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Old 01-11-2008, 09:11 AM
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Red_Chili Red_Chili is offline
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Default Mobile Ham antenna configs

So I have a soft top.

And a sunroof on what little cab I have.

There is always the hood for a ground plane, but then the antennae (2M, 70CM) are in the field of vision.

Reading through the ARRL book, I see lots of interesting (and cheap, appeals to my inner cheapskate) antenna configurations,

Has anyone run screen as a ground plane in something like a soft top? Wire arrays? How about a loop antenna around the circumference? That would be too close to what little ground plane I have though wouldn't it?

Just kicking around ideas.

Learning about antenna design, it's amazing my CB works as well as it does. Hardly any ground plane to speak of, attached to the spare tire carrier.

Cool stuff. RF does really weird things.
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Old 01-11-2008, 09:25 AM
leiniesred leiniesred is offline
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MY CB antenna is mounted in the bed of my 4runner. It fits inside the hard and soft top. Optimal? no. It is corner mounted making it directional, plus 15 inches of it is reflected by the box, and the remaining foot gets reflected by the cab.

I need an antenna design that won't make my rig more than 1 inch taller than my roofline, otherwise, the truck won't fit in the garage.

How does my tiny XM antenna work? I like it.

Shoot, cops seem to get along with <1 foot antennas mounted on the trunk lid. I see like 3-5 antennae sticking up back there.

Back in the old jeep and bronco daze, I had the good ol' 20 foot whip. Works great as a CB antenna AND is good for knocking leaves, sticks, and dead raccoons into your hair. (chicks don't dig that.)
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  #3  
Old 01-11-2008, 10:02 AM
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It depends on the wavelength.

I definitely want to be set up for minimal SWR and a quality install for this rig though. Cheap and not subject to tree branches would be a bonus!
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Old 01-11-2008, 10:32 AM
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There seems to be a misunderstanding of what a ground plane is and why you care. You say you have the ARRL book, I assume it's the study manual for the test and not the ARRL Antenna Book.

If you guys want, I've posted bits and pieces about this stuff in other places and have been trying to compile that into a single file for reference (adding diagrams for clarity) that I was going to offer to Groucho for the class. So for the sake of conciseness here I will just say that there are physical reasons why a CB antenna is more tolerant of a bad local ground plane than your VHF and UHF radio will be. You can get a fair signal out with a CB with a poor vehicle ground plane where a 2m antenna needs more care (it's also easier to actually get a good 2m ground plane). You do have a ground always in every antenna that's not shorted, it's just how ideal the current path and incident wave are. In the case of a CB, the earth itself is your ground plane and that works fine, but the path back to the radio is not ideal necessarily.

The short answer is for you Bill is that your best option is right in the middle of the cab. Best option is to drill a hole, but I know you've said you don't want to do that. I have my ham antenna on my roof, slightly rear of center. I get good range out of it and even though your roof is a little smaller, it would be a very good ground plane option. Depending on who you reference the optimal ground plane size varies from about 1/4 to 1/2 wavelength, but in general you don't see any additional benefit beyond 1/2 wavelength. So in reality if you can get about 3 feet of sheet metal in all directions you will be about ideal for 2m. That is not really possible on most cars and so I tend to shoot for 1/4, so about 18" in any direction. That's pretty close on a soft top 4Runner.

I would suggest a magnetic mount. The quality of ham equipment is much higher than the CB stuff, so before you balk at that just realize that hams have the benefit of sharing a lot of gear with commercial gear, so mag mounts on balance tend to hold better and do less damage than the junk you get at Rat Shack.

It is possible to use the hood, but I find that distracting and not much more convenient. Other options will be roof gutters, door edges and on the bumper. I had good luck with my ham antenna on the bull bar (and still have the CB out there), although geeks like me can give you pages of reasons why that should be lousy. You do have a very directional pattern on the edges of the car, but it's workable.
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Old 01-11-2008, 10:38 AM
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In the middle of my cab roof is... glass.
The biggest piece of existing sheetmetal is my hood. Fenders are 'glass'.
That's kinda why I was wondering about creating a ground plane in the soft top. I didn't know (until I read it in the ARRL test prep book) that this could be effectively done with screen or a wire array.
OR... coming up with an antenna that doesn't require a ground plane 'mirror'.
Whatcha think?
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Old 01-11-2008, 10:45 AM
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bill, I really think your taking that "idle hands" passage waaaay to far.

j
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Old 01-11-2008, 10:50 AM
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Quote:
Originally Posted by Red_Chili View Post
In the middle of my cab roof is... glass.
The biggest piece of existing sheetmetal is my hood. Fenders are 'glass'.
That's kinda why I was wondering about creating a ground plane in the soft top. I didn't know (until I read it in the ARRL test prep book) that this could be effectively done with screen or a wire array.
OR... coming up with an antenna that doesn't require a ground plane 'mirror'.
Whatcha think?
Duh, forgot about the sun roof. Does it happen to be that passivated glass? It might still work OK for a ground plane if it's conductive and got a good connection to the truck body.

To answer your question about the soft top. Yes, you can build a counterpoise into or on your soft top. This is actually a pretty common technique when dealing with fiberglass roofs and non-metallic bodied cars (Saturns, Vettes, etc.). You build a conductive plane under or over the composite using copper sheets, fine wire mesh, wire radials, etc. In the case of a 2m/70cm antenna that could certainly be a mesh ring under your antenna. I think the issue with a soft top is mostly getting a solid mechanical fit for the antenna (unless maybe you are thinking of a roll bar or something?).

You mention the so called no ground plane antennas. That's something of a misnomer, there is no such thing. But physically a 1/2 wavelength, half dipole, end fed antenna is a bit of an oddity in how it expects the return path to couple and so it appears to the radio that you don't need a ground plane. Some of this I'm sure Groucho will cover, but you don't get anything for nothing. Basically they do what they say, give you a low VSWR without a good groundplane, that is true. You give up some efficiency and change your directionality and pattern for it. It's certainly an option to keep in your back pocket.
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Old 01-11-2008, 11:20 AM
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The soft top has a stainless frame that I could mount an antenna to - with a small sealed hole through the canvas. How large of a mesh ring would be needed to form an adequate plane? How do you figure the dimensions needed?

The downside would be interference with the rack, which I now don't use nearly as much (costs me 2mpg, believe it or not).
I could put it toward the cab and pick up somewhat of a ground plane from it. The top mount will be vulnerable to trees, and Miss Chili ain't the shortest Amazon in the herd.

I prolly oughta go by Radio Service Center and jaw.
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Old 01-11-2008, 11:48 AM
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Quote:
Originally Posted by Red_Chili View Post
The soft top has a stainless frame that I could mount an antenna to - with a small sealed hole through the canvas. How large of a mesh ring would be needed to form an adequate plane? How do you figure the dimensions needed?
There is some disagreement amongst references and engineers, but on VHF generally 1/2 wavelength is considered best and anything above 1/4 wave is perfectly sufficient. So since 2 meters is what you're shooting for and the other half of your dual band is shorter 70 cm, take 1/2 of 2m = 1m or about 3 feet. That is ideal. Since that is also essentially impossible, a realistic ground plane would be 1/4 of 2m = ~ 1.5 feet. So an 18" doughnut of mesh would give you a good ground plane on 2m and a great ground plane on 70 cm. If it had to be even 12" or 15", it would still be not bad on 2m and still pretty good on 70 cm. It's more important that you have some ground plane that's above maybe 12" or so, beyond that you are starting more into optimizing than just getting a good antenna.
Quote:
The downside would be interference with the rack, which I now don't use nearly as much (costs me 2mpg, believe it or not).
I could put it toward the cab and pick up somewhat of a ground plane from it. The top mount will be vulnerable to trees, and Miss Chili ain't the shortest Amazon in the herd.

I prolly oughta go by Radio Service Center and jaw.
How about on the rack? A piece of thin sheet aluminum or copper mounted on the floor of the rack would be really good.
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Old 01-11-2008, 12:13 PM
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The rack stays home most days. I haven't even fixed the towers since I ripped it off running Kane Creek with too much weight up top last year.
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