Copyright © 2002, 2006 by Harold Melton, KV5R. All Rights Reserved.
Spacing and Impedance
Don’t worry about it. A non-resonant antenna will present a feed point impedance of 10 to 5,000 ohms, with plus and minus reactance, at various frequencies, so who cares about the exact feed line impedance? Matching the antenna to the feed line simply has nothing to do with efficiency (unless you’re using coax).
Anything from 1 to 6 inches is acceptable spacing. 1-inch #14 line is 370 ohms. 1-inch #18 line is 450. 6-inch #12 is about 600. It just isn’t at all critical - and don’t let any geezer or guru tell you different! The spacing should not be over 1 percent of the highest-frequency wavelength, and that’s the only real consideration with ladder-line spacing. On the other hand, it needs to stand-off from metal at least 2-3 times the line's width, so wider lines need longer stand-offs. I think 3" open line, with 9-12" stand-offs, is probably a good place to start.
As mentioned above, parallel feeders can pick up RF from the antenna and transport it into the shack as common-mode current (this means the two wires acting as one, in-phase). The way to avoid this is to avoid resonant lengths of ladder-line, and, if possible, bring the line away from the feedpoint perpendicular to the antenna (90 degrees) for a far as possible before turning parallel to one side of the dipole. Setting your bend-point will also help you take up slack when using a non-resonant length, without having to cut off the excess.
In other words, measure your total run, then increase that to the next available “good” (non-resonant) number, then route the line to take up the slack. A 300-foot (or 100 meter) open-reel tape measure is handy, and Harbor Freight has them for about $30.
Lengths to avoid (in feet): 32, 65, 96, 130, and 260 - and multiples of any of those. Don’t let them make you buy 100 feet when you know that’ll be too close to 96! Make them sell you 110 feet, for example.
Good lengths: Somewhere around 40, 80, 110, etc. Say you saw it here. If in doubt, consult the ARRL Antenna Book.