[Elecraft] OT - SteppIR Vertical and Elecraft Products

[WILLIS COOKE]

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 <ron at cobi.biz> wrote:

> From: Ron D'Eau Claire <ron at cobi.biz>
> Subject: Re: [Elecraft] OT - SteppIR Vertical and Elecraft Products
> To: "'Elecraft Discussion List'" <elecraft at mailman.qth.net>, "'David Wilburn'" <dave.wilburn at verizon.net>
> 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
> 
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