For an End Fed Random Wire (EFRW) it's normal to use a 9:1 balun, so you need to avoid resonance (where the impedance would drop and via the 9:1 would present a very low impedance to your system).
I've found a number of EFRW length calculators on line, usually for the USA band plans, but several of them conflict with each other.
So I've added my own one to the mix (with UK band plans). This C program should compile on most systems (builds on Windows/BSD/Linux).
It sweeps through the bands, harmonics (upto 10th), lengths, and range of velocity factors to eliminate "bad" lengths, then display the "good" lengths. It takes the longest band length (in meters) as an optional argument.
The range is the difference between the min and max lengths. When measuring long wires a bigger range means there is less chance of a measuring error (or wire stretch error) going beyond the min/max limits.
These are the results that have at least 25cm range, and are over 1/4 wavelength long.
| Longest band | Min len(m) | Max Len(m) | Range (m) | Mid len(m) | Min len(ft) | Max Len(ft) | Range (ft) | Mid len(ft) |
|---|---|---|---|---|---|---|---|---|
| 20 | 5.246 | 9.487 | 4.241 | 7.37 | 17'2.5" | 31'1.5" | 13'11.0" | 24'2.0" |
| 30 | 7.272 | 9.487 | 2.215 | 8.38 | 23'10.3" | 31'1.5" | 7'3.2" | 27'5.9" |
| 40 | 10.493 | 11.275 | 0.782 | 10.88 | 34'5.1" | 36'11.9" | 2'6.8" | 35'8.5" |
| 40 | 11.805 | 13.136 | 1.331 | 12.47 | 38'8.8" | 43'1.2" | 4'4.4" | 40'11.0" |
| 60 | 13.991 | 15.510 | 1.519 | 14.75 | 45'10.8" | 50'10.6" | 4'11.8" | 48'4.7" |
| 60 | 16.262 | 18.975 | 2.713 | 17.62 | 53'4.2" | 62'3.0" | 8'10.8" | 57'9.6" |
| 80 | 20.987 | 22.552 | 1.565 | 21.77 | 68'10.3" | 73'11.9" | 5'1.6" | 71'5.1" |
| 80 | 23.609 | 26.274 | 2.665 | 24.94 | 77'5.5" | 86'2.4" | 8'8.9" | 81'9.9" |
| 20 | 27.982 | 28.464 | 0.482 | 28.22 | 91'9.7" | 93'4.6" | 1'7.0" | 92'7.1" |
| 80 | 32.523 | 33.829 | 1.306 | 33.18 | 106'8.4" | 110'11.9" | 4'3.4" | 108'10.1" |
| 80 | 35.413 | 37.952 | 2.539 | 36.68 | 116'2.2" | 124'6.2" | 8'4.0" | 120'4.2" |
| 160 | 41.973 | 45.105 | 3.132 | 43.54 | 137'8.5" | 147'11.8" | 10'3.3" | 142'10.1" |
| 20 | 55.963 | 56.382 | 0.419 | 56.17 | 183'7.3" | 184'11.8" | 1'4.5" | 184'3.5" |
| 160 | 65.046 | 65.687 | 0.641 | 65.37 | 213'4.9" | 215'6.1" | 2'1.2" | 214'5.5" |
| 160 | 73.451 | 75.906 | 2.455 | 74.68 | 240'11.8" | 249'0.4" | 8'0.7" | 245'0.1" |
| 20 | 83.945 | 85.394 | 1.449 | 84.67 | 275'4.9" | 280'2.0" | 4'9.0" | 277'9.4" |
| 160 | 97.935 | 98.192 | 0.257 | 98.06 | 321'3.7" | 322'1.8" | 0'10.1" | 321'8.8" |