[UK-CONTEST] OT but Likely I'll get answer....

[Dave Lawley]

Hi Adrian

If you don't mind my saying, I think you're asking the wrong question! 
But to answer it, you can use an analyser, balun and variable resistor. 
Sweep the analyser on a high range (say 14-28MHz) and observe the peaks 
and troughs in impedance. Adjust the resistor until they are smoothed, 
that's the impedance.

An equally valid way is to measure the wire diameter and separation and 
use the formula.

Unless you want to run multiple bands simultaneously for a Multi-Multi, 
why run three separate long feeders? A trap dipole for 20-15-10 would do 
very nicely, and you can avoid the impedance excursions by using a 9:1 
transformer at the antenna end. Note that the impedance of a horizontal 
dipole will be nearer 70 ohms than 50, so in theory a 630 ohm line would 
give a flat match and would not carry standing waves. I wouldn't worry 
though, the whole point about open wire lines is that they can carry 
standing waves without excessive losses and if you use good quality 450 
ohm line the losses will be minimal. Main point as already noted is to 
keep the line away from metallic objects, also avoid sharp corners (90 
degrees or more) as this can introduce unwanted reflections (so I'm 
told). If you want to turn 90 degrees, do it in two 45 degree bends.

Cheers, Dave G4BUO