There's another issue that leads to greater feed line efficiency in addition to the points Jack made: keeping the SWR relatively low. That is, less than, say, 15:1 or less. All feed lines become lossy at high SWRs. One reason, as Jack mentioned, is the likelihood of dielectric breakdown. That's usually a catastrophic failure. The line is no good after that happens, especially if it's common coaxial line.
The other reason for loss at high SWRs is ohmic loss due to the extremely high currents at the current loops (points of maximum current). At even moderate power levels very large currents will occur there just as very high voltages appear at the voltage loops 1/4 wavelength away. RF flows only in the very surface of a conductor so the RF resistance of a conductor is much, much higher than its d-c resistance (unless the conductor is a very, very thin tube - probably too thin to handle). Heating at these current loops converts RF into wasted thermal energy. Holding the SWR down greatly reduces these losses, and open wire line does that even when there's no attempt to match the feeder to the antenna. Here's how that works. Consider a center fed wire. At the frequency where the wire is at 1/2 wavelength long the impedance at the center is usually about 50 ohms (in free space it would be 75 ohms, but as one moves close to the earth, the impedance drops). At the frequency where the wire is 1 wavelength long, the impedance at the center feed point will be in the range of 3500 to 4000 ohms, depending again on the height. As it becomes longer in wavelengths, those impedance extremes decrease, so the maximum range of impedance values are from about 50 to 4000 ohms. The SWR on a 50-ohm coaxial feed line, when the wire is 1/2 wavelength long, will be about 1:1. When the wire is 1 wavelength long, the SWR will soar to 80:1 (4000/50). The SWR on a typical open wire line of, say, 400 ohms, will be 8:1 when the wire is 1/2 wave long (400/50) and about 10:1 when the wire is 1 wavelength long (4000/400). So, not only is open wire line intrinsically better able to handle high voltages (and, by using large conductors) high currents more easily than coax, it has intrinsically much, much lower SWRs when used with typical multiband Ham antennas! The result is a much, much more efficient feed line. Traps and other devices are employed on many Ham antennas to provide a more consistent impedance somewhere near 50 ohms to permit the use of coax by keeping the SWR down, but at the cost of some loss in the traps and greater overall complexity of the antenna system. 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
