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Random Wire Antennas

Category: Evergreen
Tags: Antennas

Random Wire Antenna Lenghts #

For best results, chose a length from the table below as these lengths will form a non-resonant antenna for the amateur bands indicated. The antenna length should NOT be ¼ or ½ wavelength on any frequency that you transmit as the impedance will be very high (or low) and will not transfer through the matching unit at a favorable impedance to your antenna tuner. The theory of the antenna length is to make the antenna non-resonant on any amateur band so that the impedance at the antenna side of the matching unit is in the range of 200-600 ohms and when divided by 9 will be in the range of your transceiver antenna tuner.

Any length of 50/75 ohm feed line is ok (over 35 feet minimum) but longer feed lines over 50 feet may show reduced SWR on some bands due to soil conductivity, nearby objects, etc. Due to local ground conditions, antenna height and feed line length, SWR may vary and an antenna tuner may be required or some bands to bring SWR at end of feed line to acceptable levels. Use of one or more ¼ wavelength counterpoise(s) connected to the ground post of the matching unit may also improve antenna efficiency and reduce SWR on certain bands. The best counterpoise is ¼ wavelength at the lowest frequency you intend to use. The first counterpoise should be installed under the horizontal portion of the antenna for best results

Bands CoveredWire LengthMinimum Coax LengthCounterpoise Length
40, 30, 20, 1535'6”, 41’, 58’, 71’, 84’35’32'9”
40, 30, 20, 1735'6”, 41’, 58’, 71’35’32'9”
40, 30, 20, 17, 15, 12, 1041’, 71’50’32'9”
80, 40, 30, 20, 17, 15, 12, 1071’65’6'7”
160, 80, 40, 30, 20, 17, 15, 12, 10203’130’131'2”

9:1 Unun #

An unun is an impedance transformer which matches an unbalanced antenna to a feedline. Random wire antennas typically have an impedance of 450 Ω and need to be matched to a 50 Ω feedline. A 9:1 unun is often used for for this match.

References #