absorbed? I suggest you lose the ferrites and band-aids as they tend to hide design errors and inefficiencies. I still don't understand what that quarter wave whip is suppose to do that sits on top of the array I hate easy questions. If you look at the construction of the alternating coax sections, the top section will be one with the hot RF lead eventually connected to the outside of the top coax section. In other words, the outside of the coax is the radiating element. http://www.rason.org/Projects/collant/collant.htm Why bother using another coax section when it would be easier to just use a piece of wire? Look at the Fig 3 drawing and just follow the RF path from the coax entry at the left to the 1/4 wave element on the right. That might also answer your question about odd/even sections. and I think I will try to omit that in my first design (unless someone convinces me otherwise). Not recommended, but you have the test equipment to determine if it's a good or bad idea. Ummm... you were planning on testing this thing? Anyways, time to get my hands dirty and build me an antenna! Good luck, but first a little math. What manner of tolerance do you thing you need to cut your coax pieces? Let's pretend you wanted to get the center frequency accurate to 1Mhz. At 418MHz, one wavelength is: wavelength(mm) = 300,000 / freq(mhz) * VF wavelength = 3*10^5 / 418 * 0.83 = 596 mm That works out to: 596 / 418 = 1.4 mm/MHz So, if you want the center frequency accurate to within +/- 1MHz, you gotta cut it to within +/- 1.4 mm. Good luck. Like I previously ranted, you'll need a cutting fixture. A steady hand, good eye, quality coax, and plenty of patience are also helpful. Incidentally, since the top 1/4 wave element represents something close to perhaps 50 ohms, it would be interesting to measure the amount of RF that isn't radiated and actually gets to the top section of the antenna. If my analysis of the antenna is correct, the first section (near the coax connector) radiates 1/2 the power. The next section 1/4th. After that 1/8th, etc. By the time it gets to the top of the antenna, there won't be much left. However, that's theory, which often fails to resemble reality. It would interesting if you stuck a coax connector on the top, and measured what comes out. Happy Day of the Turkeys. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
A few questions about collinear coaxial antennas [radomes and dielectrics]
"Thomas Magma" wrote in message
... Thanks Barry for the in-depth enlightenment of radome material :) Do you happen to know the answer to this question: When you tune the near field effects of PVC out, what is the end result in loss? and how is this compared to fiberglass? When you tune the antenna to compensate for its surroundings, usually by shortening the elements since the real portion of the permittivity of the close surroundings is capacitive, you just have to accept the added loss due to the imaginary portion of the permittivity, the dielectric dissipation. When designing a radome, look for a material with the lowest dissipation factor (or loss tangent which is another way of describing the dissipation losses). The added capacitive reactance can be tuned out, but it is still best to use a material with a low dielectric constant. This does not say that the antenna pattern will not be affected, however, as the antenna lengths and spacings are changed. A good analogy to illustrate this is the fact that an aircraft cannot be perfectly stealthy. If the aircraft is made entirely from microwave transparent material, the difference between that material's dielectric constant and that of the surrounding air will still generate reflections that will show up on radar. In fact, if you make the aircraft out of completely absorbing material, the bow wave compression of the air will create a slight dielectric discontinuity which still reflects microwaves. Of course, the radar cross section will be far smaller. The idea is to make the radar target appear so small that it is ignored. While the dielectric constant of PVC is slightly lower than fiberglass reinforced polyester or fiberglass reinforced epoxy, its lost tangent is higher. Also, the mechanical strength of PVC is less than the fiberglass reinforced plastics, so you can make the radome thinner with the FRP materials. This benefits the detuning as well as the losses. Before closing, I would like to comment on an additional related issue brought up by my friend AE6KS... "Jeff Liebermann" wrote in message ... If you want to try a real disaster, try black drainage PVC pipe. Carbon filled. For a good acid test, try putting a pipe section in a microwave oven. I would be willing to bet that the carbon black had little to do with the losses. It only takes a tiny amount of carbon black to make the plastic quite black looking and to provide good ultraviolet light protection - just a few percent at most. I once needed a good microwave absorber for an instrument I was building. Not wanting to wait on Emerson & Cuming to sell me some of their ECCOSORB ® material, I decided to make my own. Just down the hall was our polymer testing lab where they could blend polymer chips with various additives and mold me some 4" x 4" blocks from the blend. I decided to have them make a polyethylene blend with 30% carbon black added. The finished material was a dull black, and it would even leave nice black marks when rubbed across paper. I was confident when I placed a 1/2" thick block of the material between two WR-90 10 GHz flanges that it would not let much signal through. Boy was I surprised when it offered very little attenuation! It took a while to think about what was happening. The polyethylene was a low loss dielectric material at these frequencies, and the carbon black particles were extremely small. The result was a plastic that contained conductive particles that were insulated from each other. The particles were so small that even at 10 GHz, they were far too small to couple much microwave energy into them. What I really needed was long carbon fibers that made electrical contact with each other in the plastic. I didn't have time to study this as the Eccosorb arrived and I used it in my instrument. But it taught me a valuable lesson about absorbers. By the way, http://www.eccosorb.com has a number of good technical tutorials on absorbers and dielectrics. 73, Barry WA4VZQ |
A few questions about collinear coaxial antennas
On Nov 27, 2:46*pm, Jeff Liebermann wrote:
.... Good luck, but first a little math. *What manner of tolerance do you thing you need to cut your coax pieces? *Let's pretend you wanted to get the center frequency accurate to 1Mhz. *At 418MHz, one wavelength is: * *wavelength(mm) = 300,000 / freq(mhz) * VF * *wavelength = 3*10^5 / 418 * 0.83 = 596 mm That works out to: * *596 / 418 = 1.4 mm/MHz So, if you want the center frequency accurate to within +/- 1MHz, you gotta cut it to within +/- 1.4 mm. *Good luck. *Like I previously ranted, you'll need a cutting fixture. *A steady hand, good eye, quality coax, and plenty of patience are also helpful. But why would you care to try to get it within 1MHz? With only four radiating elements, the beam 3dB width will be roughly 8 degrees if the bottom of the antenna is a wavelength above ground (30 degrees in freespace...). There's not much point in putting a lot of effort into get closer than perhaps 4 electrical degrees along the line, and I don't believe even that is necessary to get good performance. That's several mm, and should be easy with such short lengths. Using foam- Teflon coax makes it easy to do: the insulation doesn't melt when you solder things together. I cut the sections to matched lengths, use a little jig to trim the layers to the same lengths on each, and then put a wrapping of 30AWG or so silver plated wire (wire-wrap wire) around each joint to hold it while soldering. That makes it easy to adjust before soldering, and solid after. Incidentally, since the top 1/4 wave element represents something close to perhaps 50 ohms, it would be interesting to measure the amount of RF that isn't radiated and actually gets to the top section of the antenna. *If my analysis of the antenna is correct, the first section (near the coax connector) radiates 1/2 the power. *The next section 1/4th. *After that 1/8th, etc. *By the time it gets to the top of the antenna, there won't be much left. *However, that's theory, which often fails to resemble reality. *It would interesting if you stuck a coax connector on the top, and measured what comes out. There's very little loss in a half wave of decent coax at 450MHz. That means that the voltage across the lowest junction between sections is echoed up the antenna at each other junction. In freespace, by symmetry, the currents will be very nearly the same going down from the top as going up from the bottom. My model over typical ground (bottom a wavelength above the ground) shows current symmetry within a percent or so, assuming equal voltages driving each of the three junctions. If you wish, you can use the parameters of the line you're actually using to figure the differences among the feedpoint voltages, based on the loads at each junction. When I've done that in the past, the differences are practically negligible. You can iterate, feeding those voltages back into the model to find new load impedances, etc., repeating till you're happy that the models have converged. Recent versions of EZNEC even let you put the transmission line into the model, along with its loss. The supporting tube certainly will affect the feedpoint impedance, but in my experience, it does not materially affect the pattern. I deal with the impedance through a matching network; it's no trouble to adjust for a low enough reflection that I don't worry about it. Decoupling is the more interesting problem, to me. Cheers, Tom |
A few questions about collinear coaxial antennas
Jeff Liebermann wrote:
. . . Incidentally, since the top 1/4 wave element represents something close to perhaps 50 ohms, it would be interesting to measure the amount of RF that isn't radiated and actually gets to the top section of the antenna. If my analysis of the antenna is correct, the first section (near the coax connector) radiates 1/2 the power. The next section 1/4th. After that 1/8th, etc. By the time it gets to the top of the antenna, there won't be much left. However, that's theory, which often fails to resemble reality. It would interesting if you stuck a coax connector on the top, and measured what comes out. I'm intrigued by this, and would like to know what "theory" it's based on. The field radiated from a conductor is proportional to the current on it. You'll see from either modeling or measurement that the currents on all sections of a collinear array, or a long wire antenna for that matter, are nearly the same. So in those directions in which the fields reinforce, each section is contributing about the same amount to the total field as any other. Although the logic is sound for this particular situation, it can't be used in general to assign particular amounts of radiated power to particular parts of an antenna. The fields from two parts of the antenna might partially or fully cancel in some directions, even though both are producing large fields. Any part of the antenna which is carrying current is involved in the radiation process, and the total field is the vector, not algebraic, sum of those fields. So if you have a valid method of determining how much of the total radiated power comes from each part of an antenna, I'd be very interested in learning more about it. References would be welcome. Roy Lewallen, W7EL |
A few questions about collinear coaxial antennas
On Fri, 28 Nov 2008 18:55:26 -0800, Roy Lewallen
wrote: Jeff Liebermann wrote: . . . Incidentally, since the top 1/4 wave element represents something close to perhaps 50 ohms, it would be interesting to measure the amount of RF that isn't radiated and actually gets to the top section of the antenna. If my analysis of the antenna is correct, the first section (near the coax connector) radiates 1/2 the power. The next section 1/4th. After that 1/8th, etc. By the time it gets to the top of the antenna, there won't be much left. However, that's theory, which often fails to resemble reality. It would interesting if you stuck a coax connector on the top, and measured what comes out. I'm intrigued by this, and would like to know what "theory" it's based on. I just knew this would create a problem. I'm open to corrections and explanations. I'm still learning and tend to make some rather disgusting fundamental errors. It's an observation based upon my measurements with a field strength meter on similar UHF colinear antennas (using 1/2 wave stubs for phasing). Also on a center fed 2.4GHz Franklin sector antenna of similar construction. Most of the voltage peaks were at the base of the antenna, tapering off as the field strength meter was dragged to the top of the antenna. Since the current through the antenna is constant, I assumed that the bulk of the power came from the lower elements of the antenna. My explanation was a geometric decrease in radiatated power starting at the feed point. I've also seen a similar effect with relatively high gain (10dbi) 2.4GHz omni antennas in WISP applications. Any blockage of the lower sections of the antenna, had a much bigger effect on the range and measure signal strength than covering roughly an equal amount near the top of the antenna. The field radiated from a conductor is proportional to the current on it. You'll see from either modeling or measurement that the currents on all sections of a collinear array, or a long wire antenna for that matter, are nearly the same. So in those directions in which the fields reinforce, each section is contributing about the same amount to the total field as any other. I can see that on some models. I never could successfully model an antenna using coax cable sections as elements. Using a wire model, the current distribution is constant along the length as you describe. However, my field strength measurements show more RF towards the feed point. It's difficult for me to tell exactly how much more RF because my home made meter is not calibrated. I don't recall the exact numbers but I can dig out the FSM and make some measurements on some of the antennas I have hanging around on the roof this weekend. Although the logic is sound for this particular situation, it can't be used in general to assign particular amounts of radiated power to particular parts of an antenna. The fields from two parts of the antenna might partially or fully cancel in some directions, even though both are producing large fields. Any part of the antenna which is carrying current is involved in the radiation process, and the total field is the vector, not algebraic, sum of those fields. The models all show the total pattern produced by all the elements combined. I haven't found a way to show the contributions by individual elements, thus making it difficult to model my observation. So if you have a valid method of determining how much of the total radiated power comes from each part of an antenna, I'd be very interested in learning more about it. References would be welcome. Nope. I'll give in easily on this one as it's highly likely I'm wrong. However, I will double check my measurements on the roof tomorrow and see if they're reproducible. I may have simply goofed and/or drawn the wrong conclusion. Incidentally, I've been offering this observation for several years and you are the first to question it. Roy Lewallen, W7EL -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
A few questions about collinear coaxial antennas
Jeff Liebermann wrote:
. . . It's an observation based upon my measurements with a field strength meter on similar UHF colinear antennas (using 1/2 wave stubs for phasing). Also on a center fed 2.4GHz Franklin sector antenna of similar construction. Most of the voltage peaks were at the base of the antenna, tapering off as the field strength meter was dragged to the top of the antenna. Since the current through the antenna is constant, I assumed that the bulk of the power came from the lower elements of the antenna. My explanation was a geometric decrease in radiatated power starting at the feed point. There's quite a handful of potential problems with this: 1. You might have been in the near field. The relationship between field strength in the near field and the radiated far field is very complex. You can't determine the field in one based on measurements in the other. 2. If you're in the near field, the field strength you measure at a given point depends on the type of antenna used. In the far field, the field impedance (E/H) is a constant value, but not so in the near field. In various places in the near field, an antenna which responds more strongly to the E field (a "high impedance" antenna) will show higher readings where the field impedance is high, and lower where it's lower. In any case, the relationship between radiated field and local near field strength isn't simple. 3. The power applied to the antenna is radiated in all directions, although of course unequally. As I explained in my last posting, the total field is the vector sum of the fields from the individual parts of the antenna. Sampling near the antenna gives you no idea of how the fields sum at a distant point. 4. It's very difficult to make even roughly accurate measurements even at HF, let alone UHF or higher. One of several problems is that it's extremely difficult to decouple the feedline when an electrically small probe is used, so you end up not measuring what you think you are. I've also seen a similar effect with relatively high gain (10dbi) 2.4GHz omni antennas in WISP applications. Any blockage of the lower sections of the antenna, had a much bigger effect on the range and measure signal strength than covering roughly an equal amount near the top of the antenna. That's interesting, and I'd like to get some more information about it. Perhaps blocking the bottom had a greater effect on the pattern, moving the maximum away from the direction of the other end of the path? I can see that on some models. I never could successfully model an antenna using coax cable sections as elements. Using a wire model, the current distribution is constant along the length as you describe. However, my field strength measurements show more RF towards the feed point. It's difficult for me to tell exactly how much more RF because my home made meter is not calibrated. I don't recall the exact numbers but I can dig out the FSM and make some measurements on some of the antennas I have hanging around on the roof this weekend. Here's a model of a coax collinear, but using coax with unity velocity factor. This "Franklin" array model was created by Linley Gumm, K7HFD. Coaxial cable is modeled as a combination of transmission line model, to represent the inside of the coax, and a wire to represent the outside. The technique is described in the EZNEC manual. See "Coaxial Cable, Modeling" in the index. I've posted the EZNEC equivalent to http://eznec.com/misc/rraa/ as COAXVERT.EZ. The accompanying Antenna Notes file is also there as COAXVERT.txt. CM Coaxial Vertical Antenna CM CM ! Wire # 16 for I srcs, shorted/open TL, and/or loads. CE GW 1,1,0.,0.,6.76615,.02081892,0.,6.76615,.000127 GW 2,1,0.,0.,5.766841,.02081892,0.,5.725204,.000127 GW 3,1,0.,0.,4.684258,.02081892,0.,4.725896,.000127 GW 4,1,0.,0.,3.684949,.02081892,0.,3.643311,.000127 GW 5,1,0.,0.,2.602366,.02081892,0.,2.644002,.000127 GW 6,1,0.,0.,1.603057,.02081892,0.,1.561419,.000127 GW 7,1,0.,0.,.5204737,.02081892,0.,.5621104,.000127 GW 8,11,0.,0.,6.76615,0.,0.,5.766841,.00635 GW 9,11,.02081892,0.,5.725204,.02081892,0.,4.725896,. 00635 GW 10,11,0.,0.,4.684258,0.,0.,3.684949,.00635 GW 11,11,.02081892,0.,3.643311,.02081892,0.,2.644002, .00635 GW 12,11,0.,0.,2.602366,0.,0.,1.603057,.00635 GW 13,11,.02081892,0.,1.561419,.02081892,0.,.5621104, .00635 GW 14,6,0.,0.,.5204737,0.,0.,0.,.00635 GW 15,1,0.,0.,0.,.02081892,0.,.02081892,.000127 GW 16,1,208.1892,208.1892,208.1892,208.1913,208.1913, 208.1913,2.0819E-4 GE 1 FR 0,1,0,0,144. GN 1 EX 0,16,1,0,0.,1.414214 NT 16,1,15,1,0.,0.,0.,1.,0.,0. TL 1,1,2,1,50.,1.040946,0.,0.,0.,0. TL 2,1,3,1,50.,1.040946,0.,0.,0.,0. TL 3,1,4,1,50.,1.040946,0.,0.,0.,0. TL 4,1,5,1,50.,1.040946,0.,0.,0.,0. TL 5,1,6,1,50.,1.040946,0.,0.,0.,0. TL 6,1,7,1,50.,1.040946,0.,0.,0.,0. TL 7,1,15,1,-50.,1.040946,0.,0.,0.,0. RP 0,181,1,1000,90.,0.,-1.,0.,0. EN I've seen models using coax with VF = 0.82 having a good pattern. Nope. I'll give in easily on this one as it's highly likely I'm wrong. However, I will double check my measurements on the roof tomorrow and see if they're reproducible. I may have simply goofed and/or drawn the wrong conclusion. Incidentally, I've been offering this observation for several years and you are the first to question it. This isn't the first time that's happened. Roy Lewallen, W7EL |
A few questions about collinear coaxial antennas
On Fri, 28 Nov 2008 17:08:06 -0800 (PST), K7ITM wrote:
On Nov 27, 2:46*pm, Jeff Liebermann wrote: ... Good luck, but first a little math. *What manner of tolerance do you thing you need to cut your coax pieces? *Let's pretend you wanted to get the center frequency accurate to 1Mhz. *At 418MHz, one wavelength is: * *wavelength(mm) = 300,000 / freq(mhz) * VF * *wavelength = 3*10^5 / 418 * 0.83 = 596 mm That works out to: * *596 / 418 = 1.4 mm/MHz So, if you want the center frequency accurate to within +/- 1MHz, you gotta cut it to within +/- 1.4 mm. *Good luck. *Like I previously ranted, you'll need a cutting fixture. *A steady hand, good eye, quality coax, and plenty of patience are also helpful. But why would you care to try to get it within 1MHz? I don't. I wanted a number to show how accurate the cut would need to be if he wanted the minimum VSWR point to be accurate to within 1MHz. I picked 1MHz because the tolerance is easily scaled to other bandwidth and accuracy numbers. My main point was that a fixture of some sort was necessary to obtain that level of accuracy. With only four radiating elements, the beam 3dB width will be roughly 8 degrees if the bottom of the antenna is a wavelength above ground (30 degrees in freespace...). There's not much point in putting a lot of effort into get closer than perhaps 4 electrical degrees along the line, and I don't believe even that is necessary to get good performance. That's several mm, and should be easy with such short lengths. Well, at 418MHz, one wavelength (electrical) is about 600 mm. 4 degrees is: 600 mm * 4/360 = 6.7 mm Yeah, that's fairly loose and could be done with diagonal cutters and a tape measure. Normally, I would punch the numbers into an NEC model, but I couldn't figure out how to model a radiating coax cable section as an antenna element (using 4NEC2). I sorta faked it with wire segments, but got stumped on what to do with the dielectric and it's velocity factor. Here's one way to build a coax cable colinear (for 2.4GHz): http://www.nodomainname.co.uk/Omnicolinear/2-4collinear.htm Note the measurements in Fig 1 for where to measure the half wavelength sections. I'm suspicious. Of course, at 418MHz, it's less critical. Using foam- Teflon coax makes it easy to do: the insulation doesn't melt when you solder things together. I've only played with the RG6/u CATV flavor, where everything is crimped. I never have tried to solder the stuff. RG8/u with foam teflon dielectric: http://www.westpenn-cdt.com/pdfs/coaxial_spec_pdfs/50%20ohm%20cables/25810.pdf Looks nice. I cut the sections to matched lengths, use a little jig to trim the layers to the same lengths on each, and then put a wrapping of 30AWG or so silver plated wire (wire-wrap wire) around each joint to hold it while soldering. That makes it easy to adjust before soldering, and solid after. See photos of the jig at bottom of: http://www.nodomainname.co.uk/Omnicolinear/2-4collinear.htm Incidentally, since the top 1/4 wave element represents something close to perhaps 50 ohms, it would be interesting to measure the amount of RF that isn't radiated and actually gets to the top section of the antenna. *If my analysis of the antenna is correct, the first section (near the coax connector) radiates 1/2 the power. *The next section 1/4th. *After that 1/8th, etc. *By the time it gets to the top of the antenna, there won't be much left. *However, that's theory, which often fails to resemble reality. *It would interesting if you stuck a coax connector on the top, and measured what comes out. There's very little loss in a half wave of decent coax at 450MHz. That means that the voltage across the lowest junction between sections is echoed up the antenna at each other junction. In freespace, by symmetry, the currents will be very nearly the same going down from the top as going up from the bottom. My model over typical ground (bottom a wavelength above the ground) shows current symmetry within a percent or so, assuming equal voltages driving each of the three junctions. If you wish, you can use the parameters of the line you're actually using to figure the differences among the feedpoint voltages, based on the loads at each junction. When I've done that in the past, the differences are practically negligible. You can iterate, feeding those voltages back into the model to find new load impedances, etc., repeating till you're happy that the models have converged. Recent versions of EZNEC even let you put the transmission line into the model, along with its loss. I hate to admit that I made a mistake, but as you and Roy Lewallen point out, my explanation of how this antenna operates is almost certainly wrong. I'll do a fast measurement tomorrow to satisfy my curiousity, but from your explanation and Roy's, I've erred big time. With a constant current distribution along the length of the antenna, and a constant voltage at the various feed points, it's a fair conclusion that the power radiatated around each of these feed points are equal. I goofed(tm). The supporting tube certainly will affect the feedpoint impedance, but in my experience, it does not materially affect the pattern. I deal with the impedance through a matching network; it's no trouble to adjust for a low enough reflection that I don't worry about it. Decoupling is the more interesting problem, to me. Cheers, Tom Gone sulking... -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
A few questions about collinear coaxial antennas
On Fri, 28 Nov 2008 20:55:31 -0800, Roy Lewallen
wrote: 1. You might have been in the near field. The relationship between field strength in the near field and the radiated far field is very complex. You can't determine the field in one based on measurements in the other. That probably a good start. My testing was a 2.4GHz. My field strength meter was just a shottky diode, balun, ferrite/choke isolation, DC amp, and battery. Not very fancy and also not very sensitive. I tried to calibrate it against a microwave oven leakage meter, but go nowhere. My guess is that I was about 20cm away from an 8dBi vertical in one test. The antenna was a Tecom colinear. See omnis at: http://11junk.com/jeffl/antennas/tecom/ I still have some of these antennas and plan to repeat my testing. At 2.4GHz, one wavelength is about 12.5 cm, so 20cm is well within the near field. There was also a bunch of other antennas nearby, which certainly contributed some reflections. 2. If you're in the near field, the field strength you measure at a given point depends on the type of antenna used. In the far field, the field impedance (E/H) is a constant value, but not so in the near field. In various places in the near field, an antenna which responds more strongly to the E field (a "high impedance" antenna) will show higher readings where the field impedance is high, and lower where it's lower. In any case, the relationship between radiated field and local near field strength isn't simple. Umm... you lost me, but I'm not at my best right now. I'm in the last 2 weeks of radiation oncology. No problems but I currently fade fairly fast in the late evening. I'll decode it all tomorrow. 3. The power applied to the antenna is radiated in all directions, although of course unequally. As I explained in my last posting, the total field is the vector sum of the fields from the individual parts of the antenna. Sampling near the antenna gives you no idea of how the fields sum at a distant point. Agreed, but I was trying to sample what was being radiated from a single element (or antenna section). I could see some peaks and nulls as I moved along the length of the antenna, so I assumed that I was seeing the contributions of each section (at the peaks). 4. It's very difficult to make even roughly accurate measurements even at HF, let alone UHF or higher. One of several problems is that it's extremely difficult to decouple the feedline when an electrically small probe is used, so you end up not measuring what you think you are. I know. My meter is battery operated and made to be viewed with binoculars. I've used it to measure the total pattern on several antennas by hoisting it up and down a fiberglass pole (or wood barn) without any connecting wires. The problems are that it takes 2 people to operate (the 2nd to watch the meter in the binoculars). The contraption is also slightly directional, adding some additional errors. However, the big problem is that its sensitivity absolutely sucks. I need something better. I've tried to modify a Wi-Fi finder to act as a signal strength meter. That's more sensitive and works better but has a miserable 30dB(?) dynamic range. This is on the things to do list (after 100 other unfinished projects). I've also seen a similar effect with relatively high gain (10dbi) 2.4GHz omni antennas in WISP applications. Any blockage of the lower sections of the antenna, had a much bigger effect on the range and measure signal strength than covering roughly an equal amount near the top of the antenna. That's interesting, and I'd like to get some more information about it. Perhaps blocking the bottom had a greater effect on the pattern, moving the maximum away from the direction of the other end of the path? Ummm... I wasn't really able to move the tower on which the antenna was mounted. The problem was that I was stuck on the lower part of a rooftop tower. On the roof was also a parapet and HVAC box that blocked the downward view. The antenna was an overkill 12dBi something (forgot model numbers) omni. The antenna was about 3 meters from the parapet. We have a customer that was in the shadow area. From his window, we could see the top half of the antenna, but not the bottom. We installed an indoor dish antenna, but the office aesthetics committee vetoed the installation. So, I raise the base of the antenna, so that more of the bottom of the antenna was visible. The problem with this was that the top part of the antenna was in the middle of a latticework tower section used as a horizontal antenna mounting arm. The upper 25 cm of the antenna was fairly well covered. Yet, the improvement at the customers was both dramatic and adequate. I left it that way for about 2 months. When the weather improved, I replaced the antenna with a lower gain 8dBi omni, which improved the signal even more. A month later, I installed two 120 degree Superpass sector antennas (forgot exact model number), with some downtilt, and the single increased yet again. My guess(tm) was that the effects of covering the lower part of the original antenna was greater than covering approximately the same amount at the top of the same antenna. Maybe not. Here's a model of a coax collinear, but using coax with unity velocity factor. This "Franklin" array model was created by Linley Gumm, K7HFD. Coaxial cable is modeled as a combination of transmission line model, to represent the inside of the coax, and a wire to represent the outside. The technique is described in the EZNEC manual. See "Coaxial Cable, Modeling" in the index. I've posted the EZNEC equivalent to http://eznec.com/misc/rraa/ as COAXVERT.EZ. The accompanying Antenna Notes file is also there as COAXVERT.txt. Nice and thanks. Forgive my use of a different modeling program but it's one I know well, while EZNEC 5.1 is still somewhat of a mystery to me. I converted the EZ file to NEC and ran the model without modification. See: http://11junk.com/jeffl/antennas/CoaxVert/ The geometry JPG shows the current distribution, which is as you indicated, uniform. So much for my geometric decrease theory. I'll play with it some more later. I don't really understand the TL card, but will do some RTFM to see what I missed. 4NEC2 complained about wire radius ratios, but I'll fix that tomorrow. I also want to add a frequency sweep and move the design to UHF. I've seen models using coax with VF = 0.82 having a good pattern. Well, if the OP builds it with copper tubing, PTFE insulators, and air dielectric, he can use a velocity factor = 1.0. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
A few questions about collinear coaxial antennas
On Nov 25, 5:55*pm, "Thomas Magma"
wrote: Hello, I am about to attempt to build a UHF collinear coaxial antenna and am trying to finalize a design. I have done a lot of reading and am a little confused on a few things. First off I have read contradicting statements whether to use odd or even number of 1/2 wave elements. 1, 3, 5... or 1,2,4... Also I don't understand what the 1/4 wave whip is doing on the top without a ground plane (found in most designs), is this necessary for a receive antenna?. Instead of using coaxial cable, I will be building the 1/2 wave and 1/4 wave transmission lines out of ridged copper pipe with air as it's dielectric in order to maximize the velocity of propagation and therefore making true 1/2 wave elements. Does anyone see anything wrong with this approach? Thomas I just built one of these using thin wall 1 1/4" aluminum tubing and bare 20ga copper wire for inside. It was a tri band for 50, 144 & 440 mhz, section = 1/4 and 3 x 1/2 wave then top 1/4 section cut for 146mhz. SWR is higher than I like but it receives and signal reports are so much better than my factory 5/8 antenna I will now try to lower the swr. I used short pieces of PVC to couple the tubing, its freestanding and 15' tall, very light weight, I used old swmming pool cleaner telescopic pole for the elements. N4aeq unaxesc |
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