Vic, K2VCO, wrote: Keep in mind that you are measuring the current in the radiator AT A PARTICULAR POINT. --------------------- The counterpoise is tuned for maximum current at the *rig*!
The idea is that the more current flowing into the counterpoise for a given power output the lower the RF voltage at the rig. The objective is to provide the lowest impedance "RF Ground" at the rig. A well-tuned 1/4 wave wire does a very good job, of course. A 1/4 wave wire shows an impedance of about 35 ohms. Parallel several and you can reduce that proportionately. The problem is that in many situations it's not possible to carefully arrange even one resonant 1/4 wave wire, much less several. Many so-called grounds consisting of a random length wire thrown on the floor or a wire running several feet to a ground rod show an impedance at the rig in the hundreds of ohms, making the rig "hot" with RF. In that case a 35 ohm "ground" provided by a single tuned counterpoise is vastly superior for "grounding the rig". Will it change the efficiency of the antenna? Perhaps. If the antenna is an end-fed 1/2 wave wire presenting an impedance of perhaps 2000 ohms to the rig, the difference in radiation efficiency will be negligible. One can consider the two resistances in series: the ground resistance, 35 ohms in this example, and the radiation resistance of the antenna, 2000 ohms in this example. If .1 amps is flowing inn that circuit, the rig is putting out 20.35. Of that 20 watts, 0.35 watts is dissipated in the counterpoise and 20 is radiated. The advantage of the counterpoise in this case is to keep the RF voltage at the rig down. It will tend to "float" to a high RF potential if there's no ground and cause all sorts of problems. If the antenna is shortened to 1/4 wavelength, then the efficiency drops to 50% with equal current and power in the counterpoise and antenna. That sounds awful but that is, in reality a very respectable figure for such an antenna. Many short vertical antennas have efficiencies of much less than 50%. As the antenna is made shorter than 1/4 wave long (that's overall physical length, adding a loading coil doesn't help the efficiency - it only helps match impedances) the radiation resistance plummets, and with it the efficiency unless a really, really good ground system is used. That's the ultimate challenge with anyone using a "loaded" monopole of any sort. There simply is no "magic" answer to efficiency for physically small antennas - until we get some really cheap, really good room-temperature superconductors to make them from... Ron AC7AC _______________________________________________ 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
