Random Wire Antenna
How it works:
This antenna is a receiving antenna. It picks up the electromagnetic radiation from a transmitter and converts it into a very small electrical voltage..
The antenna voltage is produced with respect to the earth. The efficiency of the antenna is improved by a good earth ground system.
All antennas have an impedance, which is the amount of "resistance" that the antenna appears to have. A random wire is about 600 ohms impedance. The old TV antennas that use to populate rooftops were about 300 ohms impedance. A car antenna is about 50 ohms impedance. Thus, a random wire is a "high impedance" or "Hi-Z" antenna.
It is called a random wire antenna because it is just that: any length of wire will work. Engineers have worked out that when the length of the wire is a 1/4 wave or some odd multiple of that, the antenna works best. This is the resonant length. To compute the resonant length for a 1/4 antenna use this formula:
length in feet = 234/frequency in megahertz
Note that the lead-in wire will be part of this length. For example, at 1 MHz, the 1/4 wave length would be 234 feet. Therefore, if the lead-in wire is 30 feet, then the main antenna would need to be 204 feet.
For the first odd multiple we have 3 x 234 = 702 feet which would also be resonant at 1 Mhz. The main antenna length would be 672 feet in that case.
Random wire antennas longer than one wavelength are often called "longwire antennas" and exhibit gain. That is, they provide an amplification of the signal.
Fortunately, the nice thing about using random wires as a receiving antenna, any length greater than 50 feet will do fine for the purposes of the projects on this web page.
In another project, we will use this antenna as a transmitting antenna. On that page, an antenna tuner will be described that you can build which makes the antenna behave like it is resonant.
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