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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 |
#2
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![]() 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 |
#3
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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 |
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