Ladderline/twinlead: How close is too close? - towertalk Forum at eHam.net

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eHam.net Forum : towertalk : Ladderline/twinlead: How close is too close?

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Ladderline/twinlead: How close is too close?	Reply
by KD0JHT on November 4, 2009	Mail this to a friend!
I have JUST wide enough of a yard to erect a dipole for 80 meters. My plan is to affix two 20' 2x2's to fence posts on either side of the yard, and drop a TV twinlead feedline from the center(and possibly another support) into my 8x8' vegetable garden, which is fenced with chicken wire. From there I plan to use coax into the house, a run of about twenty feet. Will the proximity of the chicken wire pose problems? Thanks all! (KD0JHT)

RE: Ladderline/twinlead: How close is too close?	Reply
by AA4PB on November 5, 2009	Mail this to a friend!
Just keep the twin lead a foot or so away from the wire. Why are you feeding part way with ladder line for an 80M dipole? If you are going to use it on 80M then you could run coax all the way to the antenna. If you are planning on using a tuner for all band operation then the coax run to the house is going to give you loss on some bands due to the high SWR. You really should run ladder line all the way to the tuner for an all band doublet.

RE: Ladderline/twinlead: How close is too close?	Reply
by WB6BYU on November 5, 2009	Mail this to a friend!
If you feed it with twinlead all the way to a tuner in the shack, the exact length of the antenna isn't critical and you can shorten the ends 5 to 10' if necessary to make it fit (including room for ropes and pulleys at the ends of the wires.) If you want the antenna to be resonant on 80m and are short on space you bend the ends around or let them hang down to use some of the extra length. I generally let a few feet hang down from the end anyway so I can prune it without having to retie the ropes. Regarding the feedline spacing: a foot certainly should be safe, and (since the twinlead will be a tuned line anyway) if you have to get closer than that just try to cross it at a right angle to minimize the length of twinlead that is close to the chicken wire. You can even pass the twinlead through a hole in the chicken wire if necessary without much problem, though it would be best to use some sort of insulator to keep it centered, and preferably snip out a couple wire segments to make the hole larger.

RE: Ladderline/twinlead: How close is too close?	Reply
by KD0JHT on November 5, 2009	Mail this to a friend!
Oh. Wow, thanks for the answers. I thought twinlead was pretty finicky about proximity to structures and metal. I didn't think I'd have much luck running it into the house. Okay thanks for the answers.

RE: Ladderline/twinlead: How close is too close?	Reply
by W8JI on November 5, 2009	Mail this to a friend!
It also depends on how much power you run. I set a tree on fire with my ladder line and 1500 watts.

RE: Ladderline/twinlead: How close is too close?	Reply
by N5YPJ on November 19, 2009	Mail this to a friend!
Any pictures of that fire Tom?

RE: Ladderline/twinlead: How close is too close?	Reply
by W8JI on November 20, 2009	Mail this to a friend!
[quote author=N5YPJ link=topic=63516.msg413273#msg413273 date=1258686172] Any pictures of that fire Tom? [/quote] No, and the evidence is destroyed. I eventually cut the tree down. It is amazing how much voltage appears on a grossly mismatched feeder. If the SWR is only 10:1, like ladder line feeding a half wave dipole, the impedance might be 4000 ohms at some points. With 1500 watts that's 2450 volts RMS! It's only 600 V with 100 watts. Tom

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