[amsat-bb] Re: ground plane on

[Edward R. Cole]

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 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,
>
>Greg  KO6TH
>
>
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73, Ed - KL7UW, WD2XSH/45
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