Jack makes excellent points. I would like to elaborate on one comment he made:
"...open-wire feed. There we don't care about the mismatch since the feedline loss is microscopic. The RF currents balance themselves out so that the feedline doesn't radiate. But we then need a decent tuner to feed the antenna & open-wire. Here I drop back to the old tank-circuit tuners which make things "sing!" I don't like to use a balun here - it's just another device to burn up the precious RF when you hit a severe mis-match node." I have used open wire feed far more than coaxial lines for HF antennas over the past half century. The losses in open wire lines are not insignificant under many conditions. Open wire line is still subject to ohmic losses. It is far less lossy than coax because all common coaxial lines in use today are fairly low impedance - typically 50 or 75 ohms. Good open wire line will have an impedance of 500 or 600 ohms. Even the common "ladder line" has an impedance of 450 ohms. That higher impedance provides lower SWRs in most multiband antennas. For example, a doublet may show an impedance of 3000 ohms at the feed point, which is common for a center fed full wave doublet. That translates into an SWR of about 60:1 if coaxial line is used and an SWR of less than 7:1 if 600 ohm open wire line is used. That difference results in a substantial decrease in line losses. Even among open wire lines, losses are not always insignificant. It depends upon the feed point impedance (that's at the antenna or load end of the feeder). For example, a short doublet or some transmitting loop antennas can have an impedance of 10 ohms or less. With 600 ohm line that's an SWR or 60:1. The losses in the open wire line may be as great as those in coax under those conditions. One way to reduce those losses is to reduce the ohmic losses. That's where virtually all the loss in any kind of transmission line occur: RF converted to heat at the high-current points. Ladder line and "twinlead" that use small conductors is very susceptible to such losses. RF flows in the skin of the conductors; resistance is directly proportional to surface area. My open wire lines are seldom built using smaller than #12 wires. Jack is quite right about the need for a true wide-band tuner designed for such lines. My tuner can be seen sitting above my K2/100 in the my picture on QRZ.COM (AC7AC). It can handle impedances from a small fraction of an ohm to many thousands of ohms The design is similar to those suggested by Cebik in his on-line articles about ATUs. The downside are those clips for taps and knobs. Such a tuner is often called upon to handle many thousands of volts of RF even at moderate power. It needs to be big, relative to modern rigs, to avoid flashover and excessive ohmic losses. Although such a setup might be automated, it would require mechanically complex and large switching systems to tune up automatically. So, when I jump from one band to another, I have to spend 30 seconds or so readjusting the tuner: something many operators find unacceptable. They prefer a less efficient, but much faster and much, much smaller tuner. Good Ham shack design calls for compromises. Many Hams today try of find an antenna design that may not provide a low enough SWR on all the desired bands to provide a good load to the rig, but which is low enough that a less versatile but fully automatic tuner can handle. Designers like Elecraft work to provide tuners with the greatest versatility that can still be packaged into a small, efficient package. It's left up to the operator to contrive an antenna system that can work efficiently with such a tuner if he/she wants the small size and speed the automatic tuners provide. Just remember that the antenna system starts at the input to the tuner. Every element, the coils and relays in the tuners, a balun if you feel you need one, the feed line itself all consume various amounts of the RF. Every watt of RF consumed in those parts is never radiated. 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
