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Author Topic: Horizontal loop antenna  (Read 5962 times)
K4ELO
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Posts: 55




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« on: September 02, 2011, 06:45:45 PM »

I'm planning a loop antenna as described in the Wirebook V on page 180.  It's 1050' of wire in a horizontal square configuration.
I've never put up a big wire like this before so I would like some advice from someone who has.

Support masts will be Rohn H-50.  I'm going to put an end insulator at each corner and tie a support rope to it that I can run through a pulley attached to the mast for raising and lowering the loop.  The question is, how to attach the pulley to the mast?
Don't want to drill the mast and weaken it so I'm thinking of using double hose clamps around the mast and through the pulley support, but I'm not sure this will be strong enough with 250' of wire between each mast.

I'm planning on using #14 solid ccs wire but I can't find a weight for it.  #14 solid copper is 12.4 pounds per 1000' according to my research, but that seems a little light.  The copper clad steel should be heavier I expect.

Any advice from experience would be appreciated.

73
Wayne
K4ELO
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K4SAV
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Posts: 1843




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« Reply #1 on: September 02, 2011, 07:56:12 PM »

I don't have Wirebook V.  I have Wirebook IV and it isn't in there.  I wanted to see what they said about this antenna.

Being resonant at the frequency where lightning has its largest energy component should be very interesting and exciting.  It should also be great at picking up local AM stations and maybe causing intermod problems.  It doesn't show low SWR (for coax) on any band so I guess you will be using ladderline?  That means a tuner and that should help with the intermod problem.  A quick EZNEC look shows it doesn't look any better than a dipole on the low bands and on the high bands it has a huge number of very skinny lobes and nulls.

I wouldn't use anything less than copperweld wire for this because of strength and weight.  Wind loading will cause the largest force on the wire.  Try not to get copperweld with very thin plating because it will have pretty high loss on 160 compared to solid copper, especially in a very long antenna.

Jerry, K4SAV
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WB6BYU
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Posts: 13287




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« Reply #2 on: September 02, 2011, 08:06:09 PM »

The masts will certainly need a back stay to keep them reasonably vertical with that much stress on them.
The top sections of those masts are pretty flimsy - mine wiggles all around when the birds land on it.

The stress on the mast is far more than just the weight of the wire - if you pulled the wire up perfectly
horizontal the stress on the masts would be infinite.  A reasonable estimate of the tension on the ropes,
and the sideways pull on each mast, would be 1/4 of the wire weight / the sine of the sag angle (measured
down from horizontal.)  So if the rope leaving the mast runs 20 degrees below horizontal, the total
force on the mast is about 3 times the mast's share of the wire weight.  The amount of sag limits the
height of the sides of the loop - the more tension you use to pull them up into the air, the greater the
force on the masts.


I've used a number of methods to secure pulleys to my masts.  For testing inverted vees and other antennas
I use a board (perhaps a 2x2) secured to the mast with a U-bolt, and put a screw eye in each end with a
halyard through it.  That allows me to hoist two antennas at a time.  For a single pulley I'd just tie it with
baling twine:  make a loop of twine, rope, etc., pass the doubled end through the hole in the pulley and over
the pulley.  Fold the other end over itself to make a slip loop and stick it over the end of the mast.  This
method allows you to remove things as needed without untying any knots in the rope.  (A prussic knot makes
a very secure attachment to the mast.)  Or just pass a rope through the pulley, wrap it around the mast a few
times and tie it off.  If you have the slip ring for guys at each level of the mast, you can tie the pulley to that
(or use a small loop of strong wire.)  Depending on the hole sizes you may be able to find a spring clip in
the hardware store that will fit both the guy ring and the hole in the pulley if you don't want to tie knots.

For the corners I use electric fence insulators.  Generally I look for the plastic "egg" shaped ones - the ones
for electric fences have slots to put the wires into rather than holes (as are found on most ceramic compression
insulators.)  This allows me to slip the insulator over the wire, then tie the rope around the wire (on the insulator.)
If the insulator breaks, the rope is still around the wire, so your antenna doesn't come down.  This also leaves
the insulators free to slide along the wires, which is useful when you are using trees or other supports that
aren't in precisely the right spot.  (I think Radio Shack used to sell some like this - I could get a bag of 12 at
the farm supply store for less than Radio Shack wanted for 2 of them.)  They also make small electric fence
insulators that are like plastic carabiners that clip around the wire.

Good luck!
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WB6BYU
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Posts: 13287




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« Reply #3 on: September 02, 2011, 08:48:06 PM »

The choice of antenna is a separate issue.  Depending on exactly why you are considering such a loop, you
may find that a half size version (a full wave for 160m) would work nearly as well.   Some of the pieces to
consider:

The original loop matches reasonably well with a 4 : 1 balun to coax at 1.875 MHz and 3.8 MHz, but the
resonance misses 40m.  It could be fed with open wire line on all bands.

You'll probably want to feed it in a corner, where the feedline can be supported by a mast.  Feeding it in the
middle of a side will add weight and cause the antenna to sag more.

The smaller loop has maximum current at the corners on both 80m and 160m.  The larger one has current
maxima in the centers of the sides as well on 80m, where the average height will be less due to sag.  This
means it won't be as effective as a simplistic model might indicate.

The larger loop has maximum radiation off the corners on 80m, with nulls in between.  The pattern on 160m is
fairly omnidirectional, with about a 10dB overhead null.  While the smaller loop has a higher angle of maximum
radiation (straight up on 160m) the radiation at lower angles (say around 40 degrees) doesn't change that much.
So it depends whether the pattern nulls are a problem in your application on 80m.  (Of course, I'm modeling
this as purely horizontal, rather than including the sag in the wires.)

If your primary interest is local communications on 80m and 160m, the smaller loop is probably better.  If you
are putting the antenna up for DX, the larger loop might have an advantage, but it would be worth looking at
the patterns carefully, especially after taking into account any sag in the sides, to see if it really gives you
the improvement you expect.

Of course, if the objective is to put up a big antenna just to see how it works, go right ahead. 

I wouldn't cement the masts in place, however, as you may decide you want to move them around to try other
designs.  You can either get a piece of pipe that is a slip fit over the outside of the bottom section and cement
that into the ground (low enough that the lawn mower doesn't hit it), then slip the mast in and out of it as
desired.  (That reduces the mast height by 5' or so.)  Or bracket it to the side of a 4x4 or larger post.  I used
some chain link fence parts to do this, and by making the top bracket pivot on a lag screw I can tilt the mast
to get it upright (then extend it once it is vertical by standing on a tall step ladder.)
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W8JI
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« Reply #4 on: September 03, 2011, 04:28:41 AM »

Really long wire antennas are neat to look at, have, and talk about. Unfortunately they split the lobes with multiple very deep nulls, increase ground losses, and add wire resistance losses. Usually a dipole is a lot better for overall performance.

As for mechanical support, I have chain link top rail 20 feet tall that hold up a nylon string and wire arrangement that has been up for 10 years. This forms what could be a octagonal loop (except it has insulators) 350 feet diameter with 8 supports.  The single most important thing are the back stays and guys. The base doesn't do much with any tall skinny mast, except keep it from sinking in the ground and holding it while the guys are installed.
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K4ELO
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Posts: 55




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« Reply #5 on: September 03, 2011, 03:30:59 PM »

Thanks for all of the advice folks.  Well, no local AM stations out in the farm country so I don't think that will be a problem.

Yes, I was planning on feeding it with 450 ladderline near one corner.  Good tip on the insulators - looks like they are about $0.25 ea at Tractor Supply of which there are two not too far from me.

Yes, I was planning on using 4 guys on each mast and using the Rohn base plates (they just have a spike on the bottom, into the ground) so nothing is permanent.

I'll have to play with the model some more and see if I can figure out how to add the sag.

Should be an interesting experiment.

73
Wayne
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WB6BYU
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Posts: 13287




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« Reply #6 on: September 03, 2011, 04:47:08 PM »

I always pay attention to what they have in the Electric Fence department, as it is pretty cheap because it is
used in large quantities.  One way to reduce the sag around your loop would be to use aluminum electric
fence wire, which is light.  Does take a bit more work to get a good connection to copper feedline, however,
without corrosion problems due to dissimilar metals.

If anyone wonders what you are doing, say it is an electric fence for very tall giraffes.
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N4JTE
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Posts: 1156




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« Reply #7 on: September 03, 2011, 06:20:18 PM »

Used the rohn 50 many years, all I remember is that at 45 ft they bent considerably even when holding up simple 80 meter dipole.
Bob
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K4SAV
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Posts: 1843




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« Reply #8 on: September 03, 2011, 08:46:16 PM »

I'll have to play with the model some more and see if I can figure out how to add the sag.

If you have a model, compare it to a dipole at the same height as the loop on 160.  The dipole kills the loop.  Make sure you are not using a Mininec ground.  That will give large errors.  Use a "real high accuracy" ground.

The H50 is actually 46 ft in height.  If you have a set of guys at the top, it should support this OK.  If you have less than 3 sets of guys on the mast, it may buckle.

Jerry, K4SAV
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NH7O
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Posts: 126


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« Reply #9 on: September 06, 2011, 11:45:24 AM »

I am wondering what the attraction is to these loop antennas? I had a look at some models a while ago, and they have some wild and not very useful patterns on the high bands, and are so-so on the low bands. As W4RNL said:

 "Tall and Small is better than Low and Long".

Effort at going up is always better spent. There are some really good long wire antennas, but they tend to be point to point commercial installations with very tall supports. Take the huge signal from ZD8Z on 40m, for instance. N6TJ (the op) said they used stacked rhombics with very tall masts (a US spook site, I guess). The height is what it takes to reduce ground losses.
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K4SAV
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Posts: 1843




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« Reply #10 on: September 06, 2011, 01:53:52 PM »

I am wondering what the attraction is to these loop antennas? I had a look at some models a while ago, and they have some wild and not very useful patterns on the high bands, and are so-so on the low bands. As W4RNL said:

 "Tall and Small is better than Low and Long".

Effort at going up is always better spent. There are some really good long wire antennas, but they tend to be point to point commercial installations with very tall supports. Take the huge signal from ZD8Z on 40m, for instance. N6TJ (the op) said they used stacked rhombics with very tall masts (a US spook site, I guess). The height is what it takes to reduce ground losses.

Loops have a lot of myths associated with them, like bigger is better, or loops are always quieter, or loops work better than dipoles at low height.  It's much the same as many people think a very long antenna is better than a half wave one.  Those people don't look at the data for the antennas but rely on something others have said, so the myths propagate.  I find it amazing that a lot of people would rather have a testimonial about the stations worked with an antenna than the theoretical data on the antenna.

Those comments may not apply the the poster of this question.  At least he has a model.

Jerry, K4SAV
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W4VR
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« Reply #11 on: September 07, 2011, 10:45:59 AM »

I chat with OE5MSM on 40 meters from time to time.  He has a 1000-foot horizontal Delta loop up 100 feet in tall pine trees.  He uses helium baloons to bring the wires over the tree tops.  He corner feeds his and claims to have a directional pattern into North America.  He feeds it with 500 ohm line.  He always has one of the strongest signals coming out of Europe.  Check him out on QRZ.com.
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K4SAV
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Posts: 1843




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« Reply #12 on: September 07, 2011, 05:38:20 PM »

I chat with OE5MSM on 40 meters from time to time.  He has a 1000-foot horizontal Delta loop up 100 feet in tall pine trees.  He uses helium baloons to bring the wires over the tree tops.  He corner feeds his and claims to have a directional pattern into North America.  He feeds it with 500 ohm line.  He always has one of the strongest signals coming out of Europe.  Check him out on QRZ.com.

A 1000 ft loop on 40 meters at 100 ft height should have about 12 dB more gain than a 1000 ft loop on 160 meters at 40 ft height.  Of course the beamwidth is much smaller (about 15 degrees) so you have to aim it accurately.

Jerry, K4SAV
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W8JI
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« Reply #13 on: September 08, 2011, 05:04:56 AM »

I chat with OE5MSM on 40 meters from time to time.  He has a 1000-foot horizontal Delta loop up 100 feet in tall pine trees.  He uses helium baloons to bring the wires over the tree tops.  He corner feeds his and claims to have a directional pattern into North America.  He feeds it with 500 ohm line.  He always has one of the strongest signals coming out of Europe.  Check him out on QRZ.com.

A 1000 ft loop on 40 meters at 100 ft height should have about 12 dB more gain than a 1000 ft loop on 160 meters at 40 ft height.  Of course the beamwidth is much smaller (about 15 degrees) so you have to aim it accurately.

Jerry, K4SAV

The real question is not how much more gain it has over a very poor antenna, but how much gain it has over a reference dipole.  My bet is the large loop on 40, even at 100 feet, does not have any more gain than a small Yagi at the same height.

Even a Rhombic with optimum alignment, high in the air, and several wavelengths long is just about the same as a 4 or 5 element Yagi.

http://www.w8ji.com/rhombic_antennas.htm

73 Tom
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K4SAV
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« Reply #14 on: September 08, 2011, 07:04:48 AM »

The real question is not how much more gain it has over a very poor antenna, but how much gain it has over a reference dipole.  My bet is the large loop on 40, even at 100 feet, does not have any more gain than a small Yagi at the same height.

It's about the same as a three element Yagi at the same height (100 ft), but the beamwidth of the Yagi is about 3.7 times as wide.  Actually it's about the same as my 2 element XM240 at 65 ft, up to about 5 degrees elevation, then it beats me a little from 5 degrees to 23 degrees (up to 3 dB at the loops best angle) then my antenna is much better at elevation angles over 23 degrees.  A lot of that low angle gain is due to my terrain.  The beamwidth of my XM240 is about 5.7 times as wide as the loop.  With the 15 degree beamwidth of the loop, you could only cover about 1/3 of Europe and stay within the beamwidth.  If I had four 100 ft supports, I could think of several better antennas.

Jerry, K4SAV
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