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.
w8tam says
When you’re ready to QRO, the window line I picked up at Dayton is 14awg. :D It’s big stuff compared to the 16/18awg I currently have installed!
Not sure exactly how much of it I need for my vacation antenna, but once I know, the leftovers are up for grabs. :D
-T
Walter Underwood K6WRU says
2.25 dB isn’t bad, but it isn’t great, either. 6 dB is terrible. With a 100 W amp, 75 W are spent heating the transmission line!
That is substantially more loss than RG-58, for example. It is even loss more than RG-174.
http://www.w4rp.com/ref/coax.html
So window line really does get lossy when it gets wet. When dry, it is better than any coax that I’d like to pay for, basically the same as LMR-1200. But coax works the same wet and dry. Window line goes from great to bad.
Dan KB6NU says
Well, as noted, I’m using the ladder line at frequencies below 50 MHz, so the losses are closer to 2 dB than 6 dB. Also, the amount of time that the feedline is actually wet isn’t all that long, so I’m willing to make that compromise. Having said all that, I think I will make my own open-wire feedline for my next doublet antenna project.
Walter Underwood K6WRU says
Pretty much every commercial broadcast install uses coax. Heck, I helped pull Heliax up 14 story shaft for the new KTRU antenna back in the 1970’s.
Coaxial cable was a really brilliant invention, running RF in a contained transmission line that is not affected by nearby objects. That’s almost magic, but Oliver Heaviside figured it out in 1880 and Bell Labs perfected it in 1929.
David Ryeburn says
I suspect your Cobra antenna itself changes quite a bit when it gets wet. Moving from ladder line to true open wire line for the feeder will help when it gets wet, but that will do nothing for the behavior of the Cobra antenna, just the behavior of the feedline. Perhaps making the Cobra antenna with high quality spacers and wire, rather than three conductor cable, might help.
AWG 14 wire in the feedline is overkill. Loss even in AWG 18 is pretty low. Because of skin effect the effective cross-section area of wire at RF is nearly proportional to radius, not to square of radius. So AWG 14 isn’t in effect as much bigger as you would think it would be, if you are thinking in terms of behavior in say house wiring at 60 Hz.
Dan KB6NU says
I hadn’t thought about that, but you’re probably right. My purely anecdotal observation is that the antenna’s performance doesn’t drop way off when it gets wet. I do wonder what effect conductor spacing has on antenna tuning and antenna performance. I’ve been putting off learning how to use antenna-modeling software, but perhaps this would be a good reason to do so.
Mike Antio says
I’ll first admit I know little about this topic. Have been a ham since ’77, and other than one winter with a G5RV, have always had coax for a feedline. The question I’m pondering is “So are you needing to re-tune it when it dries?”
Dan KB6NU says
Yes, you do. When the feedline dries out, you have to reset the tuner to the original settings.
Thom w8tam says
http://www.karinya.net/g3txq/wet_ll/
Pretty interesting testing with more window line, wetting, wetting with an agent, and other tests…