You are absolutely right Ron. It is possible to end feed a half wavelength antenna. All it takes is very big coils and very high voltage capacitors and a lot of knowledge and usually a lot of money. Not quite so bad if you stay with QRP power levels. And when you are through you have an Antron 99 for whatever band you build for. I certainly don't recommend it to beginners. But for those of you who want to try it, lots of luck. When you are done you will have the equivalent of a center fed half wave antenna.
Willis 'Cookie' Cooke K5EWJ --- On Sat, 5/16/09, Ron D'Eau Claire <[email protected]> wrote: > From: Ron D'Eau Claire <[email protected]> > Subject: Re: [Elecraft] OT - SteppIR Vertical and Elecraft Products > To: "'Elecraft Discussion List'" <[email protected]>, "'David > Wilburn'" <[email protected]> > Date: Saturday, May 16, 2009, 2:09 PM > -----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:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: > http://www.qsl.net/donate.html ______________________________________________________________ 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
