Twisted-Pair Balun WireMarch 28th Fact-of-the-Day
Two-wire speaker cable, hand-twisted enamel wires, and other wire pairs with unknown line impedances are commonly used to wind RF baluns. The result can be balanced-to-unbalanced transformation at the expense of impedance mismatch. 100-ohm line is ideal for winding many baluns, because most radios have 50-ohm antenna connections. Two 100-ohm lines wound on a toroid can be connected in parallel at one end to match 50-ohms and in series at the other to match 200-ohms. The problem seems to be where to find 100-ohm line. 100-ohm line suitable for receiving and QRP applications actually is easy to find. The CAT 5e cable used to network computers contains four 100-ohm twisted pairs. The propagation velocity is 72 percent. The loss per 100 feet is 1.3dB at 4MHz, 2.0dB at 10MHz, 2.8dB at 20MHz, and 6.7dB at 100MHz. Losses in the small lengths needed for baluns usually are negligible. ©2005 Martek International All rights reserved.
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