Random wire antenna

Lore & Background

The random wire antenna is often inaccurately referred to as a long-wire antenna, but there is no accepted minimum size for a long wire, and the term 'random wire' avoids such confusion. Its only requirement is that the antenna length was not planned for resonance at a particular frequency. A combination of two random parallel wires of different length is actually the original zeppelin antenna as defined in the patent from 1909, since that patent does not specify any length-to-wavelength ratios. With a proper matching unit and a common-mode choke, a zeppelin antenna that is end-fed half-wave in the 17m band can be tuned to all shortwave frequencies.

Due to the great variability of the unplanned antenna structure, the random wire's effectiveness can vary erratically from one installation to another. A single random wire antenna can have wildly different reception or transmission strength in one direction compared to another azimuth direction about 70° to 140° different, and strengths and directions can be wildly different on only moderately different frequencies. The radiation pattern of a thin wire antenna is predictable using antenna modeling, but any unpredictability is caused by uncontrolled interaction with nearby matter such as soil or structures.

Reader's Guide

Random wire antennas are widely used as receiving antennas on the long wave, medium wave, and short wave bands, as well as for transmitting on these bands for small outdoor, temporary, or emergency transmitting stations, and in situations where more permanent antennas cannot be installed. When used for transmitting, a random wire antenna usually requires an antenna tuner, as it has an unpredictable impedance that varies with frequency. One side of the tuner output is connected directly to the antenna without a transmission line, the other to a good earth ground. A wire near an odd number of quarter-waves in length works best for transmitting; a wire near an even number of quarter-wavelengths long, while fine for receiving, on transmit will exceed the matching ability of most small tuners unless first fed through an unun with a large transform ratio. The ground may be chosen by experimentation, such as connecting to a cold water pipe or using counterpoise wires under the antenna. Even without a good earth, the antenna will still radiate, though poorly, by capacitively coupling to nearby conducting material.

Did You Know?

• A random wire antenna's length is chosen for convenience, not for resonance on any particular frequency.
• A wire near an even number of quarter-wavelengths long can have a feedpoint impedance of thousands of Ohms due to antiresonance.
• Operator W0IPL developed a table of usable odd multiple lengths and unusable even multiple lengths for HF amateur frequencies.