Hi Dan,
On 17/04/2011, at 15:53, KF1BUZ kf1buz@gmail.com wrote:
A Copper Jpole, has this been tried? Just thinking it might make my getting into the birds better.
Thanks Dan KF1BUZ
If I interpret this right (subject + message), you are asking if adding a Ground Plane to a J-pole antenna will improve its performance in a satellite ground station application. I believe the answer is no, though someone would need to do the modeling to understand for sure.
A J-pole is an end-fed dipole, with the "J" portion being a 1/4 wave long at the frequency of the antenna. Recalling some RF theory stuff, a 1/4 wave "matching section" has a low impedance on one end, and a high impedance at the other. One end goes to the coax feed line (low impedance), and the other is attached to the end (high impedance) part of the dipole. You will find that the single pipe section of the J-pole antenna is about a 1/2 wave long at the antenna's design frequency, and since it's connected directly to the end of the matching section, it makes for an end-fed dipole. Some designs use a 5/8 wave dipole section for a little extra gain towards the horizon.
So a J-pole antenna is actually a pretty effective satellite antenna, similar to a simple ground plane antenna but mechanically more robust. I've used both kinds. My very first satellite contact ion 1993 was using one for the uplink into RS-10, and that contact was followed by many many more. That antenna is still in service nearly 18 years later. (If you hear or use the KO6TH APRS iGate, you're using it!) I've got two SO-239-type Ground Plane antenna carcases in the garage; they didn't last. As a satellite antenna, both Ground Plane and J-pole antennas do have a null directly overhead, but very few satellite passes go directly overhead, and when they do, they spend very little time there. So don't sweat it.
A J-pole with a 1/2 wave section on top will also work as a dual band 2M / 70CM antenna, with the upper band on the antenna's 3rd harmonic. Using the antenna that way, I'm told, it has a radiation pattern that is lifted somewhat from the horizon, so it should be a good match for satellite work.
But, back to your question... The J-pole antenna is a totally balanced system in itself, and doesn't suffer for not having a ground plane below it. What putting a ground plane some distance below the end of an end-fed dipole will do to the radiation pattern, however, is a modeling task for someone at a higher mental pay-grade than me. My guess is that it will depend very significantly exactly where the ground plane is mounted. You could alter both the radiation pattern and the feed point impedance with that addition, and maybe make things worse.
Hope this helps a little,
Greg KO6TH
Greg, Dan:
One of my first antennas put up in Alaska was a simple 2m J-Pole. I used the upper portion of a TV mast and used a 1/4 wave section of copper tubing clamped 3/4 wave length below the top end of the mast. Attaching coax to the stub and mast at a point to give a 50-ohm match. Worked fine. Note that the longer part of the J-Pole continued below where the 1/4 wave stub attached for about ten feet. There was no matching problems. What affect on the radiation pattern resulted is unknown, but I observed no detrimental effects.
Adding ground radials to the bottom of the J-pole should not have a big effect on tuning. I think the only effect on radiation is that you establish a better ground for the vertical 1/2 wave dipole (perhaps lowering the pattern a little closer to the horizon.
I built a 6m/10m J-pole and it is mounted at ground level attached to a short tower set into the ground. The tower seems to not affect the tuning point. This summer I may try adding some 1/4 wave radials to the base to see if it has any affect. I set it up mainly for working 6m/10m FM, but is also usable for mode-A satellites. I have compared it to my 3-element triband yagi on A07 mode-A and the beam is better. http://www.kl7uw.com/J-Pole.htm
Sidenote: I have re-installed satellite antennas for 2m, 70cm , and 13cm on my short tower this weekend. Both 2m and 70cm antennas working well. I have not installed the 2.4 GHz LNA or downconverters, as yet. Control wiring for the satellite antennas has not been connected plus the B5400 az-el rotator control unit still requires repair so antennas not rotatable. http://www.kl7uw.com/sat.htm
73, Ed - KL7UW
At 09:49 PM 4/17/2011, Greg D. wrote:
Hi Dan,
On 17/04/2011, at 15:53, KF1BUZ kf1buz@gmail.com wrote:
A Copper Jpole, has this been tried? Just thinking it might make my getting into the birds better.
Thanks Dan KF1BUZ
If I interpret this right (subject + message), you are asking if adding a Ground Plane to a J-pole antenna will improve its performance in a satellite ground station application. I believe the answer is no, though someone would need to do the modeling to understand for sure.
A J-pole is an end-fed dipole, with the "J" portion being a 1/4 wave long at the frequency of the antenna. Recalling some RF theory stuff, a 1/4 wave "matching section" has a low impedance on one end, and a high impedance at the other. One end goes to the coax feed line (low impedance), and the other is attached to the end (high impedance) part of the dipole. You will find that the single pipe section of the J-pole antenna is about a 1/2 wave long at the antenna's design frequency, and since it's connected directly to the end of the matching section, it makes for an end-fed dipole. Some designs use a 5/8 wave dipole section for a little extra gain towards the horizon.
So a J-pole antenna is actually a pretty effective satellite antenna, similar to a simple ground plane antenna but mechanically more robust. I've used both kinds. My very first satellite contact ion 1993 was using one for the uplink into RS-10, and that contact was followed by many many more. That antenna is still in service nearly 18 years later. (If you hear or use the KO6TH APRS iGate, you're using it!) I've got two SO-239-type Ground Plane antenna carcases in the garage; they didn't last. As a satellite antenna, both Ground Plane and J-pole antennas do have a null directly overhead, but very few satellite passes go directly overhead, and when they do, they spend very little time there. So don't sweat it.
A J-pole with a 1/2 wave section on top will also work as a dual band 2M / 70CM antenna, with the upper band on the antenna's 3rd harmonic. Using the antenna that way, I'm told, it has a radiation pattern that is lifted somewhat from the horizon, so it should be a good match for satellite work.
But, back to your question... The J-pole antenna is a totally balanced system in itself, and doesn't suffer for not having a ground plane below it. What putting a ground plane some distance below the end of an end-fed dipole will do to the radiation pattern, however, is a modeling task for someone at a higher mental pay-grade than me. My guess is that it will depend very significantly exactly where the ground plane is mounted. You could alter both the radiation pattern and the feed point impedance with that addition, and maybe make things worse.
Hope this helps a little,
Greg KO6TH
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73, Ed - KL7UW, WD2XSH/45 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 144-1.4kw, 432-100w, 1296-testing*, 3400-winter? DUBUS Magazine USA Rep dubususa@hotmail.com ======================================
participants (2)
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Edward R. Cole -
Greg D.