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Author Topic: Loop Antenna  (Read 1976 times)
KF7PCG
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« on: October 27, 2012, 12:04:43 AM »

Hi guys thanks in advance for the info. Ok at present I will be able to put up a form a of a loop antenna up it'll be 13' high and in most places 3 feet above the roof about 68 long by 16 wide. I haven't learned to run the eznec program or I'll learn a better one if someone can give me a name. Anyway back to the loop it will be feed with 450 ladderline. If this works I'll try to stretch it out to be more or less a square. So I have read both ways it will work it won't work and a few maybes tossed in. So basically in your opinion will it work and if so whats a guess on how well. Oh one last thing I'll have to put new shingles on in a few years I was planning on metal as the other type doesn't seem to hold in montana. Thanks for your help KF7PCG
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WX7G
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« Reply #1 on: October 27, 2012, 04:24:31 AM »

I ran EZNEC simulations of your antenna and it will work.

1. The VSWR on the ladder line is much better if the antenna is fed in the middle of a long side.
2. If you want it to be resonant on 40, 20, 15, and 10 meters (for lower VSWR on the line) the circumference should be 142 ft. It might not tune on 80 meters with this length because a half wavelength closed loop presents a very high impedance.
3. The original length will tune on 80 meters.
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W8JI
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« Reply #2 on: October 27, 2012, 05:16:04 AM »

There should be a lot of "maybes" with that antenna.

On lower bands, like 80 or 40 meters, a 68 ft long 16 ft wide loop is more like a folded dipole than a loop. This is because wire spacing is 1/8th wave or less. Even on 20 meters the width spacing is just 1/4 wave. This is why feeding in the the middle of a long side works so much better.

Like any 80 meter 1/2 size folded dipole, it will not work well.
It should be OK on 40 and higher, but probably not so good on 30.

I'd keep it away from a metal roof.
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AB1JX
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« Reply #3 on: October 28, 2012, 10:24:53 PM »

If you haven't learned to model antennas you should.  It's lot easier to model than build and it saves money and time in the long run.  I like MMANA-GAL and I'm just starting with 4nec2 but mostly I run Unix, so that leaves just NEC2.  EZNEC is supposedly easy but the free version is too limited for much more than a dipole.  Some ARRL stuff comes with a version of EZNEC, I don't know how limited that is.

One thing about perimeter loops like this is that they're mostly NVIS antennas which means they beam your signal straight up.  If you can put one side of the loop above the other it's at least bidirectional.  The signal comes out at right angles to the plane of the loop.

Modelling yourself lets you play around with ideas.  I did a perimeter loop in an apartment I had once and it was about useless at least for listening.  I've since modeled loops around the eaves of houses, triangular loops in gable ends.  I have one in the attic now that runs along the ridge with the lower side just above the attic floor, fed in the middle of the lower side with balanced line.  Picks up trash from TVs, computers, etc. in the house but it's not bad late at night.  I don't worry much about lightning with it, which is why I put it up.

Trying to put antennas on them is one disadvantage of metal roofing.  If you can get a single mast up you could do a triangular loop with the vertex up on the mast and the lower side a foot or so above the roof.  Or two masts, but the metal is really a problem otherwise.  What about fiberglass roofing?

Are there trees near enough to be of any use?

  Alan, ab1jx
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W8JI
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« Reply #4 on: October 29, 2012, 05:33:54 AM »

He probably should model the exact antenna.

If it is an elongated loop with large length compared to much less than 1/4 wave width, it will have maximum radiation (just like a folded dipole) when fed on the long side.  It's only when wire spacing is large, approaching 1/2 wave or more,  it can significantly null the horizon. His proposed loop is only 1/2 wave wide on ten meters, so it is pretty much a fat folded dipole on 20 and down.
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K5LXP
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« Reply #5 on: October 29, 2012, 07:16:47 AM »

I will be able to put up a form a of a loop antenna up it'll be 13' high and in most places 3 feet above the roof about 68 long by 16 wide.

I think that pretty much precludes any modeling.  There'd be no way to factor in the variables of the nearby structure.


Mark K5LXP
Albuquerque, NM
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KF7PCG
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« Reply #6 on: November 03, 2012, 12:28:35 AM »

Thanks for the answers I am setting it up and should be done tomarrow so I will be able to test things out.  Thanks Lloyd
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