-----Original Message----- 6. Don't use a half wavelength at your desired frequency because it is very difficult to match an antenna with nearly infinite reactivity. You can put a coil in series to make it about 0.75 wavelength so you can match it, but it will not be easy.
----------------------------- Not at all. I do it all the time. The reactance of a 1/2 wave wire is zero (A 1/2 wave is, by definition, resonant. Resonant means it has zero reactance). The impedance in a "real world" 1/2 wave antenna is something in the range of 4000 or 5000 ohms, tops, and often much less. It's affected by the length/diameter ratio. That's only the resistive value since the reactance is zero. Such an antenna is often referred to as a "Fuchs" antenna, since he popularized it in the 1930's. What I *have* noticed is that most commercial matching networks (ATUs) today won't handle an impedance of several thousand ohms. Either they simply don't have the range of adjustment needed or they'll arc over inside. That's because of the very high RF voltages that are present when the impedance is in the thousands of ohms. That's why you see *big* air variables or even vacuum capacitors were commonly used in the ATUs from the 1930's and 40's just as they were in the high-impedance "tank" circuit at the output of a vacuum tube power amplifier. Overall, it's wonderfully efficient antenna, as is any end fed, vertical, inverted L or otherwise, that has a very high feed point impedance compared to the impedance of the ground connection. Remember that the RF current is divided between the antenna (almost all of it becoming electromagnetic waves) and the ground connector according to Ohm's law. The two impedances are in series, so the idea is to reduce the ground impedance as far as possible while *raising* the radiator's impedance to a value as high as possible. Since we're often constrained by a given radiator impedance, we're usually struggling to reduce the ground impedance to improve the ratio and so the efficiency. Ron AC7AC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
