Perhaps you are being a bit "hard" on the poor half wavelength, Jim! Actually, the "villain" in your scenario is the 1/4 wavelength wire, not the 1/2 wavelength wire. But they are both really good "guys" around the shack!
A wire that has RF flowing along it with show a range of impedances from high to low, depending upon its length in wavelengths. The earth shows a low impedance to RF. A wire connected to the earth will show an impedance of, essentially, the resistance of the connection to the earth. At HF and using a simple ground rod, you can expect this resistance to be several hundred ohms, typically. If you connect a wire to that ground and feed RF into the other end of the wire, current will flow into the ground provided the wire is very short in terms of wavelengths - say less than 0.1 wavelength long. That's about 40 feet on 160 meters, 20 feet on 80 meters, but only 5 feet on 20 meters and 2.5 feet on 10 meters. Those are *maximum* practical lengths for a ground wire if you don't want the length to play a significant role in the circuit. As the wire is made longer, you must consider the length to understand what to expect. With one end of the wire grounded, 1/4 wave away from the ground the impedance, and RF voltage, will be very high. At the highest it can be under the circumstances, actually. One quarter wave further along - 1/2 wave from the ground connection - the impedance and the RF voltage will be the same as it is at the ground connection! That is, whatever impedance is present at one end of a half wave length of wire, it will be "repeated" every 1/2 wave along the wire. So a 1/2 wave "ground wire" would be perfect! But only where the wire is 1/2 wave long. If that were the case, say, we used a 32-foot ground wire on 20 meters, it would be an effective ground connection, but on 40 meters the 32-foot wire is 1/4 wave long. Remember, the impedance will be very high 1/4 wave from the ground, so now the rig would be at a high RF voltage because of that same 'ground' wire that worked so well on 20. Suppose we disconnect that 32-foot wire from the ground stake when we're on 40 meters and isolate the far end. Isolating (insulating) the far end forces it to be at a high impedance instead of the low impedance it saw connected to the earth. If the impedance at the far end is high, then the impedance at the rig *must* be low! Now that wire provides a rather good RF ground for the rig! For most of us, it's easier to arrange a 1/4 wave ground for each band that is isolated at the far end than it is to arrange 1/2 wave length wires on each band that are connected to the earth, but either one will work equally well. Remember, the impedance repeats every half wave along the line: what you find at one end repeats at the other end. That's why both ends of a half wave antenna are at a high-impedance point and the center, 1/4 wave from each end, is at a low(er) impedance point. The impedance of a 1/4 wave wire inverts. That is, if it's high at one end (insulated) it will be low at the other end. Ron AC7AC -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of JIMMY D HARRIS Sent: Tuesday, January 23, 2007 4:40 PM To: [EMAIL PROTECTED]; elecraft@mailman.qth.net Subject: [Elecraft] RE: In Shack Radials and Ground Don, Maybe my thinking is too simple. But here goes. I believe that we both have indicated that quarter wavelength can eliminate RF in the shack among other RF problems caused by less than a perfect antenna system. I believe that we also agree that a half wavelength does not do that. Therefore, a half wavelength ground wire has no positive influence on RF problems. That indicates to me that I should avoid half wavelength ground runs as they have no positive influence on RF problems. I would guess in the world of amateur radio there are RF problems that are not recognized. Half wavelength grounds runs do nothing to clear up those problems. Of course, the real solution is to have an adequate antenna system. You can bet that in commercial systems a proper antenna system is used. Jim, AB0UK _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
