Ham antenna - Rubicon Owners Forum
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post #1 of 72 (permalink) Old 03-16-2019, 04:50 PM Thread Starter
Rubicus Maximus
 
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Ham antenna

Hijack for any ham thoughts that come up, but I have a couple of questions. I’m going with an antenna that is “less” dependent on a ground plane and will probably mount it in the rear. In height, I’ll at least mount it at the top of the tub. I understand that using a metal extension will have an influence on the tuning of the antenna.

If I extend the antenna with, let’s say pvc pipe or fiberglass road markers, will that change antenna tuning?
How much is gained by putting more antenna over the height of the hardtop?

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post #2 of 72 (permalink) Old 03-16-2019, 05:57 PM
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1) Search for a top loaded antenna. The coil is up high and the short whip above is tuned for SWR. Trouble is the coils for VHF seem to be big and heavy, quite unlike the CB version. Theory is that the signal rx/tx will be above the loading coil??
2) Buy a no ground plane antenna for somebody like PCI race radio.
3) Stick a quarter wave antenna on the roof with a homemade ground plane.
4) Mag mount with 5/8 whip stuck on the hood. Pretty darn good - maybe better than the above??
What I use for my VHF is a front fender mount. Excellent in one direction and less 180 deg due to body. Reason simple and cheap. Used with the standard tuned 5/8 mast.
In any event it probably does not matter what ever you choose. Just get a 80 watt or higher radio for best results.
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post #3 of 72 (permalink) Old 03-16-2019, 09:07 PM Thread Starter
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My questions come off a previous search where I found this...https://www.rubiconownersforum.com/f...m-antenna.html.

In this post John Canefield lists a few ďno ground plane antennas.Ē Iíve chosen the diamond NR770HB at this point.

My questions are regarding the mounting of this antenna. I was being less specific about brand because others are listed.

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post #4 of 72 (permalink) Old 03-17-2019, 08:39 AM
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At VHF and UHF frequencies there is no need to add capacitance or inductance to an antenna to achieve a 50 ohm resistive match unlike CB antennas. 1/4 wavelength at 145 Mhz is 20" and 1/4 wavelength at 27 Mhz (CB) is 109". For the short CB antennas, designers must add in inductance and capacitance so it will match the 50 ohm transmitter output.

That article I wrote specifies two antennas that need no counterpoise or ground plane to work effectively, all of the others need to sit on some metal -or- you could add three pieces of wire to the shield side of the coax that are about 20" long. One point to remember is ALL antennas (and I mean ALL) are some variation of the basic dipole. It might not be obvious but it is true.

A general rule of thumb is to keep any antenna in the clear as much as possible for a couple of reasons - one of which is a mass of metal near the radiating element could detune it (shift its point of resonance or match) and the other reason is you have a less than desirable radiation pattern. A vertical antenna like we are discussing should have an almost perfect omnidirectional radiation pattern - place it near a mass of metal and that ideal omni pattern is changed.
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post #5 of 72 (permalink) Old 03-17-2019, 10:10 AM Thread Starter
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Iíve learned a lot of things, but this subject is like an unfurnished area in my head. Iím liking it.

I think the answers to my questions about raising the antenna higher on the vehicle with pvc or fiberglass are...

Yes, I could raise it up. Height in mounting is your friend, but itís not practical for what Jeeps do.

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post #6 of 72 (permalink) Old 03-17-2019, 01:18 PM
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Raising the VHF antenna (or CB for that matter) a couple of feet for reasons other than moving it away from a mass of metal (i.e., you want better performance and improved line-of-sight) wouldn't matter very much - it's really not worth the trouble. Now if you are talking about a directional (beam) antenna for 2 Meters (145Mhz), moving it from say 8 feet to 50 feet would make a huge difference. Here's a link if you want to read more about line of sight at VHF and higher frequencies.
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post #7 of 72 (permalink) Old 03-18-2019, 07:04 AM
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I have mine in front, on the A pillar. In the rear my dead coyote rack is just too much mass of metal. The A pillar mount is not ideal or optimal, as seen on an SWR meter, and it does suffer the directional issue, but it works just fine for my purposes. Using the same Larsen 2/70 antenna on an NMO mag mount in the middle of the hood gets excellent SWR though. And putting it up in the middle of my coyote rack works superbly. So on my remote travels I just keep a mag mount base buried in my hope to never need stuff.

But I did briefly consider pulling out one of the hood snubbers and just permanently mounting it in the middle of the hood .



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post #8 of 72 (permalink) Old 03-18-2019, 10:04 AM
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Yes too close to the A. Mine is forward about 18" on the fender/hood crack and my SWR is not great due to the reflected signal relation to the A. Mid hood fixes that issue as will moving mine farther fwd.
You will get a similar result mounting in the tail light location, but not as bad as a near A location. In the back mid vehicle would be better.
Get an SWR that give fwd and reflected and you can see why the SWR is poor.

Quote:
Originally Posted by DAA View Post
I have mine in front, on the A pillar. In the rear my dead coyote rack is just too much mass of metal. The A pillar mount is not ideal or optimal, as seen on an SWR meter, and it does suffer the directional issue, but it works just fine for my purposes. Using the same Larsen 2/70 antenna on an NMO mag mount in the middle of the hood gets excellent SWR though. And putting it up in the middle of my coyote rack works superbly. So on my remote travels I just keep a mag mount base buried in my hope to never need stuff.

But I did briefly consider pulling out one of the hood snubbers and just permanently mounting it in the middle of the hood .
- DAA
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post #9 of 72 (permalink) Old 03-18-2019, 05:48 PM Thread Starter
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What do you guys like for an SWR meter?

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post #10 of 72 (permalink) Old 03-18-2019, 07:44 PM
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