Topband: FEEDING AN END-FED LONGWIRE

[Donald Chester]

> > Well, the radiation may or may not be as described,
> > but one thing is for sure:  common mode feeder current
> > is exactly equal to the current feeding the "antenna".
> > That's physics.
>
>That's an excellent way to put the problem in perspective. That's why
>J-poles have considerable feedline radiation, and why "no-radial" verticals
>and end fed Zepps have such horrid problems with RF in the shack.
>
>However with an exact half-wave antenna and an exact 1/4 wl feeder, the
>problem is minimized because the antenna can have a few thousand ohms
>impedance (low current) and the opposing feeder wire at least has
>differential current in somewhat the right distribution to reduce the RF
>problem in the shack, IF the tuner has a real floating output (a Johnson
>Matchbox won't be good for example because of the grounded capacitors). The
>tuner has to look virtually like a perfect open circuit to ground even with
>optimum lengths or the Zepp idea "falls apart".

Decades ago, I had excellent performance on 80m. from the 3nd floor apt in 
an old 3-decker in the Boston area, using an end-fed zepp running from the 
roof of the building across the street to a tree.  The antenna was carefully 
cut to an electrical halfwave on the operating frequency.  The feedline was 
a little less than a quarter wave long, and I used a small rotary inductor 
in series with one side of the feedline.  It was connected directly to the 
swinging link of the pushpull final.  The variable inductance was adjusted 
for optimum loading with the link as far out of the tank coil as possible.  
The link, variable inductor, feedline and antenna all floated free from 
ground.  The antenna took power without any problem, and there was no rf in 
the shack anywhere, and I could set a TV right on top of the transmitter and 
pick up local stations while transmitting with no TVI.

I  later moved the entire  station to Houston.  I had equally good results 
there with a similar end fed zepp, but I replaced the simple rotary inductor 
with a full  fledged series-tuned balanced link coupled tuner with split 
stator capacitor.  Unlike the Johnson matchbox, the whole thing floated 
above ground. The tuner seemed to perform about the same as the simple 
variable inductor.

The only problem with the antenna near Boston was that several times a year 
one of the old wooden three-deckers on the street would catch fire and the 
fire engines would knock down my antenna with their ladders.  I was glad to 
get out of that neighborhood before it was my building's turn to burn to the 
ground.

Don K4KYV

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