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Old 04-11-2016, 02:40 PM   #10
NN5I
Carl, nn5i
 
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Join Date: Sep 2011
Location: Tallahassee, FL
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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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