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#1
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Help with Sleeve Dipole
I am a little confused with calculation of appropriate length of metal
tubing used in a sleeve antenna design (the 1/4 wave section). I understand that as RF travels through various mediums, Velocity of the wave slows down relative to Er of the material. So for coax the wavelength would be say 0.66 Lambda. With the tubing over a coaxial cable though it seems there is Vp related to the dielectric formed between the sleeve and the outer sheath of the coax cable but also the Metal has an Er value. 1. Do you need to count for both these when calculate how the tube should be. 2. Is there relationship that needs to be considered between sleeve length and distance between sleeve and coax (ie. If the sleeve is made larger diameter does it effect the length required for the tube or is this still defined strictly by Er) also, Does the diameter of the tube effect the impedance ? The specific example is RG174 Coax cable vertical dipole. The top radiating element is 1/4 long. At the bottom of this element is a length of RG174 Coax. 1/4 wave length of this coax has a 5/32 Brass sleeve. The sleeve is soldered to the RG174 coax at the point where the coax meets the top 1/4 wave element. I need to understand what needs to be used to calculate the length of Brass tubing sleeve given the tubing diameter is 5/32 and besides air, the outer sheath of the RG174 is also between the sleeve and Earth braid. Is the diameter of the tubing critical (will it impact adversely on SWR looking into this antenna). The final point would be if I place some ferrite rings across the coax below the sleeve, will this improve SWR ? Thanks Regards David |
#2
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On Sun, 02 Oct 2005 07:16:01 GMT, David wrote:
I need to understand what needs to be used to calculate the length of Brass tubing sleeve given the tubing diameter is 5/32 and besides air, the outer sheath of the RG174 is also between the sleeve and Earth braid. Is the diameter of the tubing critical (will it impact adversely on SWR looking into this antenna). Hi David, The Er of the metal is not something that comes quickly to mind (especially this late at night). Besides, it sounds like what you are describing is the radial component which would be fantastically small (many decimal places, not simply tens of percent). It also seems like your application is for UHF (you need to be more specific). This makes such "calculations" little better than SWAGs. It doesn't take much to be way off. What is even more confounding is your problem contains a hidden trap. The sleeve of brass that drops below the feed point is embracing the jacket of the RG174. THIS constitutes the dielectric constant of interest and is a material that is different from the insulator of the inner line. Even more, and as you note above, there is some air mixed in to really gum up the back of the napkin calculations. This is the G of the SWAG. Worst yet, some jacket material is actually quite lossy, or so it has been reported - another G of the SWAG. All-in-all, what you need to do is build one and measure it. No doubt that will lead to another trail of tears. On the other hand, there is nothing so stimulating to learning than pain. Most of my best subjects centered on projects I thoroughly screwed up. As the saying goes: "If you haven't failed, you aren't trying hard enough." 73's Richard Clark, KB7QHC |
#3
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Richard,
Thanks for the feedback. No problem with the failing issue, plenty of that going on. The way I see it though is that if I can add some knowledge to my experimentation then at least I can "play" with some hope of moving to a solution in the right direction. Richard Clark wrote: On Sun, 02 Oct 2005 07:16:01 GMT, David wrote: I need to understand what needs to be used to calculate the length of Brass tubing sleeve given the tubing diameter is 5/32 and besides air, the outer sheath of the RG174 is also between the sleeve and Earth braid. Is the diameter of the tubing critical (will it impact adversely on SWR looking into this antenna). Hi David, The Er of the metal is not something that comes quickly to mind (especially this late at night). Besides, it sounds like what you are describing is the radial component which would be fantastically small (many decimal places, not simply tens of percent). It also seems like your application is for UHF (you need to be more specific). This makes such "calculations" little better than SWAGs. It doesn't take much to be way off. What is even more confounding is your problem contains a hidden trap. The sleeve of brass that drops below the feed point is embracing the jacket of the RG174. THIS constitutes the dielectric constant of interest and is a material that is different from the insulator of the inner line. Even more, and as you note above, there is some air mixed in to really gum up the back of the napkin calculations. This is the G of the SWAG. Worst yet, some jacket material is actually quite lossy, or so it has been reported - another G of the SWAG. All-in-all, what you need to do is build one and measure it. No doubt that will lead to another trail of tears. On the other hand, there is nothing so stimulating to learning than pain. Most of my best subjects centered on projects I thoroughly screwed up. As the saying goes: "If you haven't failed, you aren't trying hard enough." 73's Richard Clark, KB7QHC |
#4
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David wrote:
I am a little confused with calculation of appropriate length of metal tubing used in a sleeve antenna design (the 1/4 wave section). I understand that as RF travels through various mediums, Velocity of the wave slows down relative to Er of the material. So for coax the wavelength would be say 0.66 Lambda. With the tubing over a coaxial cable though it seems there is Vp related to the dielectric formed between the sleeve and the outer sheath of the coax cable but also the Metal has an Er value. 1. Do you need to count for both these when calculate how the tube should be. 2. Is there relationship that needs to be considered between sleeve length and distance between sleeve and coax (ie. If the sleeve is made larger diameter does it effect the length required for the tube or is this still defined strictly by Er) also, Does the diameter of the tube effect the impedance ? The specific example is RG174 Coax cable vertical dipole. The top radiating element is 1/4 long. At the bottom of this element is a length of RG174 Coax. 1/4 wave length of this coax has a 5/32 Brass sleeve. The sleeve is soldered to the RG174 coax at the point where the coax meets the top 1/4 wave element. I need to understand what needs to be used to calculate the length of Brass tubing sleeve given the tubing diameter is 5/32 and besides air, the outer sheath of the RG174 is also between the sleeve and Earth braid. Is the diameter of the tubing critical (will it impact adversely on SWR looking into this antenna). The final point would be if I place some ferrite rings across the coax below the sleeve, will this improve SWR ? Thanks Regards David David; That's why they call it the ART of antenna design. First you make the best mathematical calculations you can then you start cutting wire/tubing. Make your lengths slightly longer than the math says then prune the lengths until the antenna works the way you think it should. There are so many variables in antennas that this is the only practical way to make a new one. Once you find your optimal lengths you can make more to the same specs. However remember that just like in housing antenna's depend on three things: location, location, location and have to be custom adjusted for each site. Dave WD9BDZ |
#5
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With the tubing over a coaxial cable though it seems there is Vp related to the dielectric formed between the sleeve and the outer sheath of the coax cable but also the Metal has an Er value. From what I understand, the RF will be traveling on the outside of the sleeve, and won't "see" the internals at all. So, the sleeve ends up as a 1/4 wavelength stub. Is the diameter of the tubing critical (will it impact adversely on SWR looking into this antenna). It shouldn't be The final point would be if I place some ferrite rings across the coax below the sleeve, will this improve SWR ? Not if the sleeve is doing it's job. The sleeve's open end should look like a short at the operating frequency, and keep RF off the feedline. Thanks Regards David David; That's why they call it the ART of antenna design. First you make the best mathematical calculations you can then you start cutting wire/tubing. Make your lengths slightly longer than the math says then prune the lengths until the antenna works the way you think it should. There are so many variables in antennas that this is the only practical way to make a new one. Once you find your optimal lengths you can make more to the same specs. However remember that just like in housing antenna's depend on three things: location, location, location and have to be custom adjusted for each site. Dave WD9BDZ |
#6
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On Sun, 2 Oct 2005 12:26:00 -0500, "Dave VanHorn"
wrote: Not if the sleeve is doing it's job. The sleeve's open end should look like a short at the operating frequency, and keep RF off the feedline. If you wish to keep common mode off the feedline you will need to add decoupling. That's the old AEA ISO-Loop had that extra sleeve. Danny, K6MHE email: k6mheatarrldotnet http://users.adelphia.net/~k6mhe/ |
#7
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I am still confused on this one.
I see a 1/4 wave radiating element joined to another 1/4 wave radiating element that just happens to also act as a choke to reduce RF on the return braid of the RG174 coax. The point where they meet is supposed to look like the centre of a 1/2 wave dipole so the impedance must be around the 72 Ohms If the Sleeve is acting as a choke(Stub) then does it not need a second element as the return (ie is it not acting like a 1/4 length of coax where the tube is the outer braid and the RG174 cable running up the centre outer sheath is the dielectric and the outer braid of the RG174 becomes the inner conductor of the stub )? If this is correct then the electrical length of the sleeve must be modified by the presence of the RG174 coax outer sheath. The sleeve is sort of performing a dual function. As it is difficult to cut the tube once soldered in place, I will need to make heaps of these to get the length right. At least if I had a reasonable starting point it would minimize the number of iterations. The other issue is that I read about cutting lengths for elements and not that even an element in free space is trimmed down by about 3% to account for some "end effect" ? Would this also apply to my sleeve ? If so, then the length may be difficult to get right because if I make allowance for the end effect then the choking effect of the sleeve will not work as well. I do not want to reinvent the wheel here as these antennae are sold already being sold commercially and I am sure there are numerous people who have successfully made these. Therefore if I can learn from other people's mistakes and success, the starting point is much closer to the solution. I have a practical application but want to understand also. The requirement is that I have a small antenna that can be built into the end of a RF connector and plug directly onto a radio modem operating in the 915MHz band. The power is 1 Watt max. The design needs to be repeatable and not require trimming at every location (being portable I have no control over where the unit is located), as there is no decent ground plane, I need a unit that does not require radials or large ground plane. The low radiation angle and no requirement for large ground plane made the J-Pole look like a good contender but it was physically difficult to mount in this application. After looking around and opening up some commercial units, I noted many used the Choke Sleeve, 1/2 wave dipole antenna. This is why I am now wanting to understand how these work and have a good starting point for making them. The commercial units used molded construction that was difficult for me to emulate. I have started with RG174 Cable, 5/32 Brass tube all attached to SMA plug and used 1/4" nylon tubing as a radome. It goes together well and looks good but does not perform very well. Thanks for any further info. Dave VanHorn wrote: With the tubing over a coaxial cable though it seems there is Vp related to the dielectric formed between the sleeve and the outer sheath of the coax cable but also the Metal has an Er value. From what I understand, the RF will be traveling on the outside of the sleeve, and won't "see" the internals at all. So, the sleeve ends up as a 1/4 wavelength stub. Is the diameter of the tubing critical (will it impact adversely on SWR looking into this antenna). It shouldn't be The final point would be if I place some ferrite rings across the coax below the sleeve, will this improve SWR ? Not if the sleeve is doing it's job. The sleeve's open end should look like a short at the operating frequency, and keep RF off the feedline. Thanks Regards David David; That's why they call it the ART of antenna design. First you make the best mathematical calculations you can then you start cutting wire/tubing. Make your lengths slightly longer than the math says then prune the lengths until the antenna works the way you think it should. There are so many variables in antennas that this is the only practical way to make a new one. Once you find your optimal lengths you can make more to the same specs. However remember that just like in housing antenna's depend on three things: location, location, location and have to be custom adjusted for each site. Dave WD9BDZ |
#8
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On Mon, 03 Oct 2005 02:02:13 GMT, David wrote:
If this is correct then the electrical length of the sleeve must be modified by the presence of the RG174 coax outer sheath. The sleeve is sort of performing a dual function. Hi David, I think I pointed that out already. The effects of this jacket are variable across many suggestions offered here over the years. As it is difficult to cut the tube once soldered in place, I will need to make heaps of these to get the length right. At least if I had a reasonable starting point it would minimize the number of iterations. Ah yes, the pain of it all. I mentioned that too. You may as well grit your teeth and set to it, there is no shortcut to this all. The other issue is that I read about cutting lengths for elements and not that even an element in free space is trimmed down by about 3% to account for some "end effect" ? Would this also apply to my sleeve ? If so, then the length may be difficult to get right because if I make allowance for the end effect then the choking effect of the sleeve will not work as well. You might want to work this backwards. That is start off with a successful choke section of the sleeve. Let that dictate what follows because its isolation will mediate what I describe. If you have any experience with what is called an Offset Center Fed Dipole, you would find that it offers a closer match to 50 Ohms. You are using the dipole as a variable match by finding the 50 Ohm portion along its full length. This means that the portion above the sleeve's attachment to the drive point may not be the classic length (and certainly not accounted by the 3% of the "end effect" - not even close). That is neither here nor there because at that point you will have simultaneously achieved resonance, a match, and isolation; and yet by no fixed formula pulled out of a hat, nor accredited by an institution of higher learning. And, by the way, it isn't going to pull together in the first pass. I hope you have a stack of tubing, so put on a happy face and get down to it. This is why I am now wanting to understand how these work and have a good starting point for making them. The commercial units used molded construction that was difficult for me to emulate. I have started with RG174 Cable, 5/32 Brass tube all attached to SMA plug and used 1/4" nylon tubing as a radome. It goes together well and looks good but does not perform very well. How do you know? It might be doing the best job you could ask of any design. Your statement requires FAR MORE qualification than a rather subjective toss-off. In other words: 1. To what parameters? 2. Compared to what? 3. By what measure? 73's Richard Clark, KB7QHC |
#9
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Richard,
With regard to performance. Doing an A/B comparison between the commercial antenna and my version. The range achieved by the commercial antenna was approximately 4 times the range of my version. The range was determined by loss of using one end as a beacon and moving the test unit further away until it could no longer receive valid packets, then move in and out of range several times to confirm consistency. The same was done with my version. Before the test, I monitored the return port on the directional coupler on a Spectrum analyzer while seeping frequency from the sig. gen and noted the SWR on the commercial unit was lower than my version. (Though the measurement was not calibrated I could see the SWR was better but did not determine how much). Previously I designed a Microwave movement detector that operated at 1.2 GHz using Doppler principle. It used a microstrip tank circuit connected to a negative resistance oscillator. The design was published (RF Design magazine Dec 1986). Though there were many technical issues regarding the oscillator (how to adjust impedance in the base for optimal negative resistance looking into emitter, effect of moving the antenna, ground plane under the antenna etc etc), others were able to make the design just as per my prototype and it worked. They could then proceed to"play" with the design to change its performance. I really would have expected that a similar scenario would exist with antenna design (Someone has made one just like what I am wanting to make and is able to share with me the details on what they did that worked, then I would play with the design). I am grateful for the many responses I have received from my postings to this group and am confident there is much good information in what I have received but mixed with people's actual experience is a bunch of theoretical information that makes it hard to pick out what is useful. (ie. some of the information has conflicted and left me not understanding what is actually happening here). If someone has actually made these types of antennas and has some practical experience with them, that would be a fantastic starting point for me. (ie. Low power, UHF, portable). Richard Clark wrote: On Mon, 03 Oct 2005 02:02:13 GMT, David wrote: If this is correct then the electrical length of the sleeve must be modified by the presence of the RG174 coax outer sheath. The sleeve is sort of performing a dual function. Hi David, I think I pointed that out already. The effects of this jacket are variable across many suggestions offered here over the years. As it is difficult to cut the tube once soldered in place, I will need to make heaps of these to get the length right. At least if I had a reasonable starting point it would minimize the number of iterations. Ah yes, the pain of it all. I mentioned that too. You may as well grit your teeth and set to it, there is no shortcut to this all. The other issue is that I read about cutting lengths for elements and not that even an element in free space is trimmed down by about 3% to account for some "end effect" ? Would this also apply to my sleeve ? If so, then the length may be difficult to get right because if I make allowance for the end effect then the choking effect of the sleeve will not work as well. You might want to work this backwards. That is start off with a successful choke section of the sleeve. Let that dictate what follows because its isolation will mediate what I describe. If you have any experience with what is called an Offset Center Fed Dipole, you would find that it offers a closer match to 50 Ohms. You are using the dipole as a variable match by finding the 50 Ohm portion along its full length. This means that the portion above the sleeve's attachment to the drive point may not be the classic length (and certainly not accounted by the 3% of the "end effect" - not even close). That is neither here nor there because at that point you will have simultaneously achieved resonance, a match, and isolation; and yet by no fixed formula pulled out of a hat, nor accredited by an institution of higher learning. And, by the way, it isn't going to pull together in the first pass. I hope you have a stack of tubing, so put on a happy face and get down to it. This is why I am now wanting to understand how these work and have a good starting point for making them. The commercial units used molded construction that was difficult for me to emulate. I have started with RG174 Cable, 5/32 Brass tube all attached to SMA plug and used 1/4" nylon tubing as a radome. It goes together well and looks good but does not perform very well. How do you know? It might be doing the best job you could ask of any design. Your statement requires FAR MORE qualification than a rather subjective toss-off. In other words: 1. To what parameters? 2. Compared to what? 3. By what measure? 73's Richard Clark, KB7QHC |
#10
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On Mon, 03 Oct 2005 03:14:52 GMT, David wrote:
Doing an A/B comparison between the commercial antenna and my version. The range achieved by the commercial antenna was approximately 4 times the range of my version. The range was determined by loss of using one end as a beacon and moving the test unit further away until it could no longer receive valid packets, then move in and out of range several times to confirm consistency. The same was done with my version. Hi David, This suggests your problems are indeed very profound. A 4 times range difference would suggest a massive mismatch. Before the test, I monitored the return port on the directional coupler on a Spectrum analyzer while seeping frequency from the sig. gen and noted the SWR on the commercial unit was lower than my version. (Though the measurement was not calibrated I could see the SWR was better but did not determine how much). Then I would bet that your system topology is to blame. That is, your line from the coupler and test equipment to the antenna is not normalized out of the measurement as a network analyzer would do. And when you shift from measurement to application, your line length shifts such that your test match goes to hell. This implicates the isolation of the sleeve - which is already in serious jeopardy by many variables anyway. Your test scenario does not reveal the efficacy of this isolation, it simply affirms that using the spectrum analyzer establishes a suitable, but serendipitous arrangement. You might add ferrite beads to the lead going to the antenna/sleeve and move them along its length to test this hypothesis. This prophylactic measure is also commonly employed as a necessary adjunct to design. 73's Richard Clark, KB7QHC |
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