The "grounding" issue with end feeding a half wave radiator is in keeping the rig at a low RF voltage to avoid "RF in the Shack".
Unlike a short monopole (1/4 wavelength long or less), very little RF current flows into the end of a half wave radiator so the "ground" has very little impact on its efficiency. Short monopoles have relatively high RF currents at the feed point so the "ground" has a huge impact on their efficiency. The RF current at the feed point is divided between the radiator and the ground connection according to their relative impedances. Where only a tiny current is flowing into the radiator, only a very tiny current flows into the ground connection. With a relatively bad ground with an impedance of, say 300 ohms, connecting an end half wave antenna with a typical impedance of 3000 ohms or so means that about 90% of the RF power is delivered to the antenna and only 10% is lost in the ground impedance. Using the same ground with a 1/4 wave monopole with, say, 35 ohms impedance means that about 90% of the RF power is absorbed in the ground resistance and only 10% is delivered to the antenna. With shorter monopoles the impedance drops very fast in a non-linear fashion. It's not unusual to see feed point impedances of 2 or 3 ohms for short "loaded" antennas. In that case 98% or more of the RF power is consumed in the "ground" and only 1% or 2% is radiated. 73 Ron AC7AC ______________________________________________________________ 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
