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Some thoughts about transmission-line myths

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One of the myths floating around hamdom is that you don't want to use open-wire transmission line because it causes TVI. I think this got started in the early days of TV when poorly designed amateur transmitters often fed open-wire feeders. Open-wire line was blamed for the faults of the transmitters. The cure was to use coaxial cable in conjunction with low-pass pi networks. This allowed amateurs to continue using simple single-ended amplifier designs. (Admittedly, building a balanced amplifier feeding a balanced low-pass network isn't a trivial task, but such a transmitter could yield a very clean signal.)

I often wonder why some amateurs are looking for 75-ft transmitting twin lead. Given the heights of most amateur dipoles, 50-0 coax is actually a better match to the dipole's feed point. (Let's ignore the problem of finding a modern transceiver that wants to see a 75-f1 load...) Although the loss of that twin lead is 0.4 dB/100 feet less than that of RG-213 (about 1.3 dB) at 30 MHz, this figure applies only under ideal conditions (perfect impedance match, no bends or interaction from nearby objects). The power rating of the twin lead is less than that of matched RG-213: It's about 1000 W at 30 MHz as opposed to approximately 1800 W for RG-213.

Perhaps because of its high price, Teflon-dielectric coaxial cable is mistakenly assumed to have lower loss than its polyethylene dielectric counterpart. Actually, the polyethylene dielectric used in coaxial cables is less lossy than the Teflon dielectric. True, the Teflon dielectric does handle more power, but this is because of Teflon's high melting point: It can withstand more heat before melting.

Finally, there's the question of what constitutes acceptable line loss. Sorry, I can't supply a magic number. If you want a calculated number, you could figure out the path loss between your station and the station you want to work and see what line loss gives you the desired radiated signal strength. For instance, I've found that 50 mW to my low antennas is enough to work Europe on 20 meters when conditions are good. Therefore, if I ran 100 W, I could use 4100 feet of RG-213 or 2200 feet of RG-58 for a feed line and wind up with the same radiated power. Now, if only I owned a piece of property to test this calculation.

Zack Lau, ARRL Lab Engineer