On Sat,6/10/2017 11:40 AM, Stan GW3SRM wrote:
I note that the subject of counterpoises has been debated many times in this forum but I have not seen the subject of tuneable artificial grounds discussed.
Fundamental concepts like this are very well addressed in the classic references on antennas. Your use of the word "counterpoise" is precisely correct. Your use of the word "ground" is absolutely and totally WRONG.
A connection to Mother Earth does NOT make a transmitting antenna work better. The earth is a big resistor. Shoving TX RF into it wastes TX power by warming worms.
What IS needed, no matter where you operate, is a return for antenna current -- SOMETHING to act as the other half of the antenna. Antennas like a dipole inherently provide that; with an end fed antenna, we must supply that "other half," and the correct name for it is "counterpoise."
Transmitters are designed to work into some defined load Z, usually 50 ohms, but with some tolerance. That tolerance is often defined by the apparent SWR as measured at the transmitter. The function of an antenna tuner is to transform the Z at the transmitter end of the feedline to the Z that the transmitter want to see -- that is, the Z into which it will provide the most power, and do so without stressing the output transistors or tubes. The so-called "artificial grounds" provide the same function, but in a different way -- they vary the electrical length of the counterpoise so that the combination of the "intentional antenna" (the end fed wire) and the counterpoise make the Z at the transmitter as close as possible to 50 ohms. This allows the TX to put power into the antenna. That counterpoise, whatever it is, is PART OF THE ANTENNA. And that's true whether we CALL it "ground" or counterpoise.
Another point. ALL end-fed antennas must put their return current SOMEWHERE. If we don't provide a "good" path (like a good counterpoise), mother nature will take over and put the current where SHE wants it to go, like anything attached to the chassis of the transmitter. This can be a coax shield, the power system protective earth (green wire), a wire going to a ground rod, even telephone wiring or CATV wiring connected to that ground rod. (Remember, the earth is a big resistor). KK9H, a fairly smart engineer who is one of my old buddies in Chicago, intentionally uses the HVAC air ducts in his home as a counterpoise for a 160M wire. At my last Chicago QTH, I used a big wrought-iron fence running around the front of my lot as a counterpoise on 160 and 80 for a Tee-vertical.
A connection to mother earth is important for only two reasons. First, and most important, for lightning protection and electrical safety. Second, for SOME receiving antennas, where the resistance of the ground makes the antenna work, or where the loss of efficiency caused by adding the resistance to an RX antenna doesn't matter.
SO -- let's stop using (or thinking about) the word "ground" in conjunction with TX antennas, and in stead, talk (and think) of counterpoise.
73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
