Random wire antenna

A random-length wire antenna is a type of radio frequency antenna typically chosen more for convenience than any particular design criteria. This antenna sometimes is called the zig-zag antenna, as it may be strung back and forth between trees just to get enough wire into the air. For example, an antenna for 3mhz might be 20m - 40m long.

Usually, it is a long (at least one quarter wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Folding (to fit in space available) will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) If used for transmitting, a random wire antenna usually will also require an antenna tuner, as it might have a random impedance that varies nonlinearly with frequency.

Typically this antenna is constructed from a number 12 or 14 AWG (1.6 to 2.0 mm diameter) wire of nearly any length. Such an antenna can be used for transmitting on practically any frequency with a properly tuned matching network.cite book
last = Straw
first = R. Dean
title = The ARRL Antenna Book, 20th Edition
publisher = The ARRL, Inc.
date = 2003
location = Newington, Connecticut, USA
pages = 944
isbn = 0-87259-904-3] Although random wire antennas can be made from nearly any length of wire, one-quarter wavelength works best, and one half wavelength will work poorly with most tuners. [ [http://kb6nu.com/the-almost-random-wire-antenna/ The "Almost Random" Wire Antenna] ] [ [http://www.w0ipl.net/ECom/NVIS/random-l.htm Random length wire] ]

The antenna is fed directly from output of the matching network or a tuned circuit, without a feed line. Since the antenna is located very close to the transmitter, RF feedback can be an issue. RF feedback can be minimized by selecting a wire length that causes the low feed-point impedance at a current loop to occur at the transmitter. Alternately, a remote tuner can be fed with feedline, and the tuner located on the antenna.

The ground for a random wire antenna may be chosen by experimentation. Grounds could be returned to the transmitter, a nearby cold water pipe or a wire that's approximately one-quarter wavelength long.

References