[Elecraft] Twinlead or ladder line antenna matching

Fri Dec 5 12:29:10 2003

Vic,

your foil idea is an excellent one.=20

   Dave

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Vic Rosenthal
Sent: Friday, December 05, 2003 10:18 AM
To: Elecraft Reflector
Subject: [Elecraft] Twinlead or ladder line antenna matching

I have one tip I'd like to add to the discussion of multiband dipole=20
antennas fed with twinlead or ladder line.  If you have a single antenna =

that you want to use on all bands, it can be hard to find a feedline=20
length that will present a matchable impedance on all bands.  Cut and=20
try can be frustrating, and it's nice to have the feedline be just long=20
enough to reach from the antenna to the tuner.

Here's what I do: Start with a dipole antenna such that the length of=20
one side PLUS the length of the feedline is at least 1/2 wavelength at=20
the lowest frequency you want to use.  For example, I used a dipole with =

33 ft. (10m) legs and a feedline length of about 80 ft (24m) on all=20
bands 80 through 10 meters (including WARC).

My tuner was an old Johnson Matchbox, which doesn't even have settings=20
for WARC bands, and I found that there were some frequencies on which it =

was unable to provide a match.  What I did was take a piece of aluminum=20
foil about 1 foot (30 cm) long and wrap it around the feedline.  Then I=20
slid it up and down (MUCH easier than cutting and extending!) the=20
feedline until the Matchbox was able to handle all bands.

The idea is that the foil is in effect a capacitor across the line at=20
the point that it is at.  If it is near a voltage loop for a particular=20
band, it will have a great effect on the impedance that the line=20
presents for that band; if it is near a current loop it will have much=20
less effect.

I found that it was possible to bring all bands into range with just one =

piece of foil, but there's no reason you can't use more than one if it=20
helps.  The foil itself doesn't add significant losses, but of course=20
you want to try to adjust the system to make impedances as reasonable as =

possible to reduce losses in the balun or tuner.

This might also be useful in the field, or to make it possible to use an =

existing antenna with a tuner that has limited range, like the KX1's =
tuner.

Vic K2VCO

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