Random wire antenna
| Random wire antenna | |
|---|---|
| Type | End-fed, non-resonant wire |
| Feed | 9:1 unun at the feed point, or directly to an end-fed tuner |
| Matching | Antenna tuner always needed |
| Bands | Several HF bands from one wire |
| Installation | |
| Lengths to avoid | Close to any multiple of a half-wave on a band in use |
| Counterpoise | Required: radials, a counterpoise wire or a good ground system |
| Popular for | Portable, temporary and low-profile stations |
A random wire antenna is a length of wire fed at one end and deliberately not cut to resonance on any band. Instead of matching the wire by its length, the station relies on a matching transformer and an antenna tuner to load it on whatever band is wanted. One wire, a support at the far end and a tuner can put a signal out on most of the HF bands, which makes the random wire a favorite for portable operating, temporary setups and homes where a full-size antenna isn't possible.
How it works
The impedance at the end of a wire depends on how its length compares with the wavelength. On one band it may be a few hundred ohms, on another a few thousand, and it is almost never 50 ohms. The usual approach is to put a 9:1 unun (an unbalanced-to-unbalanced transformer, described under Balun) at the feed point. It divides the impedance by nine, bringing the widely varying values into a range that a tuner can handle. The 9:1 unun doesn't create a match by itself; the tuner, at the radio or at the feed point, finishes the job.
An alternative is an automatic tuner designed for end-fed wires, mounted outdoors at the base of the wire with no unun at all. Either way, coax carries the signal from the feed point to the radio.
A random wire is different from an end-fed half-wave antenna, which is cut to be a half-wave long on its lowest band and uses a 49:1 or 64:1 transformer. The random wire takes the opposite approach and avoids half-wave lengths on purpose. The term "long wire" is often used loosely for any end-fed wire, though strictly it means one at least a wavelength long on the band in use.
Choosing a length
When a wire is close to a half-wave, or a multiple of a half-wave, the impedance at its end becomes very high, and even a 9:1 unun and a good tuner may fail to match it. The usual rule is to choose a length that is not close to any half-wave multiple on the bands to be used. A half-wave in feet is roughly 468 divided by the frequency in megahertz. Lists of suitable lengths have been published, and commonly suggested values include about 29, 35.5, 41, 58 and 71 feet.
Longer is generally better. A wire much shorter than a quarter-wave on the lowest band, about 65 feet on 80 meters, is inefficient there, though a tuner may still load it. Get as much of the wire as possible high and in the clear.
Counterpoise and ground
An end-fed wire is only half of an antenna. The other half is the counterpoise, the connection on the ground side of the unun, and without one the antenna uses whatever it can find, usually the outside of the coax shield. The usual answer is one or more wires attached to the ground terminal of the unun, laid on the ground or run along a fence, and several wires of different lengths generally work better than one. A ground rod alone makes a poor counterpoise at HF, though one is still needed for lightning safety, as described under Grounding and lightning protection.
RF in the shack
Random wires have a reputation for putting RF where it doesn't belong. If the counterpoise is poor, current flows on the coax and into the shack, causing tingling metal surfaces, distorted transmit audio, and computers or USB devices that misbehave during transmission. Running the wire directly into the shack to the tuner makes the station's own wiring part of the antenna and is the worst case.
The remedies are to feed the wire outdoors, give it a proper counterpoise and add a common-mode choke where the coax enters the building. The end of the wire can also carry high RF voltage, so keep it out of reach of people and pets, and do an RF exposure evaluation for any wire that runs near living spaces.