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Author Topic: 80 M Loop Dimensions  (Read 1982 times)
KE7DJQ
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« on: September 03, 2012, 09:38:53 AM »

In my search for HF antennas, I've decided on an 80 M loop.  I  have several possible locations.  The most limiting factor is an access for farm machinery, which includes large tractors, combines and semi trucks.  They have to be able to get onto my property to the farm behind my house.  The farm is mine.  I can put masts in several configurations.  

Using 280 as the perimeter:
1. Span the access road with a rectangular configuration of 80,80,60, 60.   Blue rectangle.

2. Put the antennas up over my garden and animal area in the same 80,80,60,60 or as 100,100,40,40.  Green rectangle.

3. Use my garden area, plus a couple of tweaks to allow access of 76,70,69,59.  This gives me 274 feet.  This is as close to a square as I can get.  I have a bit of latitude here with the first two poles,  but not with the last two.  If I move the last two anymore west they might be hit by tractors.  Brown rectangle.

Location Diagram:  http://www.frontiernet.net/~blsplus/hamradio/images/HouseAndLot.jpg


Is the 100 ft or 80 ft distance too long?  How much will the wire droop in the middle?

Any  help will be appreciated.

I guess I could also only put up a 40 M loop, then I have a whole bunch of location options and don't  have to worry about tractors and semi-trucks.  I would then need a second antenna for 80 M.

I should have had all the antennas shown slightly askew, as I did with #3, since I have power lines running east-west just north of the house.
« Last Edit: September 03, 2012, 09:44:42 AM by KE7DJQ » Logged
AD5ZC
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Posts: 69




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« Reply #1 on: September 03, 2012, 09:55:46 AM »

#3
Smiley

You might find that you need a second antenna for 80 anyway since the one your putting up is going to shoot your 80M transmissions almost straight to the sky but it will work very well on 40.  IF you put up a 40 meter loop it will shoot 40 meters up at the sky so don't do that if you want 40 DX. 
You will be a shoe in for local 80 meter roundtables but not gonna have a lot of DX.

Easiest way to get into 80 or 160 DX is the L.  You've got nice room for some recieving antennas as well.  You should give it a shot.
« Last Edit: September 03, 2012, 10:02:14 AM by AD5ZC » Logged
WB6BYU
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« Reply #2 on: September 03, 2012, 10:19:14 AM »

What radiation patterns do you want on the higher bands? 

Where are you feeding the antenna?

How are you feeding the antenna?


All those things things factor into the decision.

For example, on 40m the 100 x 40 loop will radiate more off the short ends than off
the long sides, while a square loop is about the same in all 4 directions.

On 80m the same loop will be around 35 ohms when fed in the center of the short
side and 150 ohms in the middle of the long side.  (Depending on height above ground,
etc.)  The resonant frequency shifts 100kHz when moving the feedpoint.

Radiation patterns on the higher bands are more complex.  I've used a loop square fed in
one corner for Field Day, and it gives good radiation in the corner directions on 40m and
up.  The radiation patterns are different when feed in the middle of one side.

But a loop doesn't have to be symmetric, or even rectangular.  I've put up loops around
my property making use of trees, a chimney, a TV mast, boards nailed to a fence, etc.,
with 6 or 7 legs, and concave in places (which isn't optimum, but sometimes that is the
only way you can get it to fit.)  So you may find that putting one support near (but not
in the way of) the road will ensure that the wires are as high as possible at that point,
even if it results in an irregular shape.  Then you live with whatever radiation pattern
you end up with.  Generally, making it as close to circular (enclosing as much area as
possible) is probably a good thing, but there is no sense in taking it to an extreme.


How much do wires sag?  That depends on the span, wire weight, tension, and whether
the wire contains a feedpoint.  VK1OD has a calculator here: 

http://www.vk1od.net/calc/awcc/awcc.htm
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KE7DJQ
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« Reply #3 on: September 03, 2012, 11:16:50 AM »

What radiation patterns do you want on the higher bands?

I want to do some DX, but I also want to talk locally on 40M.

Where are you feeding the antenna?   

At the NE corner, closest to the house.

How are you feeding the antenna?   

4:1 balun fed with coax, then to a box with lightning protection, then into the shack.
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WA9UAA
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« Reply #4 on: September 03, 2012, 12:19:49 PM »

This may answer your question, the gain of the loop antenna is a function of the area inside the loop so a circle is best, followed by a square, delta shape etc. There are also loops with the plane in a vertical attitude which work best in two directions at the fundamental  frequency.  HTH
73,
Rob WA9UAA
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WB6BYU
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« Reply #5 on: September 03, 2012, 12:30:42 PM »

A 80m loop used on 40m will tend to have a null overhead, so won't be as good for
local NVIS contacts (out to 100 miles or so).  Depending on height above ground,
it will be better at 300 miles.  It may still be adequate, however.

I hope you've read some of the late W4RNL's articles on loop antennas like this one:

http://www.cebik.com/content/a10/wire/horloop.html
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KE7DJQ
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« Reply #6 on: September 03, 2012, 04:20:42 PM »

Another ham, about 25 miles away has one like this, and he is able to talk locally on it.  His is up just above his roof line, about 21 feet up.  I used his as the model for mine, but I'm not putting it up over my house like he did.  He has a much smaller lot.
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W4OP
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« Reply #7 on: September 03, 2012, 04:58:24 PM »

A 80m loop used on 40m will tend to have a null overhead, so won't be as good for
local NVIS contacts (out to 100 miles or so).  Depending on height above ground,
it will be better at 300 miles.  It may still be adequate, however.

I hope you've read some of the late W4RNL's articles on loop antennas like this one:

http://www.cebik.com/content/a10/wire/horloop.html

The difference in gain between a 1 lambda round and square loop is less than 0.5dB. And remember, this is  the on axis gain- not a very useful direction (for a horizontally deployed loop) for gain except NVIS

Dale W4OP
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WA4FNG
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« Reply #8 on: September 03, 2012, 07:44:06 PM »

If I had to put up only 2 antennas it would be an 80m loop and an 80m doublet, both as high as possible and fed with ladder line and a tuner. Well, I might replace the 80m doublet with an inverted L with a base mounted auto tuner. The other option for a 2nd antenna might be a ZS6BKW.

The actual length won't be that critical with ladder line and a tuner, and actually is easier to match on the even harmonics if the length is a bit long at the fundamental freq. The big loop works rather well for the higher frequencies with an overhead null. There will be some gain at lower angles, but also nulls in between those lobes.

That being said, I had pretty decent results on 80/40 with my loop for local work. Having two antennas gives you a few options.

-Milt
« Last Edit: September 03, 2012, 07:47:12 PM by WA4FNG » Logged
KE7DJQ
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« Reply #9 on: September 04, 2012, 05:06:52 AM »

I understand that a  ladder line fed antenna has advantages.  I am concerned about lightning strikes and haven't seen how to protect against it with a ladder line feed.  Maybe I have missed something somewhere.  All the stuff I have read has been for coax.
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WA9UAA
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« Reply #10 on: September 04, 2012, 05:59:13 AM »

I switch my ladder line to ground when not in use for lightning protection. There are also two 100K Ohm wire wound resistors to ground to bleed charges off of the line. I have heard of power pole connectors being used to manually switch ladder line as well. HTH
73,
Rob WA9UAA
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WA4FNG
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« Reply #11 on: September 05, 2012, 05:56:33 AM »

You don't always have to run the ladder line into the shack. Many (myself included) have the balun outside and a short run of coax to the tuner. However, if you were using a true balanced line tuner you'd run the ladder line into the shack. The old style knife switches can be used to disconnect the line.
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KB8VIV
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« Reply #12 on: September 13, 2012, 07:29:52 AM »

I'm using an 80m horizontal loop meself, up about 20 ft.  I think you want to get it as close to square (round) as possible.  I just feed mine with coax (I know, not optimal) with a few loops at the antenna feedpoint.  I've had that antenna up for close to 10 years, and it works like a champ. 

You can work everything from 80 through 10, not as good as a vertical on 15, 20, or 10, but it works.  It really plays well on 80 and 40 as you can imagine.  You will need a good tuner.  I'm not all that knowledgable when it comes to antennas but when I was researching, this seamed to be the best option for me.  If my memory serves me, it's aprox 275 ft long.  Good luck.

Kb8viv
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KE7DJQ
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« Reply #13 on: September 13, 2012, 09:12:02 AM »

Thanks for the responses.  I'm working on getting the masts up this week.  Next week I'll order the wire and other materials and actually make the antenna.
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K4SAV
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« Reply #14 on: September 13, 2012, 11:04:10 AM »

Using 280 as the perimeter:
1. Span the access road with a rectangular configuration of 80,80,60, 60.   Blue rectangle.

2. Put the antennas up over my garden and animal area in the same 80,80,60,60 or as 100,100,40,40.  Green rectangle.

3. Use my garden area, plus a couple of tweaks to allow access of 76,70,69,59.  This gives me 274 feet.  This is as close to a square as I can get.  I have a bit of latitude here with the first two poles,  but not with the last two.  If I move the last two anymore west they might be hit by tractors.  Brown rectangle.


The SWR all of these configurations is close except for the 100x40 on 80 meters.  On 80 the feedpoint impedance should be close to 10 ohms.  If it was fed in the center of the 100 ft wire, instead of at a corner, the feedpoint impedance on 80 would be about 39 ohms.  There would be no clear-cut winner as to where to feed this for SWR on the other bands since some are better and some are worse.  A tuner will probably be required for all of these configurations on some frequencies.

The radiated pattern of all these antennas is very strange on the high bands.  There are lots of lobes and nulls.  The radiated pattern changes a lot depending on where you feed the antenna.  It's difficult to pick a winner because of the huge difference in patterns between all the possible combinations.  You will find that some stations are very loud in some directions while others in different directions are very weak.

What radiation patterns do you want on the higher bands?

I want to do some DX, but I also want to talk locally on 40M.

I assume the antenna is pretty low since you are using poles to support it.  That will limit the DX performance on the low bands.  On the higher bands the radiated angle drops and you should be able to work some DX.  The limitation there will be the radiated pattern which has lots of lobes and nulls. 

If locally on 40 means 100-500 miles then the antenna should do OK even though there is a null overhead.  Signals in that range are also subject to solar conditions and time of day.  Local signals are usually stronger than DX so an antenna with less gain usually works OK. 

I understand that a  ladder line fed antenna has advantages.  I am concerned about lightning strikes and haven't seen how to protect against it with a ladder line feed.  Maybe I have missed something somewhere.  All the stuff I have read has been for coax.

You can add surge protectors or resistors to handle static buildup but protecting a loop or a dipole from a direct strike can be problematic in most cases because the wires and feedline cannot handle a direct strike without disintegrating.  I have tried to answer this question several times on different forums and present a possible solution (although painful to implement), but I have offered no solution that I would consider to be reliable when the loop or dipole is near the house.  I could see a possible solution if the loop was made from #4 wire and had several ground rods at each of the 4 metal masts, and grounding implemented the same as for a tower, but few people would consider that.  I haven't seen any solution in print that has any chance of working, other than something I wrote.

Ladderline is particularly difficult to handle because you can't ground the shield at the base of the tower (or mast) and you can't ground it at the entrance panel, and you can't run it under ground.  That's the only reason I don't have any ladderline fed antennas.

Jerry, K4SAV
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