End-fed half-wave or random wire?
Both antennas are a single wire fed at one end, which makes them popular for portable operating and small gardens where a centre-fed dipole won't fit. They work in different ways, though, and need different transformers.
An end-fed half-wave (EFHW) is cut to exactly half a wavelength on its lowest band. At the end of a half-wave wire the impedance is very high, typically 2,000 to 4,000 ohms, so a 49:1 or 64:1 transformer brings it down close to 50 ohms. Because the wire is also a whole number of half-waves on harmonics, one wire can match on several bands without a tuner. A 40m EFHW of about 20 m also works on 20m, 15m and 10m.
A random wire does the opposite. It's deliberately not a half-wave on any band you want to use, so its impedance stays moderate, usually a few hundred ohms. A 9:1 unun brings that down to something an antenna tuner can match on almost any band. The catch is that you need a tuner, and you must avoid lengths that land near a half-wave, where the impedance rises beyond what the 9:1 can handle.
Choosing an unun
| Ratio | Use it for | Needs a tuner? | Notes |
|---|---|---|---|
| 49:1 | EFHW | Usually not on the design bands | The most common EFHW transformer. Good for 2,450 ohm feed impedance. |
| 64:1 | EFHW | Usually not on the design bands | Suits higher feed impedance, around 3,200 ohms, such as wires very high in the clear. |
| 9:1 | Random wire | Yes | Pair with a non-resonant length from the tool above and a counterpoise. |
| 4:1 | Some random wires, loops | Yes | Less common for end-feds. Better suited to off-centre-fed dipoles and loops. |
Check the power rating carefully. Transformer ratings are usually for SSB, and FT8 and other data modes put far more continuous heat into the core. A common rule is to use around a third of the SSB rating for data modes.
Counterpoise and chokes
An end-fed wire needs something to push against. With an EFHW, the outside of the coax braid usually does the job, and a choke a short distance down the coax (around 0.05 of a wavelength on the lowest band) stops the rest of the cable becoming part of the antenna. With a random wire, add a separate counterpoise wire, often about 5 to 8 m long, laid on the ground or run along a fence. There's no magic counterpoise length: any wire that gives a good match on your bands is fine, and more than one is often better.
How the random wire tool chooses lengths
For every ticked band, it works out the wire lengths that would be a whole number of half-waves anywhere in that band, then adds a margin of 5% either side. Those are the red zones. Anything left over is shown in green and listed by how much room it has either side, so the top suggestion is the one least likely to give your tuner trouble. Real-world height, ground and nearby objects will move things a little, so start in the middle of a green zone.
Questions
How long is a 40m EFHW?
About 20 m (66 ft) for 7.1 MHz with a 0.95 shortening factor. It will also match on 20m, 15m and 10m. Commercial 40m EFHW kits typically use a wire of roughly 20 m, often with a small loading coil or capacitor to fine-tune the higher bands.
Can I use a 9:1 unun for an EFHW?
Not well. A half-wave wire's end impedance is around 2,500 ohms, and a 9:1 is designed for around 450 ohms, so the SWR will be high. Use a 49:1 or 64:1 instead.
Do I need a tuner with an EFHW?
Usually not on the bands where the wire is a whole number of half-waves. A tuner helps to cover the edges of wide bands like 80m and 10m, and it lets you use the WARC bands where the wire isn't resonant.
What length random wire is best?
It depends on which bands you want. With every band from 40m to 10m ticked, including the WARC bands, the clearest lengths up to 45 m are around 8.7 m, 12.3 m, 18.1 m and 21.5 m. Tick your own bands in the tool above to see what suits your station.