[Elecraft] Twinlead or ladder line antenna matching

[Vic Rosenthal]

I have one tip I'd like to add to the discussion of multiband dipole 
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 
length that will present a matchable impedance on all bands.  Cut and 
try can be frustrating, and it's nice to have the feedline be just long 
enough to reach from the antenna to the tuner.

Here's what I do: Start with a dipole antenna such that the length of 
one side PLUS the length of the feedline is at least 1/2 wavelength at 
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 
bands 80 through 10 meters (including WARC).

My tuner was an old Johnson Matchbox, which doesn't even have settings 
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 
foil about 1 foot (30 cm) long and wrap it around the feedline.  Then I 
slid it up and down (MUCH easier than cutting and extending!) the 
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 
the point that it is at.  If it is near a voltage loop for a particular 
band, it will have a great effect on the impedance that the line 
presents for that band; if it is near a current loop it will have much 
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 
helps.  The foil itself doesn't add significant losses, but of course 
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