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Author Topic: Can a full-wave loop be "too large" for higher band operation?  (Read 2726 times)
KG4NEL
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Posts: 373




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« on: July 26, 2012, 08:31:27 AM »

Hey all -

I'm planning out my antenna work in the fall, and the more I read about full-wave loops fed with balanced line, the more I'm thinking about going in that direction. Have plenty of room, could fit a full-wave loop for 160m with the lowest point at 25' and the highest at 60' or above. I realize this is way too low to do much good at low elevations on Top Band, but I figure if I can get a few points during contests, that's good enough for me.

However, is going with a 530' loop or so to cover 160m going to cost me significantly for the higher bands? I haven't heard much discussion about loops being "too large", but I figured I'd ask before I spend a weekend fighting branches to loft the wire.

Thanks!
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WB6BYU
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Posts: 13343




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« Reply #1 on: July 26, 2012, 09:07:14 AM »

It can be too large, depending what you want it to do.

The late W4RNL has a good article on multi-band use of large loop antennas here:

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


I've put up and used a number of loop antennas.  Some have worked well, some
haven't.  I've had better results on 40m and 80m than on the higher bands, and some
of that might be due to antenna height.

For NVIS work, a horizontal loop is great on the fundamental, but has a null overhead
on even harmonics.  Fortunately the null is fairly narrow, perhaps +/- 15 degrees, so
it can still cover moderate distances.

The pattern develops more lobes and nulls on the higher band, as shown in the plots
in W4RNL's article.  For the last couple of years I've put up a full wave 80m loop for
Field Day, carefully arranged so the major lobes run North, East and South (which is
optimum for Oregon.)  After plotting the radiation pattern on a map, I discovered that
the lobes were a bit narrow on 20m, and considerably so on 15m and 10m.  I could hear
and work strong signals on 15m, but there weren't a lot of them, and only in specific
directions.  This year I was on crutches and couldn't put up the big loop, so settled
for a 40m doublet fed with twinlead and did about as well operating 40, 20 and 15m.
It would be interesting to compare maps of the results:  the loop worked well overall
and made lots of contacts (more than the other stations combined) but had to be
aligned fairly carefully to keep from aiming the null right at the major concentrations
of stations.

So large loops can work in many directions on the higher bands, but will also have
a corresponding number of nulls.  It's not a bad overall antenna when you aren't
particularly concerned with the direction you are working, or where you can align
it for those areas of particular interest to you.  You may find that you want to have
a second antenna to help fill in the nulls.
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NH7O
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« Reply #2 on: July 26, 2012, 10:23:29 AM »

I would suggest looking at an inverted L for 160m using that 60' tree. It will work just fine for local contacts and much better for DX than the NVIS loop. Try a trap in it for 80m. Put up a simple doublet of some reasonable length and feed it with open line for the higher bands.

Not much more work, but with a more useful result than one big loop.
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AD5ZC
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Posts: 72




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« Reply #3 on: July 26, 2012, 03:00:53 PM »

Agree with checking out Cebik.

I think when you see the plot for the 80 meter loop you will reconsider.

Here is how I see it.

On 160 it is an NVIS antenna.  Period.  (but so is any other horizontally oriented antenna at a height a normal human can put up for 160.
On 80, where you probably would have found the NVIS feature useful, it is now capable of working some DX but probably no better than a dipole at whatever height you've been able to raise your loop.  Well, a 2WL loop is a little better on TO angle than a same height dipole but not much.
On 60.  Gangbusters.
On 40.  Gangbusters.  But you've already got lobes.  Luckily the nulls on loops aren't as deep as a doublet but you've already got them.
On 30 I'd still call this thing a winner but your starting to have to take luck of the draw on where stations fall within your pattern.
On 20, well, if they are in a lobe they're gonna think your using a beam but your already in a totally luck of the draw situation with that antenna.  Lots of lobes and nulls.
No need to go further.  You can match the antenna up to 6 meters but where your talking is a total crapshoot.

Don't get me wrong though, you will fill a logbook with a 160 meter loop.  I threw up 400 ft of wire strung through my oak trees and talked all over the place.  On the higher bands though, it really was a crapshoot as to whether or not I could hear what someone else was hearing on, oh, let's say a DX spotter.  (hate to admit using it but, oh well)  All I could do was turn on the radio and see if they fell in the pattern.

Now, if you MUST have 160 and you've got lots of natural trees laid out so it's not a hassle to put up a loop compared to a dipole then definitely go for the 160 loop.  Impedances at the feedpoint stay pretty mild and the nulls on a loop like I said before aren't as bad as a 160 dipole on the upper bands and TO angle is slightly better for the same height.

But if you have to put up a bunch of supports and stuff like that, just put up a dipole.  If your stuck on balanced line and a tuner, the above is my best advice. 
NH7O earlier mentioned an inverted L.  If you want DX on 160, this is the way to go but be prepared to install a decent set of ground radials and get yourself an autocoupler.  Your not gonna feed an L with balanced line.  Well, I guess where theres a will theres a way but...
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NA0AA
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Posts: 1042




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« Reply #4 on: July 26, 2012, 08:04:29 PM »

I've been using a 160 meter horizontal loop for a couple of years.

I have not operated on 160, but mainly on 80 and 40 with this antenna - I find it excellent for those bands.  Performance on 20 is uneven due to the lobing and does not improve as you go higher.

IF you want to use a loop for the higher bands as well, then I would do two of them, no reason you could not make a 160 and hang a 40 inside of it, using a separate open feed line. 

I'm not sure that at 30' like mine is, it's a great DX antenna, but it's the best I've got at the moment.  An 80 vertical is still just a bit beyond my reach yet. 
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W0BTU
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« Reply #5 on: July 27, 2012, 07:02:27 AM »

Take a good look at what the big 160m contest stations (the ones who score big) use. Almost without exception, it's a vertical or inverted-L. Likewise, no AM broadcast station uses anything except a vertical, because at MW, a vertical is superior. A loop or dipole would have to be VERY high to work as well.

"The fact is....... an Inverted L with 20 or more radials at least 50 feet and hopefully 100 feet long will absolutely smoke any normal height loop antenna or dipole antenna at nearly any distance on 160 meters. The possible exception is between 20 and 200 miles." --quoted from W8JI at http://www.w0btu.com/160_meters.html
« Last Edit: July 27, 2012, 07:03:58 AM by W0BTU » Logged

KD8GEH
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Posts: 470




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« Reply #6 on: July 29, 2012, 03:35:32 AM »

Hi,

I have a 544 ft loop fed with  stub of 450 ladder line at 40 feet avg height, feed point in at about 35 feet. Awesome on 160, so so on 80, awesome on 40 & 20.

Very quiet compared  to the dipole.  I love it!!!!

Big antenna nice receive.... I would try it just for fun see how it works for u.

73  Dave  KD8GEH
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W8JI
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« Reply #7 on: July 29, 2012, 06:43:04 AM »

How we "feel" an antenna works really is very subjective. Feelings usually have a limited amount to do with how a particular antenna really works.

If we call someone we hear and they reply, then it is the best antenna ever. If we can't hear something we think we should, or nobody ever answers, then it is poor. :-) I have fallen for this myself, and felt average antennas were awesome just because of a few stations I worked with them.

The technical answer to your questions is "yes, a loop antenna can be too large".  There are two things that make "too large" not good:

1.) As the antenna becomes larger, the path along the wire is longer. This increases losses, and reduces average gain (efficiency). More power is wasted as heat because conductor length, and losses in the earth below the wire, increase.

The efficiency of a 40 foot high full wave loop on 160 meters is about 38%. This is because the antenna is so low, and so long around the perimeter. On ten meters, the same antenna is 71% efficient. The increase is because the antenna is higher in wavelengths.

2.) The antenna splits into an increasing number small narrow useful maximums, and more wasteful nulls scattered all around. This makes it difficult to "aim" the useful lobes in useful directions, without planting a null on another desired direction.

On 160 the antenna has one lobe straight up, and one null along the horizon at low angles.

On 10 the antenna has 28 very narrow lobes with very modest gain in the best lobes compared to a dipole. On each side of the lobes it has deep nulls, the nulls are just six degrees off most peaks.

Statistically on ten meters, someone is just as likely to fall into a -15 dB null as a 9 dBi average peak.

A dipole has about 8-9 dBi gain, a very wide smooth peak, and only TWO deep nulls!

Any reasonable antenna will make contacts. If we don't get emotional attached to them, it becomes very difficult to beat a properly constructed dipole.

73 Tom
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