Regarding open wire line, I generally use something in the vicinity of 450 ohms. That is an easy size to buy or fabricate and produces a fairly low SWR when used to feed most doublets. While open wire lines are low-loss, they are not "lossless". SWR does matter, just no where near as much as when using coaxial cable.
Typically, the voltage loop (highest impedance) point of a wire antenna won't exceed 3,000 or 4,000 ohms impedance, so the SWR on the line won't reach 10:1 even when fed at a voltage loop (such as when the doublet is a full wavelength long). At the frequency at which it is 1/2 wave long, the feedpoint impedance is about 50 ohms which produces an SWR of less than 10:1 again. Everywhere in between will produce impedances somewhere inside that range. That's what keeps the losses on open wire low. The greatest losses in well-made open wire lines are ohmic: the RF resistance of the wire. RF resistance is directly proportional to the diameter of the wire, since RF currents flow only on the outer surface of the conductor. So I like to use a much larger conductor than most commercially-made wires. That is a significant help when I load up the antenna at a frequency below which it is 1/2 wave long. For example, using my 66 foot doublet on 80 meters. It's an efficient antenna when used that way provided feeder losses are kept low. But when loaded on a band where the wire is only 1/4 wave long (e.g. a 66 foot wire used on 80 meters) the impedance is rather low so the SWR on 450 ohm line might be 20:1 or greater. That's one reason I shun ladder line or twin lead with small conductors. Using open wire line to feed other low-impedance antennas, such as small loops presents the same issues of high SWR and increasing losses. As I mentioned before I make my own open-wire for most work using #14 (or larger) copper wire and well-spaced low-loss insulators. Actually, the common "dog bone" end insulators work well providing a spacing of about 2.5 inches. Hold 'em in place with a small ty-wrap threaded through the same hole and snugged down tight to provide a tight friction-grip with the wire (most ty-wraps do disintegrate in three or four years from UV but I make a point to check my installation at least that often). Another way to hold the wire is to loop a short length around the end of the insulator and wrap it tightly around the feeder directly above and below the insulator. Those will last indefinitely. One insulator every three or four feet is plenty. It is not necessary for them to hold the wires exactly equidistant: only to keep them from touching. I don't try to run my open wire at a low SWR, so the actual impedance or impedance bumps along the line aren't important. Open wire line made that way using white insulated house wire and white dog-bone insulators is actually harder to see from several feet away than the typical brown window line or twin lead. There are lightweight plastic "dog bone" end insulators as well. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
