Last week, we had a couple of rainy days here at KB6NU, and what that means is that my homebrew Cobra antenna needed to be retuned once the ladder line got wet. It is kind of annoying to have to do this, but since this is about the only negative thing I have to say about this antenna, I think that I can put up with it.
I did get to thinking about this, though, and decided to do a little Googling as to why and how much the rain actually affects the characteristics of the 450 Ω ladder line feedline. One of the first references was to an an item in the Elecraft forum on Nabble. It, in turn, linked to an article by Wes Stewart, N7WS, “Balanced Transmission Lines in Current Amateur Practice Taking a closer look at “ladder line” and its application.”
This article had just the information that I was looking for. Not only does Wes describe what’s going on with today’s ladder line, but also gives us a little history of the use of open-wire feedline. N7WS purchased and tested four samples of ladder line that he purchased from The WireMan, as well as a sample of air- insulated line that he made from 16 AWG enameled wire spaced 0.75-in.
Because the lowest frequency he could test at was 45 MHz, he tested the samples at 50 MHz. Nevertheless, he had some very interesting results:
- The characteristic impedance of the dry samples actually varied from a low of 359 Ω to 405 Ω. The characteristic impedance of the open-wire feeder was 399 Ω.
- The loss in dB/100-ft. ranged from 0.33 dB to 0.41 dB. The loss of the open-wire feeder was 0.30 dB.
- When wet, the characteristic impedance of the ladder line samples decreased by a little more than 4%, and ranged from 343 Ω to 387 Ω. The characteristic impedance of the open-wire feeder was unchanged.
- The loss in dB/100-ft. ranged from 4.8 dB to 6.1 dB. The loss of the open-wire feeder was unchanged.
The first thing to note that the maximum loss is only 6 dB. That’s only 1 S-unit, and this loss only occurs when the feedline is wet. Not only that, because the measurements were made at 50 MHz, the loss at say 7 MHz will be lower. The equation that N7WS gives to approximate this loss at other frequencies is:
If you plug in 7 MHz into that equation, you’ll find the approximate loss to be about 2.25 dB, which is less than half an S-unit. So, if someone tells you that they don’t like ladder line because it gets lossy when it gets wet, as someone did to me a couple of weeks ago, you can pretty much tell him that he’s all wet!
Having said that, I may just try to make my own open-wire feeder at some point. It would eliminate having to re-tune the antenna when it gets wet, and it would be fun to do, I think.