[Elecraft] Twinlead or ladder line antenna matching

[w2bvh]

I've been using the antenna described at 
http://www.qsl.net/w5dxp/notuner.htm with my K2 for about 3 years and am 
very happy with it.

73,
Lenny W2BVH

Vic Rosenthal wrote:

> 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
>
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