Random wire length calculator
Why the length is not random at all
The name misleads: the length matters, precisely because it must not be resonant. If the wire turns out to be a multiple of a half wave on some band, the feedpoint impedance climbs to several kilohms, and neither a 9:1 unun nor a tuner will save you. The calculator searches through lengths and shows the ones that sit furthest from every resonance at once.
| Operating bands | |
| Minimum length (m) | |
| Maximum length (m) | |
| Search step (m) | |
| Check your own length (m) | |
📐 How the search works
For every length L and every band you select:
λ = 299.792458 / f
n = L / (λ/2)
distance from resonance d = | n − round(n) |
score = the smallest d across all bands
The higher the score the better. The maximum is 0.25: you cannot get further from a multiple of λ/2 than that.
How to read the score:
| Score | Verdict |
|---|---|
| ≥ 0.20 | good |
| 0.12…0.20 | acceptable |
| 0.06…0.12 | risky |
| < 0.06 | bad, the impedance will be very high |
Where this calculator stops:
- The calculation is geometric: it ignores height, ground and nearby objects, all of which move the real resonance
- Band centre frequencies are approximate; on the wide 80 and 10 m bands resonance wanders across the segment
- The score says nothing about the antenna’s efficiency — only whether a tuner will be able to match it
- A counterpoise or radials are needed regardless: without them the tuner’s “ground” is the cable
🎯 In practice
- A portable antenna for all of HF. A 9:1 unun, a tuner and a well-chosen length of wire.
- Checking a wire you have already put up. Enter your length and see which band it comes closest to trouble on.
- Your own set of bands. If you do not need 160 m, switch it off and the list of best lengths changes.
Practical advice:
- Take the longest acceptable length: the low bands like a long wire.
- The counterpoise should be at least a quarter of the wire length, or the cable becomes part of the antenna.
- A choke on the cable right after the unun is not optional — it is needed here even more than on balanced antennas.
- The classic “good” lengths from the handbooks (13, 17.5, 22, 29, 35.5 m) will appear at the top of the list by themselves if you select the 80–10 m bands.
© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.
How to cite this calculator
UR3PKI. «Random wire length calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/antennas/random_wire (licence CC BY-NC-SA 4.0).The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.