[Elecraft] OT - SteppIR Vertical and Elecraft Products

[David Cutter]

Have a look at QEX this month.  W6WO shows an excellent method I have not 
seen before.  He feeds 10% in from the end and uses a simple 4:1 
transformer.  His capacitor is just a compression trimmer.  It is coax fed 
via a choke and runs to 100W.

Also look at AA5TB website for end feds.  Kite flyers use them a lot.

The attraction for me is that I can have a single lightweight wire supported 
by a flimsy fishing pole and the heavy coax is near the transmitter at 
near-ground level.  The 10% tail end does not need to be in-line, it can be 
bent out of the way for safety and tidiness.  I've modelled several 
configurations and they are mostly good for NVIS unless vertical.  I 
modelled 30ft horizontal at 4ft above ground then 34ft vertical (up the 
fibreglass pole) and that gives a small f/b of 6dB and useful low angle take 
off.  Great for mini-dxpeditions and back-packers, especially good next to 
the sea of course.

David
G3UNA

----- Original Message ----- 
From: "Stephen W. Kercel" <kercel1 at suscom-maine.net>
To: <wrcooke at flash.net>
Cc: "'Elecraft Discussion List'" <elecraft at mailman.qth.net>
Sent: Sunday, May 17, 2009 3:37 AM
Subject: Re: [Elecraft] OT - SteppIR Vertical and Elecraft Products


> 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
>
>