[Elecraft] OT - SteppIR Vertical and Elecraft Products

Sat May 16 22:37:43 EDT 2009

Willis and others:

There is a much cheaper trick than big coils and capacitors that ought 
to work, and could easily handle high power.

Although I have never tried end feeding a half wave antenna, I have had 
great success in center feeding full wave antennas. The trick is to use 
an open wire feedline as a quarter wave transformer. I used the old 
fashioned "ladder line" with plastic spreaders and bare copper wire 
rated at 450 Ohms. Since the velocity factor is practically 1, I cut the 
length to the theoretical free space quarter wavelength. One end feeds 
the center of the antenna, and the other end goes to a 1:1 balun. It 
presents a very low SWR to 50 Ohm coax. I have done this on various 
bands with great success.

I have not tried the "end fed Zepp" configuration implied by an end 
feeding scheme, but for the sake of this discussion, I have run a quick 
simulation.

Here's what the math says:

I ran the "Backyard Dipole" File that Roy provides with  with EZNEC. I 
Moved the feedpoint to one end, changed the ground conditions to 
"Real-Medium" and included the copper losses in the wire.

The computed end point impedance at 14 MHz (where it is a half 
wavelength) 1282-J1791.

If you use 450 Ohm window ladder line in the TLW program and use a 14 
foot length of line in the traditional single ended Zepp fed 
configuration, the impedance at the input of the transmission line is 
43.15-J18.06. Connecting RG-8 to that works out to an SWR of about 1.5 
on the coax.

In the simulation the transformer length did not turn out to be exactly 
a quarter wavelength. I had to tweak it a bit to find a workable 
transformation. In practice, the ground losses will not be exactly the 
same as the simulation, and one might have to play a bit  with the 
transformer length when you try it for real.

However, my experience with the "double Zepp" scheme above is that it is 
not that hard to find the right line length and the antenna is quite 
forgiving in practice. I see no reason why a single ended Zepp scheme 
should be significantly more difficult to do in practice.

Some posters may object that the antenna's behavior is impractical to 
predict because  the end point impedance can vary over a wide range. 
That is true, but by using a quarter wave transformer with relatively 
high characteristic impedance, that wide range at the high-Z end is 
transformed to a narrow range at the low-Z end.  Thus, in practice, with 
perhaps a bit of tweaking, using this scheme will give you acceptable 
SWR in the coax, even if your actual impedance numbers are not the same 
as my numbers.

73,

Steve Kercel
AA4AK

WILLIS COOKE wrote:
> 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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