Part of our Building Your First Ham Radio Station guide.
“Random wire” is a bit of a misnomer: the length isn’t actually random if you want it to work well (see my full breakdown of magic lengths and lengths to avoid for the math behind that). But the idea is simple: one long wire, one counterpoise, and a 9:1 transformer to bring the wire’s high, swinging impedance down into a range your antenna tuner can handle. It’s cheap, it’s flexible, and it’s a great option when you don’t have room for a full dipole.
What you’ll need
- One toroid core: FT140-43 handles up to about 100W, FT240-43 if you want more headroom, or a small FT50/FT82-43 for QRP builds
- About 8-10 ft each of three separate strands of enameled magnet wire (same gauge, e.g. 18-22 AWG for smaller cores)
- A weatherproof enclosure
- An SO-239 or binding post for the coax side, plus binding posts for the wire and counterpoise/ground
- An antenna wire, see the length guidance below
- A counterpoise wire
- An antenna tuner (external or your radio’s built-in ATU), this is not optional with a random wire
Step 1: Wind the transformer
The common approach is a trifilar winding: three wires wound together simultaneously, 9 to 10 turns around the core. Twist the three wires loosely together first (this keeps the coupling tight and consistent), then wind them as a bundle through the core, keeping the turns evenly spaced.
Once wound, you’ll have three separate windings, each with two ends (six leads total). The connections that turn this into a 9:1 autotransformer:
- Connect the finish of winding 1 to the start of winding 2, and the finish of winding 2 to the start of winding 3. This chains all three windings in series to form the secondary (a 3:1 turns ratio across the full winding).
- The start of winding 1 and the junction between windings 1 and 2 become your coax connection (hot and ground).
- The far end of winding 3 becomes your high-impedance connection to the long wire; the coax ground side also connects to your counterpoise.
If you’re working from a kit, it’ll usually come with color-coded wires and a diagram for this exact connection. Follow it, since a wrong connection here just gives you a dead short or a wrong ratio, not a fire hazard, but it won’t work.
Step 2: Pick your wire length
This is the part people skip, and it’s the part that actually matters most. A 9:1 unun doesn’t magically make every wire length work on every band. It just brings a high impedance down to something your tuner has a fighting chance at. Certain lengths land on feedpoint impedances that are brutally hard to match on specific bands (multiples of a half wavelength are the usual troublemakers).
Lengths that are well documented to behave well across most HF bands:
- 29 ft: good compact option, works with most tuners
- 41 ft
- 58 ft
- 71 ft
- 84 ft: a classic long-wire length going back to 1930s QST articles, often paired with a 17 ft counterpoise
Pick whichever fits your space. Longer generally means better low-band performance, but any of these will get you on the air.
Step 3: Add a counterpoise
A random wire without a counterpoise is a random wire that doesn’t work well. Run at least one counterpoise wire, roughly a quarter wavelength of your lowest intended band (about 17-25 ft), connected to the transformer’s ground terminal. Lay it out away from the radiator, along a fence, under a deck, wherever it fits. More than one counterpoise, laid in different directions, helps even more.
Step 4: Deploy and match
Run the wire up and out at whatever angle your space allows, sloping, zigzagged, it doesn’t much matter for a random wire. Connect the transformer, then let your antenna tuner do the rest of the work band to band. Don’t expect a flat 1:1 SWR without the tuner in line; that’s the tradeoff for a single wire that covers everything from 80m to 10m.
A note on safety
Random wires can develop high RF voltage at the far end, especially on higher-impedance bands. Keep the far end well out of reach while transmitting, and don’t route it near metal gutters, railings, or anything someone might touch.