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Re: [TowerTalk] Par Electronics - End-fed 1/2 wl antennas

To: <>,"Bill Tippett" <>
Subject: Re: [TowerTalk] Par Electronics - End-fed 1/2 wl antennas
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Mon, 18 Oct 2004 14:21:40 -0400
List-post: <>
>          I'm sure that is true, but the current must be
> low.  This is probably summarized by AA5TB (not Par) here:

> almost always exists anyway. Therefore, a small return
wire may be
> necessary although there is usually enough stray
capacitance from one side
> of the circuit to the other to satisfy this requirement
without any
> additional return wires.

He implies the entire problem is displacement current, and
it only tries to flow back across the feedpoint to the thing
everyone thinks is the antenna. This goes to the heart of
what I am trying to get across. People just don't understand
the system.

The coax from the radio to the antenna is no different than
the antenna when a good solid reference or counterpoise is
missing and we end-feed a wire. It IS the antenna, along
with the thing we call an antenna.

If the antenna requires 100mA of current to end feed, the
current on the outside of the feedline shield could wind up
being an ampere or more 1/4 wl from the antenna!

As a matter of fact if you look at

and scroll down to the I-max antenna, it isn't much
different than the PAR antenna for feedline excitation.

If the antenna is exactly 1/2 wl long it does not behave
differently. It is the feedline length that controls feed
line current! As a matter of fact, a 1/2 wl end fed requires
an extraordinary choke at the feedpoint area to divorce the
feeder from common mode current because the voltage is so

Now you can find a sweet length that minimizes peak feeder
current along the line, but you can't stop the feeder from
being part of the antenna system without a counterpoise. The
optimum length of feeder is 1/2 wl when the feeder is
grounded. In that case maximum feeder common mode is right
at the antenna. The worse length is a 1/4 wl or odd multiple
when the feeder is grounded in the shack.

It a simple system. I'm really amazed so many people don't
grasp the concept that without a good counterpoise or good
isolation device the entire length of the feeder is also an

73 Tom


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