Topband
[Top] [All Lists]

Re: Topband: FEEDING AN END-FED LONGWIRE

To: topband@contesting.com
Subject: Re: Topband: FEEDING AN END-FED LONGWIRE
From: "Donald Chester" <k4kyv@hotmail.com>
Date: Fri, 05 Mar 2004 01:44:04 +0000
List-post: <mailto:topband@contesting.com>

> 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

_________________________________________________________________
Frustrated with dial-up? Lightning-fast Internet access for as low as $29.95/month. http://click.atdmt.com/AVE/go/onm00200360ave/direct/01/


_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband

<Prev in Thread] Current Thread [Next in Thread>