Random Wire Antenna: Length, 9:1 Unun and Counterpoise
A random wire antenna is the oldest trick in the portable operator’s bag: throw a length of wire over a branch, connect one end to a matching device, add a second wire on the ground, and you are on the air on several bands. It costs almost nothing, packs into a pocket and works surprisingly often. It also hides a few traps that cause high standing wave ratios, RF in the shack and endless disappointment. This guide explains what a random wire really is, how to choose the length, why a 9:1 unun or a tuner is needed, what the counterpoise does and how to keep the result safe and quiet.
What a Random Wire Is
Following the standard description in the random wire antenna reference, it is a long wire suspended above the ground, whose length does not bear a particular relation to the wavelength of the radio waves used. The length is chosen for a convenient fit between the available supports, or according to the wire at hand, rather than to be resonant on any particular frequency. The wire may run straight or zigzag between trees or walls simply to get as much wire into the air as possible.
Random wires are fed at one end against a counterpoise, which can be the earth, a wire laid on the ground or the metal of a vehicle or equipment case. They are used as transmitting antennas for small outdoor, temporary or emergency stations and as receiving antennas on long wave, medium wave and shortwave. The same description points out that the effectiveness of a random wire varies from one installation to another, and that the pattern can differ greatly between directions and between frequencies.
A word about names. People often call a random wire a long wire, but a true long-wire antenna is at least a wavelength long at the working frequency, which most random wires are not on the lower bands. The distinction matters mainly because it tells you what to expect: a random wire is a convenient compromise, not a high-gain antenna.
Why It Needs a Matching Device
A resonant half-wave dipole presents about 70 ohms at its feed point, and an end-fed half-wave presents several thousand ohms. A random wire can present almost anything, from a few ohms to thousands of ohms with large reactive parts, depending on length, frequency, height and the counterpoise. Direct connection to a transmitter that expects 50 ohms therefore results in high reflected power on most bands. You have two options.
- A 9:1 unun at the feed point. An unbalanced-to-unbalanced transformer with a 9:1 impedance ratio converts an impedance around 450 ohms to 50 ohms. A random wire in a typical range of lengths often presents an impedance near that value on several bands, so a 9:1 transformer can give a tolerable standing wave ratio without any tuning. It is simple and weatherproof, and it is a popular choice for portable kits.
- An antenna tuner. A tuner near the transmitter, or better, at the antenna feed point, can match a much wider range of impedances than a fixed transformer. Our article on choosing an external antenna tuner explains when a tuner is the right answer.
The two are often combined: a 9:1 unun at the feed point to bring the impedance into a range the tuner can handle, and a tuner in the shack to remove the last bit of mismatch. An unun is not a magic device. It does not create a resonant antenna, and on some bands the wire will still look like a poor match. Our guide to baluns and ununs describes the transformer types and their limits.
Choosing the Length
Although the wire is random in principle, some lengths work much better than others. A wire that is close to an even multiple of a quarter wavelength on a band, which means a multiple of a half wavelength, has a very high feed point impedance on that band. The reference notes that the feed point can show thousands of ohms of both reactance and resistance at that point, which is beyond the range of most matching devices. The efficiency of the antenna is not necessarily bad, but you cannot get power into it.
The reference also records a practical compromise. An amateur operator with the call sign W0IPL worked out a table of lengths that were usable and unusable across the HF amateur bands, and chose 74 feet, which is about 22.6 metres, as a good work-any-band length. Another commonly favoured length is 100 feet, or about 30.5 metres. These are starting points and not guarantees, because the height, the wire insulation and the counterpoise all change the impedance. Test them with an antenna analyser and cut or add a metre if one of your favourite bands falls on a bad value.
| Length | Source or reason | What to expect |
|---|---|---|
| About 22.6 m (74 ft) | Compromise length proposed for the HF amateur bands | Usable on most bands with a 9:1 unun or tuner |
| About 30.5 m (100 ft) | Another favoured length for general use | More wire on the lower bands; check the upper bands |
| Near a multiple of half a wavelength on a band | Feed point impedance becomes very high | Avoid for any band you plan to use; trim or extend |
| Under about 0.6 wavelength | Single main lobe in the pattern | Predictable pattern; long wires split into several lobes |
The last row follows from the radiation pattern of a thin straight wire: below about 0.6 wavelength it has a single main lobe, and as the wire gets longer the pattern breaks into several lobes at angles to the wire. On the higher bands, a long random wire therefore has sharp peaks and nulls, and you may find a good direction on 15 metres where the same wire is weak in another direction.
The Counterpoise Is Half the Antenna
A random wire is an unbalanced antenna. Current that flows up the wire must return through something, and that something is the counterpoise. If you do not provide a good one, the return current finds its own path: along the outside of the coax shield, through the transmitter case, along the mains lead and into the shack. The result is RF on the microphone, a tingle on the metal case and a distorted pattern. The counterpoise concept is the standard answer: a wire or set of wires at the base of the antenna that provides a return path with a known geometry.
- Length. A common rule is to start with a counterpoise wire about 5 metres long for the higher bands and add additional wires or longer wires for the lower bands. Several wires of different lengths often work better than a single wire.
- Layout. Lay them on the ground, or just above it, running away from the feed point in different directions. Insulated wire is better than bare wire when people will walk across it.
- Tuning by measurement. The best counterpoise length changes with the band. Measure the standing wave ratio on each band you use and adjust the length until the match is acceptable. A change of a metre can make a large difference.
- Do not rely on the mains earth. The protective earth of the mains supply is not a radio ground.
The counterpoise is where portable use often fails. In a vehicle or a tent, the case of the radio and the coax shield can become the whole counterpoise, and you will feel it as RF burns or hear it as noise. A little time spent laying out wires is worth far more than a better transformer.
Taming RF in the Shack
Even a good random wire installation sends some RF back along the feedline. A common-mode choke on the coax, placed close to the matching unit and another at the shack end, suppresses that current. Our article on common mode chokes explains how to build or choose one. Use low power until you have checked for RF on the equipment, and keep the feedline away from the wire and the counterpoise where possible.
Also think about safety. An end-fed wire has high voltage at the far end and at the feed point when the match is poor. Keep both out of reach of people and pets, and do not touch the wire or the unun during transmission. Install the wire well away from power lines and take it down in a storm.
Building and Deploying a Portable Kit
A practical kit has five parts: a 9:1 unun in a small weatherproof case, a spool of lightweight insulated wire, a handful of shorter counterpoise wires, a length of coax with a choke and a throwing weight with a line to get the wire over a branch. Pack a small analyser if you have one, because it saves time on the first deployment.
- Pick the supports. A tall tree branch or a fibreglass pole gives the most height, and sloping the wire down is perfectly acceptable.
- Raise the wire, with the feed point near the ground or at the operating position.
- Lay out the counterpoise wires, in different directions and lengths.
- Connect the unun, add the choke and the coax, and connect the radio.
- Measure the standing wave ratio on the bands you plan to use. Adjust the counterpoise and, if necessary, the wire length.
- Tune with the radio’s internal tuner or an external tuner, and keep power low until the reflected power looks sensible.
If you want a more efficient antenna for a single band, a resonant end-fed half-wave is a better choice. Our guide to the end-fed half-wave antenna and its 49:1 unun compares the two approaches. The random wire is more flexible, while the half-wave is more predictable.
A Random Wire for Listening
Random wires are widely used as receiving antennas on long wave, medium wave and shortwave, and that use is the easiest place to start. A few metres of wire strung outdoors, away from the house, will usually pick up far more than the short whip supplied with a portable receiver. For listening, the match is much less critical than for transmitting, because a mismatch costs only a little signal, and the noise from the surroundings is often a bigger limit than the sensitivity of the receiver.
If you listen on the lower bands, keep the wire as far as you can from buildings, power lines and electronics, and bring the feedline in through a choke so that house noise does not travel along the outside of the cable. A wire that is much quieter than the main transmitting antenna can turn a noisy band into a usable one, and it costs next to nothing to try. Many operators keep a spare random wire in the car for exactly this purpose.
The Bottom Line
A random wire is a flexible, cheap and surprisingly capable antenna, as long as you treat it as a system of wire, matching device and counterpoise. Choose a length that is not a multiple of half a wavelength on the bands you care about, use a 9:1 unun or a tuner, build a counterpoise that works on every band, add a choke and always measure. Do that and a few metres of wire can give you contacts from the roadside, a hotel room or the back garden.
Frequently Asked Questions
What is a random wire antenna?
It is a long wire whose length is not chosen to be resonant, fed at one end against a counterpoise or ground. It is used for multiband operation with a matching unit.
What length should a random wire be?
Commonly used compromise lengths are about 22.6 metres (74 feet) and about 30.5 metres (100 feet). Avoid lengths close to a multiple of half a wavelength on the bands you plan to use, and test with an analyser.
Do I need a 9:1 unun for a random wire?
Not strictly, but a 9:1 unun at the feed point often brings the impedance into a range the radio or tuner can handle. Many operators use both a 9:1 unun and a tuner.
Why does a random wire need a counterpoise?
The antenna is unbalanced, so the current needs a return path. Without a counterpoise the current flows on the coax shield and equipment, which causes RF in the shack and a poor pattern.
Is a random wire the same as a long wire antenna?
No. A long wire is at least about a wavelength long at the operating frequency, while a random wire can be any convenient length, including much shorter than that on the lower bands.
Can I transmit with a random wire safely?
Yes, if you keep the wire and the feed point out of reach, stay clear of power lines, avoid thunderstorms and use a choke to prevent RF on the equipment. Keep power low until you have checked for RF burns or interference.