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From: jhs@mitre-bedford.ARPA
Newsgroups: net.ham-radio
Subject: care and (mostly) feeding of end-fed antennas
Message-ID: <2324@brl-tgr.ARPA>
Date: Tue, 22-Oct-85 11:23:35 EDT
Article-I.D.: brl-tgr.2324
Posted: Tue Oct 22 11:23:35 1985
Date-Received: Thu, 24-Oct-85 05:48:46 EDT
Sender: news@brl-tgr.ARPA
Lines: 53

Hot off the presses (or perhaps just reinvented):

We at the MITRE-Bedford Amateur Radio Club (MBARC)  wanted to feed a 160-meter
half-wave at the end supported by our tower, to avoid draping feed lines
across a parking area.  We came up with the following scheme, which can be
adapted to other bands by the obvious scaling of dimensions.

We observed that an end-fed antenna exhibits a high impedance, which our
antenna gurus here at MITRE say should be maybe 1200 to 1800 ohms, and of
course it would be purely resistive if the antenna is exactly resonant.
The exact resistance depends on height above ground and other factors.
Anyway, we noted that a quarter wave transformer to match 1800 ohms to 50 ohms
needs to have a characteristic impedance of exactly 300 ohms.  (In general,
the square root of the product of the given impedance and the desired
impedance.)  So we made up a special cable consisting of a convenient length
of RG-8/U soldered to a quarter wavelength of "Radio Shack's Finest" black
foam-filled TV twinlead, which incidentally, I have had good luck with even
when running the full legal limit through it at high VSWRs.  The hot side
went to the end of the half-wave antenna and the "ground" side we connected
via a water-pipe ground clamp to a cross leg of our Rohn tower.  We figured
that at 160 meters this was more or less "ground".  Purists would want to
add a 1:1 balun at the coax-to-twinlead junction, I suppose.

We fired the thing up and voila! a match of 1.5 to 1 or better was had over
much of the 160-meter band, though it did get up to 3:1 at 2 MHz.  Presumably
this could be tweaked in to cover the band segment of interest, and of course
it would work fine with a tuner even at 3:1.  Signal reports have been
outstanding.

In summary, a quarter wave transformer made of 300 ohm twinlead seems to work
well as a matching network for end-feeding a half-wave antenna.  A balun
between the coax and the twinlead appears to be unnecessary.  Soldering the
twinlead directly to the coax and covering the joint with a heavy wrap of
electrical tape makes a mechanically satisfactory connection, though shrink
tubing (I like to call it psychiatrist tubing) would doubtless be more
permanent.  Take care to connect the twinlead correctly so that the coax
shield goes to ground and the coax center lead goes to the antenna.  Of course
if you use a balun this is not a concern.  Hint: Crimp-on "butt" connectors
seem to work well for attaching more durable wires to the fragile #20 or
whatever is in the twinlead.

If you use this idea, take care to keep the twin lead away from metal objects
as much as possible.  We used rope tied around the twinlead to support the
top end, and rope to support it clear of metal objects at the bottom end just
before the coax.  Twinlead is not as strong as coax and should not be
subjected to unnecessary tension.

One final note:  The antenna ought to work reasonably well also at half the
design frequency, with maybe a 2:1 VSWR, easily handled with a tuner.


						73,
						John S., W3IKG