[amsat-bb] Re: ground plane on
Edward R. Cole
kl7uw at acsalaska.net
Sun Apr 17 23:41:27 PDT 2011
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.
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.
73, Ed - KL7UW
At 09:49 PM 4/17/2011, Greg D. wrote:
> On 17/04/2011, at 15:53, KF1BUZ <kf1buz at 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,
>Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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73, Ed - KL7UW, WD2XSH/45
BP40IQ 500 KHz - 10-GHz www.kl7uw.com
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