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Antenna article
On Mon, 17 Feb 2014 18:04:16 -0500, Fred McKenzie
wrote: In article , Jeff Liebermann wrote: I hate to admit it, but I think I've built them backwards over the years. Maybe that's why J-poles have such a lousy reputation and why I think they suck? Please pardon my ignorance. I understood that a J-Pole is a half wave antenna connected to a quarter wave stub. The transmission line is connected to a low impedance point (50 Ohms?) on the stub. Yep, something like that. In the literature, it is usually fed as an unbalanced antenna, but it is not. Therefore, it should not matter which side has the center conductor or shield - they are both wrong! Fred K4DII Well, if it really were a balanced feed, I guess(tm) there should be no difference in the pattern, gain, VSWR, etc between the two unbalanced methods of feeding the antenna. Yet the author of the original article shows large differences in the antenna models. See Pg 10 thru 13: http://www.cvarc.org/tech/antenna_myths/antenna_myths.pdf -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna article
On Tue, 18 Feb 2014 18:10:39 -0500, "Ralph Mowery"
wrote: I don't know which one would be the best but I have seen 3 methods of feeding the J-pole. If you insulate the bottom then you hook the feedline to the bottom with the center of the coax to the long side. If you do not insulate the bottom you tap up the matching segment so that you get a 50 ohm (if that is the coax used) match with the center of the coax connected to the long leg. Then there is the balun made out of coax that is hooked up to the matching segment so that a low swr is obtained. There's also the American Legion J-Pole or the Silicon Valley Emergency Communications Systems J-Pole: https://picasaweb.google.com/112916124640757906440/NonarthopodicAntenna#5459396072666399154 https://picasaweb.google.com/112916124640757906440/NonarthopodicAntenna#5459396111364421106 This design does one thing right that none of the other J-Pole mutations seem to consider. The length of the wire between the coax connector center conductor and the driven element is an inductor. In order to tune out this inductance, one needs a series capacitor, with the inductor and capacitor tuned to the operating frequency. In other words, a gamma match. The series capacitor is formed by the insulated turns of electrical wire wrapped around the driven element. Another thing this design does right is use the zero current point at the bottom of the antenna as a ground. The problem is that it also extends the length of the center wire, which makes using a gamma match all the more important. I think putting the 50 ohm feed point and the corresponding ground close to each other were either to reduce the inductance of the connecting wire, or some manner of mutation from when it was fed by a balance line. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna article
"Jeff Liebermann" wrote in message ... ... The length of the wire between the coax connector center conductor and the driven element is an inductor. In order to tune out this inductance, one needs a series capacitor, with the inductor and capacitor tuned to the operating frequency. Yes. I've been making copper pipe and 2-wire transmission line J-poles for almost 20 years. With pipe, I usually fasten the two feed points with clamps and slide the connections up and down. I'll get a VSWR low-point in-band but early-on, I discovered that the best VSWR was often about 1.7:1. I had read about (but never built) a gamma match, so I'd heard about the series cap to tune out the inductance. I tried a series cap at the feed and it helped. 70 - 100 pF seems to be about right at 2m and I can often get a 1:1 reading somewhere in the band. Does such a 0.3 dB improvement matter? That's not my call. When I'm essentially playing with the technology, I can take more time than if I'm working, like to a deadline or a budget. "Sal" |
Antenna article
On Wed, 19 Feb 2014 21:16:19 -0800, "Sal" salmonella@food
poisoning.org wrote: "Jeff Liebermann" wrote in message .. . ... The length of the wire between the coax connector center conductor and the driven element is an inductor. In order to tune out this inductance, one needs a series capacitor, with the inductor and capacitor tuned to the operating frequency. Yes. I've been making copper pipe and 2-wire transmission line J-poles for almost 20 years. With pipe, I usually fasten the two feed points with clamps and slide the connections up and down. I'll get a VSWR low-point in-band but early-on, I discovered that the best VSWR was often about 1.7:1. I had read about (but never built) a gamma match, so I'd heard about the series cap to tune out the inductance. I tried a series cap at the feed and it helped. 70 - 100 pF seems to be about right at 2m and I can often get a 1:1 reading somewhere in the band. I'm not sure of the frequency of your J-pole. The inductance of about 6 cm of #12AWG solid wire is about 0.05 uH. http://www.consultrsr.com/resources/eis/induct5.htm To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be about 200 pf. Does such a 0.3 dB improvement matter? That's not my call. When I'm essentially playing with the technology, I can take more time than if I'm working, like to a deadline or a budget. 0.3dB is about 6.7% loss. Probably not important or toss a coin? -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna article
"Jeff Liebermann" wrote in message ... I'm not sure of the frequency of your J-pole. The inductance of about 6 cm of #12AWG solid wire is about 0.05 uH. http://www.consultrsr.com/resources/eis/induct5.htm To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be about 200 pf. That's enlightening. I need to admit that no such science was applied. I just tried small caps from my stash to see what would happen. When something good happened, I remembered it for next time. The j-pole that seems to benefit most is the 2m version, which I've built the most of. Perhaps we're not dealing with just the inductance of that 6 cm of wire. There could be some residual inductance in the rest of the antenna which is being brought to resonance. Maybe when my element lengths are not optimal, some inductance would be found there. Not sure why the residual reactance would always be inductive. The first time I add a cap and it gets worse, then I'll know it isn't so. Maybe I should experiment with varying the spacing across the stub and see what that does, with and without a cap. "Sal" |
Antenna article
On Thu, 20 Feb 2014 20:38:22 -0800, "Sal" salmonella@food
poisoning.org wrote: "Jeff Liebermann" wrote in message .. . I'm not sure of the frequency of your J-pole. The inductance of about 6 cm of #12AWG solid wire is about 0.05 uH. http://www.consultrsr.com/resources/eis/induct5.htm To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be about 200 pf. That's enlightening. One must suffer before enlightenment. I need to admit that no such science was applied. I just tried small caps from my stash to see what would happen. When something good happened, I remembered it for next time. Well, I was a bit more scientific when I tried it about 10 years ago in an effort to improve the "American Legion" J-pole. This is from my memory, which is notoriously unreliable. I had the help of a return loss bridge, sweep generator, and oscilloscope. https://www.google.com/search?q=return+loss+bridge&tbm=isch I could tweak the antenna, and see the results to the VSWR immediately. I was finding that the J-pole has a rather narrow operating frequency range and I wanted to know if something could be done about it. Mounting the connector on the base resulted in the narrowest usable bandwidth (at VSWR = 2:1) but also the lowest VSWR. Using the more conventional feed points furthur up from the base, the minimum VSWR was horrid, but the bandwidth was wider. Inserting a series trimmer or wrapping the insulated wire around the 1/4 wave rod reduced the minimum VSWR, but narrowed the bandwidth slightly. Replacing the 1/4" dia aluminum rods, with 3/4" copper water pipe increased the usable bandwidth somewhat, but made the insulated wire wrap capacitor very small and critical. This may sound very scientific, but in reality, it was all done in about an hour, with no photos or recorded data. I had to return the equipment that afternoon. About a year ago, the local radio club had a beginners session on building "emergency antennas", which means a VHF Slim Jim and J-pole type antennas made from 300 ohm twinlead. The only test equipment available was an HT and VSWR bridge. I predicted problems and was invited not to show up. I did anyway, near the end. The problem was that the antennas were VERY sensitive to their surroundings. It was very difficult to cut one to length for minimum VSWR, while people and metal objects moved around the room. The j-pole that seems to benefit most is the 2m version, which I've built the most of. Perhaps we're not dealing with just the inductance of that 6 cm of wire. There could be some residual inductance in the rest of the antenna which is being brought to resonance. Maybe when my element lengths are not optimal, some inductance would be found there. Not sure why the residual reactance would always be inductive. The first time I add a cap and it gets worse, then I'll know it isn't so. Dunno. I can throw together a 4NEC2 model of a j-pole and see what might be the feed point impedance for various styles and locations. No time for that for a few days. Maybe I should experiment with varying the spacing across the stub and see what that does, with and without a cap. It's more fun to fire up the 4NEC2 optimizer, and see what it comes up with for the best possible antenna given a collection of restraints. Yet another project... -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna article
On Friday, February 21, 2014 12:07:31 PM UTC-6, Jeff Liebermann wrote:
It's more fun to fire up the 4NEC2 optimizer, and see what it comes up with for the best possible antenna given a collection of restraints. Yet another project... For EZNEC users, AutoEZ (available from ac6la.com) is an EXCEL-based optimizer - a 30 segment free demo version is also available. (I'm a satisfied EZNEC/AutoEZ user with no monitary connections.) -- 73, Cecil, w5dxp.com |
Antenna article
"Jeff Liebermann" wrote in message .. snip Dunno. I can throw together a 4NEC2 model of a j-pole and see what might be the feed point impedance for various styles and locations. No time for that for a few days. Maybe I should experiment with varying the spacing across the stub and see what that does, with and without a cap. It's more fun to fire up the 4NEC2 optimizer, and see what it comes up with for the best possible antenna given a collection of restraints. Yet another project... Too many projects, not enough me. And I face a big non-hobby project of cleaning and recoating the patio stones. Yuck! "Sal" (KD6VKW) |
Antenna article
"W5DXP" wrote in message ... On Friday, February 21, 2014 12:07:31 PM UTC-6, Jeff Liebermann wrote: It's more fun to fire up the 4NEC2 optimizer, and see what it comes up with for the best possible antenna given a collection of restraints. Yet another project... For EZNEC users, AutoEZ (available from ac6la.com) is an EXCEL-based optimizer - a 30 segment free demo version is also available. (I'm a satisfied EZNEC/AutoEZ user with no monitary connections.) -- 73, Cecil, w5dxp.com I downloaded and used the demo version of EZNEC and I can say it was pretty easy to get up and running with simple structures. I used it for a 20 m dipole and I was quite happy when I raised the antenna at FD for its first outing at full 30-ft height. I keep meaning to buy the full version of EZNEC. Maybe this will push me into it. "Sal" |
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