Random Wire Antenna with 9:1 Unun
A random wire antenna is about as simple as HF gets: one non-resonant wire, run out at any convenient length and angle, fed through a 9:1 unun and matched the rest of the way by an antenna tuner. It won't outperform a resonant dipole on any single band, but it'll get you transmitting on nearly every HF band from one wire and one feedpoint, which is exactly why it's a mainstay for hams working with a single tree, a tight yard, or a temporary field setup. This guide covers picking a wire length that plays nicely with a tuner, building or wiring in the 9:1 unun, and getting a clean match across bands.
Why "random" doesn't mean "any length is equally good"
A random wire is non-resonant on purpose — instead of cutting to 468/f for one band, you pick a length and let a tuner handle matching on whatever band you operate. But some lengths land on a very high impedance point on a particular band, which can push the match outside what your tuner can handle. Lengths like 29, 41, 58, 71, and 84 feet are commonly recommended because they avoid landing on a problematic half-wave multiple on most of the popular HF bands at once.
a wire near a multiple of this length on a given band is the length to avoid on that band
How a 9:1 unun does its job
The unun is an autotransformer wound on a toroid core, and the impedance transformation ratio is the square of the winding turns ratio. A 9:1 impedance step means a 3:1 turns ratio — simple to wind, and it brings a random wire's typically high feedpoint impedance down into a range your shack tuner can comfortably finish matching to 50 ohms.
9:1 impedance ratio = 3:1 turns ratio (3² = 9)
Why you still need a tuner
The unun transforms impedance magnitude, but it doesn't cancel the reactance a non-resonant wire presents on most bands. That's the tuner's job — matching a random wire without one will usually leave you with an SWR your radio won't tolerate at full power, even with the unun in place.
Why a counterpoise still matters here
Even though this isn't a two-arm dipole, the unun needs a return path to complete its transformer action, and a counterpoise (or a few radials, or a solid ground connection) gives it one. Skipping the counterpoise is a common cause of poor matching and RF feedback on this design, not a step you can shortcut.
| Section | Length (feet) | Length (m) | Notes |
|---|---|---|---|
| Radiator, option A | 29 ft | 8.8 m | Compact option, good for small yards, works 40m-6m |
| Radiator, option B | 41 ft | 12.5 m | Common all-around length, works 80m-6m |
| Radiator, option C | 58 ft | 17.7 m | Better low-band performance, needs more space |
| Radiator, option D | 71 ft | 21.6 m | Strong 80m/40m performer |
| Radiator, option E | 84 ft | 25.6 m | Longest common recommendation, best low-band capture |
| Counterpoise | 25 ft | 7.6 m | Run away from the radiator wire, elevated or ground-laid |
Random Wire Antenna with 9:1 Unun Dimension Calculator
Materials for Random Wire Antenna with 9:1 Unun
Building the Random Wire Antenna with 9:1 Unun
This is a light, forgiving build — most of the work is picking a good length and getting the unun and counterpoise right, not precision construction.
Choose your wire length
Pick a length from the dimensions table based on the space you have and the bands you care about most — longer generally favors the low bands, shorter is easier to fit and still works fine from 40m up.
Build or wire in the 9:1 unun
Wind the toroid with a 3:1 turns ratio (for example, 3 trifilar turns to 9 turns, or scaled proportionally) and mount it in a weatherproof enclosure with connections for coax in, wire out, and a ground/counterpoise post.
Prepare the wire ends
Attach an end insulator at the far end of the radiator wire and a matching termination at the feedpoint end for connection to the unun's wire-out terminal.
Prepare the counterpoise
Cut the counterpoise wire to length and terminate one end for the unun's ground post.
Deploy the wire
Run the radiator wire up and out at whatever angle your available supports allow — sloping into a single tree, an inverted-L over a mast and fence line, or a random slant across the yard all work. Keep it as straight and clear of metal as practical.
Connect the counterpoise and coax at the unun
Connect the counterpoise to the ground post and route it away from the radiator wire, laid on the ground or elevated a short distance. Connect your coax to the unun's input.
Route the feedline with a common-mode choke
Run the coax to your shack entry point, adding a common-mode choke where the feedline enters the building.
Initial tune-up
With the tuner in line, run a tune cycle on each band you plan to use and confirm you can reach an acceptable SWR. If one particular band won't tune well, that's the length landing on a difficult impedance point for that band specifically.
Log your tuner settings
If your tuner supports memories, save a setting per band for quick recall. If not, note the approximate settings somewhere handy near the radio.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Tuner can't find a match on one specific band | Wire length lands on a difficult impedance point for that band | Compare against the recommended lengths in the dimensions table | Try a slightly different length, or add/adjust the counterpoise length |
| Hot mic or RF feedback while transmitting | Missing or insufficient common-mode choking | Check for a choke at the shack entry point | Add a choke, or add turns to an existing one |
| Noisy receive / high noise floor | Poor counterpoise, or wire routed near noise sources | Compare noise with the counterpoise connected vs. disconnected, and try a different wire run | Improve the counterpoise or ground, and route the wire away from house wiring and electronics |
| Tuner won't find a match on any band | Continuity problem in the wire, unun, or coax | Check continuity from the coax connector through the unun to the wire and counterpoise | Re-solder or replace the faulty connection |
| Weaker signal reports than expected | Wire deployment shape or nearby metal absorbing energy | Check for sharp coils of excess wire, or runs close to gutters or siding | Straighten the run and keep it clear of metal where possible |
| Corroded or failed connection at the unun after time outdoors | Weatherproofing failure | Inspect the enclosure seals and terminal connections | Reseal or rebuild the affected connection |
Does the length really not matter?
It matters less than a resonant antenna, but not zero — some lengths land on high-impedance points that are hard for a tuner to match on a given band. The lengths in the dimensions table are chosen to avoid the worst of that across most popular bands at once.
Do I need a counterpoise?
Yes. The unun needs a return path to do its job, and skipping the counterpoise is one of the most common causes of poor matching and RF feedback on this design.
Can I use this without a tuner?
Not reliably. The 9:1 unun brings the impedance into a friendlier range, but it doesn't cancel reactance the way a resonant antenna's own length does — you'll need a tuner to finish the match on most bands.
Why 9:1 and not 4:1 or 49:1?
A 9:1 ratio is a middle-ground transformation well suited to the moderate, variable impedances a random-length wire presents. A 49:1 unun is built for the very high impedance at the end of a resonant end-fed half-wave instead, which is a different antenna design with a different matching problem.
How high or what shape should I deploy it?
Higher and straighter generally performs better, but this design is forgiving — a sloper into one tree, an inverted-L, or a bent run around a yard will all get you on the air.
Will this work on 160m or 6m?
It can, within your tuner's matching range, but don't expect strong performance at either extreme — 160m asks a lot of a wire this short relative to the wavelength, and 6m performance depends heavily on your specific length and deployment shape.