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Author Topic: 450 Ohm Ladder line Vs Distance from rain gutter.  (Read 998 times)
KC2PHJ
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Posts: 21




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« on: April 29, 2007, 04:32:10 PM »

How far from rain gutters should you run the ladder line? Would you have problems if you run it close?

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KC5CQD
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Posts: 25




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« Reply #1 on: April 29, 2007, 05:00:18 PM »

I think the rule of thumb is two to three times the width of the ladder line.  I'm using homebrewed open-wire-feeder that is three and a half inches wide and so I made sure that I had ten and a half inches of clearance when passing over my rain gutters.  I actually have about two feet of clearance.  Hope this helps.
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K0ZN
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Posts: 1531




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« Reply #2 on: April 29, 2007, 07:49:43 PM »


Hi.

Over time, here on eham, there has been a lot of opinion offered on this, but I doubt there has been any SERIOUS study. I think it mostly falls into the common sense category. Remember that a parallel line like 450 ladderline does have a field around it and there is NO shielding...the line works due to cancellation by two waves 180 deg. out of phase.

The "rule of thumb" seem to be a mininum of several times the line spacing. Personally, I would want a minimum of 6" and try for 12", but I have no scientific data to support that....and I doubt neither does anyone else. There may be some theoretical equations that could define this...but I don't know them.

The more serious problem is when the line runs parallel to a large metalic object like a gutter. If you cross it perpendicular, and it is just a small place, I would not worry about two inches....it is the parallel run that is a problem. There is obviously some capacitive coupling between the line and the gutter...and the longer the run the higher the capacitance so in an overly simplistic sense, it is like connecting small capacitors between the two conductors...and this is obviously not a good idea.

I can tell you this as fact: commercial and military HF installations keep parallel lines a long way from parallel "foreign" conductors; typically many FEET.

If you want, I guess you could install it temporarily about 12" or 16" away and note the SWR (presumeably, 1:1 when the tuner achieves a good match)....then start moving the line close to the gutter. At the point/distance the SWR starts to increase, you clearly have some interaction/coupling with the gutter and that point would be "too close". I would probably run this experiment on both 40 and 15 M just to see what effect frequency has on it.

Ultimately, it is pretty easy to fabricate some long stand-off insulators out of PVC pipe or some other non conductor to keep it spaced away from the gutter.

It will be interesting to see what other opinions you get.

73,  K0ZN
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KC2PHJ
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Posts: 21




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« Reply #3 on: April 29, 2007, 08:09:46 PM »

Thanks for your response. My problem is that I have extra ladder line. I had to have to run it parallel next to rain gutter.I brought an Buxxcomm #406nt dipole and it has 60 ft. of 450 ohm ladder line. Could you cut the extra ladder line so that it would be shorter say 40 ft long?
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W5DXP
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« Reply #4 on: April 30, 2007, 06:27:13 AM »

They are probably using a fixed optimum tuned feeder length so probably not a good idea to change it. Keep the ladder-line at least six inches away from the gutter and put a twist in it every couple of feet. I use electric fence insulators for that purpose.
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73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K0ZN
Member

Posts: 1531




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« Reply #5 on: April 30, 2007, 10:11:18 PM »


You may or may not have a problem if you shorten the line. Kind of depends on your tuner's range, etc. One drawback to tuned lines is that some lengths are hard to match. Now that we have the WARC bands it is even a little harder. Usually a change in line length by 1/8 to 1/4 wave on the problem band will fix the problem.

Personally, I would try cutting it to length first and see how it loads... if that doesn't work, then I would splice in the line I cut off. This doesn't mean this is better or right...just what I would try. Experimentation is one of the fun and fundamental parts of Amateur Radio.

Whatever you do, DO NOT coil or pile up the extra ladderline in some kind of "rats nest". You cannot treat ladderline casually like coax. It must be kept reasonably neat, and not bent back tightly upon itself, etc. Spread the "extra" in a spaced out, neat manner.

73.
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