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Old 04-05-2016, 08:39 PM   #1
ke0me
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Default counterpoise vs ground radial discussion

From previous posts, we had lots of discussion about how to mount various mobile/rv vertical antennas, typically 1/4 wave verticals (usually using loading coils to compensate for short length). I am interested in this due to working with my new Tarheel antenna on my fifth wheel.

Assuming the following definitions:

Counterpoise -a counterpoise is a network of SUSPENDED wires or cables (or a metal screen), used as a SUBSTITUTE for an earth (ground) connection in a radio antenna system. It is used with radio transmitters or receivers when a normal earth ground cannot be used because of high soil resistance or other reasons. It usually consists of a single wire or network of horizontal wires, parallel to the ground, suspended above the ground under the antenna, connected to the receiver or transmitter's "ground" wire. The counterpoise functions as one plate of a large capacitor, with the conductive layers of the earth acting as the other plate. (thanks to Wikipedia, capitals are mine)


Ground Radial - grounding wires to be buried in the soil in a radial pattern, in order to have a proper ground plane for the vertical radiator element. Obviously, this has a DIRECT EARTH GROUND CONTACT.

So, any mounting ON a vehicle would have to be considered a counterpoise since it is not connected to earth ground.

Therefore, is it correct to assume that I would NOT CONNECT the ground lug of the tarheel antenna mount to an earth ground connection? I would connect to the RV chassis to make the chassis part of the counterpoise.

Also, I assume after proper bonding of almost everything that is metallic to the chassis, I might have a reasonable counterpoise.

Comments please!!
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Old 04-06-2016, 01:11 AM   #2
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Dick, can you provide a link to that Wikipedia article? My initial impression is that the article is fundamentally incorrect. But I'd have to read and ponder it.
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Old 04-06-2016, 10:20 PM   #3
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Carl,

I copied from separate Wikipedia articles, one for radials and one for counterpoise.

I'll have to see if I can find them again. Might be a couple days, I have to work tomorrow :-(.

gotta have cash to buy more "toys".
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Old 04-07-2016, 11:05 AM   #4
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Dick, can you provide a link to that Wikipedia article? My initial impression is that the article is fundamentally incorrect. But I'd have to read and ponder it.
I agree with that.
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Old 04-07-2016, 01:20 PM   #5
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Well, let's see what we can figure out on our own. You mention two categories: radials, and counterpoises. "Radials" is actually two subjects, one of which is radials that are ground planes and the other of which is decoupling radials, short ones like the λ/4 radials at the bottom of a Ringo Ranger. These can also be considered as being counterpoises, but they're not ground planes (and "ground-plane antennas" are misnamed). Let's think about each of these several, very different things.

GROUND PLANE

By "ground plane" I mean a large reflecting surface, multiple wavelengths in each direction, under an antenna. This may consist of the earth around the antenna site, the surface of the sea, or "ground radials" that are long wires extending away from the antenna. At VHF and UHF, but not HF, it might be the top of a big RV. What we want this sort of "ground plane" to do is reflect back upward any of the signal that would otherwise be lost in the signal-absorbing things (such as dirt) under the antenna. The part of the signal that it reflects upward interacts with the upward-going part of the antenna's signal, and changes the antenna's radiation pattern. In general, the farther the ground plane is beneath the antenna (that is, the higher the antenna is mounted), the more signal will be radiated at low angles (good for working DX and not good for working nearby stations).

Most dirt isn't a particularly good conductor, and therefore not a particularly good reflector. Much of the downward signal from the antenna will be absorbed instead of being reflected, which isn't what we want. We lose the power in that part of the signal, and we lose the desirable interaction between the direct and the reflected signals. So, we may substitute what we hope will be a better reflector: the wires we commonly call "ground radials". It is incorrect to say we ought to bury these wires. They will work best if they're on top of the soil. What they're supposed to do is prevent the signal from entering the soil at all, so the soil won't absorb it. The wires don't need to be connected to the soil, and many systems use insulated wire for the "ground radials" because insulated wire doesn't corrode as quickly.

Having wires crossing your yard on top of the grass tends to muck up your lawn mower and ruin your fun playing croquet, so most folks bury them a little way; but they should be as shallow as you can manage. On a new lawn, lay them down before laying your sod.

There's no need to cross-connect the "ground radials" to each other, either. The currents in any ground system run toward and away from the antenna. This is true only for vertically-polarized antennas, but no one puts "ground radials" under horizontal antennas anyway.

Think of your "ground radials" as a reflecting plane that shields the dirt, keeping your signal out so it won't be absorbed in the soil.

Such a system may also act, to some extent, as a "counterpoise" (see below), but is generally impractical for HF mobiles.

COUNTERPOISE

So what's a counterpoise? It's a very poor name, for one thing.

Bottom-fed vertical antennas that aren't a full λ/2 long will have significant current at the feed point. If they're exactly λ/2 long, that's a very high-impedance point so the current will be very small.

The feedpoint current flows in and out of the center conductor of the feedline, and the feedline would like to have current flowing in the shield, too. A "counterpoise" -- poorly named -- is some wire, or sheet metal, or RV frame, or soil, that presents an impedance not so high as effectively to block the coax-shield current at the feedpoint. If the counterpoise is just λ/4 long, and open-circuited at the far end (it can be one or more λ/4 rods as in many VHF antennas, or a sheet-metal disk of diameter λ/2, for example), then it will present a very low impedance to the end of the coax shield. That's good, for the impedance of the counterpoise appears in series with the impedance of the antenna itself and can muck up the match to the coax.

You can, in fact, think of a half-wave (λ/2) dipole as a λ/4 antenna with a λ/4 counterpoise, even though perhaps no really sane person would choose to think of it that way. However, it does lead to the (correct) conclusion that counterpoises radiate.

"GROUND-PLANE" ANTENNAS

Many VHF antennas are miscalled "ground plane" antennas. That's because they have (typically) three or four little wires (or rods, or whips) sticking out sideways at the bottom, which the unworthy think constitute a ground plane. These are λ/4 long and, of course, open-circuited at the free end. Thus they're an RF short circuit at the attached end where they're connected to the coax shield; and RF flowing down the outside of the feedline gets shorted out and goes no farther, thus not burning your fingers when you fondle your transmitter, also not crashing your computer and burning out your telephone. In other words these "radials" on a ground plane antenna aren't a ground plane but instead work as a choke. They work pretty well, too.
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Old 04-08-2016, 07:24 PM   #6
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Carl,

Thanks, that gives a clearer description of what I'm trying to understand.

I will need to read it several times to absorb it, But thanks for taking the time to write it.
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Old 04-09-2016, 08:25 AM   #7
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When you get into the higher freqs radial design becomes an issue because a longer radial can become dominate in relation to the desired radiator. The other factor is take off angle so it's not just the creation of a "ground plane".
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Old 04-09-2016, 10:05 AM   #8
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When you get into the higher freqs radial design becomes an issue because a longer radial can become dominate in relation to the desired radiator. The other factor is take off angle so it's not just the creation of a "ground plane".
Actually it is. The takeoff angle is determined by the location of the ground plane (how far it is below the antenna) and by its reflection characteristics (how good a reflector it is). This is true for both vertical and horizontal antennas.

At UHF and above, and sometimes for VHF, we can build such things as rotatable parabolic reflectors that can be aimed both in elevation and in azimuth. For these there is no ground plane; but at HF such antennas are impractical. For HF antennas that aren't on spacecraft there is always a ground plane, and it always determines the takeoff angle.

This oversimplifies a bit -- there isn't just one "takeoff angle". There is a radiation pattern that gives varying signal strength at different takeoff angles. That pattern, in its variation by elevation angle, is determined almost exclusively by the location (and reflection characteristics) of the ground plane.
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Old 04-09-2016, 06:36 PM   #9
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True but corner reflector is approaching a beam with the driven element inside the focal point of the reflector. Very effective antenna but directional.
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Old 04-11-2016, 02:40 PM   #10
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Originally Posted by NN5I View Post
COUNTERPOISE

So what's a counterpoise? It's a very poor name, for one thing.

Bottom-fed vertical antennas that aren't a full λ/2 long will have significant current at the feed point. If they're exactly λ/2 long, that's a very high-impedance point so the current will be very small.

The feedpoint current flows in and out of the center conductor of the feedline, and the feedline would like to have current flowing in the shield, too. A "counterpoise" -- poorly named -- is some wire, or sheet metal, or RV frame, or soil, that presents an impedance not so high as effectively to block the coax-shield current at the feedpoint. If the counterpoise is just λ/4 long, and open-circuited at the far end (it can be one or more λ/4 rods as in many VHF antennas, or a sheet-metal disk of diameter λ/2, for example), then it will present a very low impedance to the end of the coax shield. That's good, for the impedance of the counterpoise appears in series with the impedance of the antenna itself and can muck up the match to the coax.
I think I could have made this clearer by saying that a "counterpoise" is anything that gives the current on the inside surface of the coax somewhere to go. A "ground plane" can do that, if the coax shield is connected to it. So can an RV chassis. The RV chassis, if you use it as a "counterpoise", still won't serve as a "ground plane" of course -- nothing can make it do that.

If there's nowhere else to go, the current on the inside of the shield will turn the corner and go down the outside of the shield (which, because of skin effect, is a totally separate conductor at RF). A connection to the "ground plane" -- which is the surrounding terrain under an RV -- can help avoid that. So you can use the planet as a (poor) reflector or "ground plane", and also as a (just as poor) "counterpoise".

It's a tough life for ORR guys.,
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