Ladder line: What lengths to avoid?
I have been having difficulty in tuning my 10m long ladder line to my 40m nearly folded dipole antenna. Searched and found a good reason on KV5R.COM. Lengths to avoid in feet have been mentioned as 32, 65, 96, 130, and 260 – and multiples of any of those. Ten meter would be 32.8 feet, very close to the 32 feet mentioned in the list. Some good lengths mentioned are 40, 80, 110 feet. The principle is to avoid resonant lengths on ham bands which will pick up RF from the antenna and transfer it to the shack. May be I can improve my performance by increasing the length by about 8 feet. But that will add a joint to the ladder line. Currently my ladder line and antenna are formed by a single wire of 60m length with no joints. Reason for keeping the ladder line perpendicular to the antenna at feed point as far as possible is also to prevent picking up RF from the antenna.
Yet my ladder line – antenna combination works extremely well on 20m. Recently I worked VU2YK in New Delhi who gave me a report of 5, 9+15 dB from around 2000 km. I have never been able to get such reports before on 20m at that distance. He was coming here 5, 9+20 dB with his cubical quad and linear. I was on barefoot with 100 W from my FT-710 radio. I could have a similar QSO with VU2GNL at 283 km, with similar reports at both sides. He was on barefoot and inverted V dipole. That would have been a short skip propagation as discussed earlier.
Updates
Read that ladder line length should be an odd eighth of a wavelength. Even eighths should be avoided. In that case my 10m ladder line will be an even eighth of 40m. That is 5m x 2. By the same rule, it should not work well for 20m for which it is 2.5m x 4! Still it works well for 20m. So there are other factors involved as well, in my setup.
Online calculators for calculating impedance of ladder line:
https://wpcalc.com/en/engineering/parallel-wire-impedance/
https://cecas.clemson.edu/cvel/emc/calculators/TL_Calculator/index.html
Tried the first calculator which is simpler. With 2mm wire diameter and 49 mm spacing between wire, I got 466.494 ohms as the impedance for my homebrew ladder line. That is pretty close to 450 ohms and the matching transformer should be 9:1 rather than the 4:1 which I have used!