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A random wire antenna consists of a simple piece of wire of a nonspecific length. Many people consider it a type of monopole antenna; however, it is not in tune with any specific frequency, which is one of the defining aspects of a monopole antenna. Random wire antennas typically find use in locations or situations where it is not possible, practical, or cost effective to construct a proper antenna.
When an antenna encounters an electromagnetic field, it resonates, creating an electrical signal directly proportional in frequency to the field. This is how an antenna receives a radio signal. Similarly, when an antenna transmits, sending an electrical signal to it causes it to resonate, creating an electromagnetic field directly proportional in frequency to the electrical signal. The physical length of an antenna determines at what frequencies it will resonate.
The act of tuning an antenna is to adjust the physical length of an antenna so that it will resonate at a particular frequency, allowing it to send or receive a signal efficiently at that frequency. Random wire antennas are not any specific length; therefore, they are not in tune with any specific frequency. As a result, they tend to function poorly and convert only a small amount of the available energy when receiving or transmitting a signal.
All antennas exhibit a trait known as impedance. In monopole and random wire antennas, impedance is the amount of electrical resistance measured between the end of the antenna connected to the radio and ground. In a monopole antenna, the impedance is constant at a specific level as a function of its width and mass. As a result, a monopole antenna’s impedance matches ideally with the circuitry of a radio. This allows a radio to receive and transmit a signal as efficiently as possible through the antenna.
As a random wire antenna is by nature random, there is no predetermined amount of impedance to match the radio. Additionally, because the antenna is not of a length in tune with any particular frequency, the impedance of the antenna changes depending on the frequency of the signal. This mismatch and variation in impedance causes the antenna to work poorly with the radio, which further degrades its ability to receive a signal at any given frequency.
Despite its shortcomings, a random wire antenna can be a very useful device. For example, in an emergency, one can be built quickly and easily from literally any wire available, or if the personnel available lack the skill to build a properly tuned antenna. In any situation, a random wire antenna will always perform better than no antenna.