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Author Topic: Why aren't full-size HF loop antennas more popular  (Read 6554 times)
AG4DG
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« on: March 06, 2002, 12:40:44 PM »

It seems to me that a full-size loop antenna is more space-efficient than a full-size dipole or full-size monopole.  And there should be little, if any, extra cost.

Full-sized horizontal dipoles are .5 wavelengths long and must be at least .5 wavelengths above the ground in order to radiate at the low angles needed for DX.  At 40m, this means 66 feet of horizontal space and 66 feet of vertical space are needed.  While vertical dipoles require little horizontal space, they still need .5 wavelengths (66 feet) of vertical space.

Full-sized monopole antennas are .25 wavelengths tall (33 feet for 40m).  But they require a good RF ground to work near peak efficiency, and the radials require lots of horizontal space.  The general rule is to make the radials at least as long as the antenna is tall, and this means that .5 wavelengths of horizontal space (66 feet for 40m) is needed.

A 1-wavelength loop antenna can be in the shape of a square that is .25 wavelengths long.  So for 40m, this antenna requires no more than 33 feet of vertical space and 33 feet of horizontal space.  For certain configurations (depending on the location of feedpoint, whether the antenna is horizontal or vertical, etc.), the antenna can radiate at low angles for DX even if it is not mounted at a great height (which is required for a horizontal dipole).  Unlike a .25-wavelength monopole, a loop antenna does NOT need a ground plane or radials in order to have high efficiency.  Unlike small antennas, a full-size loop has plenty of radiation resistance.  If the SWR is too high, one can use ladder line (for lower losses) and a tuner (to keep the SWR low at the transmitter).

A .5-wavelength loop antenna takes up even less space.  A square configuration would be only .125 wavelengths wide, and the antenna would require no more than 17 feet of vertical space and 17 feet of horizontal space.
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AC5E
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« Reply #1 on: March 06, 2002, 01:05:20 PM »

Hi: Very, very briefly, a "full sized HF loop" is an excellent antenna. A single resonant loop is about as good as a much higher dipole. On 160, 80/75, and to a lesser extent on 40, a single  quad or delta loop may be the best possible choice.

 Of course, quad loops can make a fine parasitic array. Generally speaking, a 2L quad at 40 is about as good as a 3L Yagi at 70. But an optimized 4 el quad occupies a space 18 by 18 by 45-50 feet. Volume wise, that's about the same space as three railroad boxcars!

Quads are good antennas especially for short towers. But they are bulky, they take a lot of work to optimize, and they take a lot of maintainence. If you enjoy antenna work they do very well. But if you want something to put up and forget - there are better choices.

For more information, check out some of the standard reference work on loops and quads.

73  Pete Allen  AC5E
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K3AN
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« Reply #2 on: March 06, 2002, 01:12:50 PM »

The loop does not have to be a square. Motivated by the October 1984 QST article by Doug DeMaw (SK) titled "The Full Wave Delta Loop at Low Height," I built and installed one at a prior QTH. It was cut for the low end of 80 Meters, and fed with about 40 feet of RG-213 and a 4:1 balun at the feedpoint. The autotuner in my TS-850 could match it on every band except 10.1 MHz. This triangular loop was probably the best wire antenna I have ever had. Low noise, multi-band operation, no radials or counterpoise, relatively compact- if you have the typical suburban lot and you can't put up a beam, then IMHO the full wave loop is your next best option.
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WB2WIK
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« Reply #3 on: March 06, 2002, 01:19:32 PM »

A couple of fallacies:

-The "full sized monopole" (1/4-wavelength, base-fed vertical) does not normally require a radial field which is 2x the width of the vertical radiator.  1/4-wave, base-fed, ground-mounted verticals are typically optimized with a large number of very short radials.  A 40m vertical 33' tall would normally have 20-30 radials each ~15' long.  Empirically, there is no evidence that making those radials longer improves anything other than the profit of the local wire supplier.

-Also, a "full sized monopole" that is elevated above ground to lose ground dependency works best with sloping radials.  A 45 degree slope below the horizon will compress the area of the circle they occupy, and actually improve antenna match by raising feedpoint Z.

-But the biggest fallacy, I think, is in the next statement: "So for 40m, this antenna requires no more than 33 feet of vertical space and 33 feet of horizontal space."  Where did that theory come from-?

A 40m loop vertically oriented and literally laying on the ground (which is what you're describing) is not really a full-wave loop, since the ground closely coupled to the lower horizontal leg of the antenna severely impacts the antenna in many ways.  Fed on a vertical side, this antenna won't work well.  It will be a 3/4-wavelength, off-center fed device whose resistance and reactance can only be calculated if the feedpoint location is precisely known.  Fed at the center of the uppermost horizontal leg, it will be a 3/4-wavelength dipole terminated in earth on both ends -- not a good antenna, either, and one that has low radiation resistance and a lot of capacitive reactance.  A vertically oriented full-wave loop needs to be elevated above ground to lose its ground dependency and start behaving as a loop, and that blows your theory about it only occupying 33' of vertical space.

The same loop oriented horizontally needs to be as high as a dipole (~1/2-wavelength) to lose most of its ground dependency and provide a low angle of radiation.

Most actual users of full-wave loops orient them horizontally and install them at less than 1/2-wavelength height, and take advantage of the benefits of the antenna's aperture and low-noise (receiving) characteristics.  

WB2WIK/6
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AG4DG
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« Reply #4 on: March 06, 2002, 01:49:43 PM »

< -The "full sized monopole" (1/4-wavelength, base-fed vertical) does not normally require a radial field which is 2x the width of the vertical radiator. 1/4-wave, base-fed, ground-mounted verticals are typically optimized with a large number of very short radials. A 40m vertical 33' tall would normally have 20-30 radials each ~15' long. >  
You can still have respectable efficiency with shorter radials if you have more of them, but that will still likely be several dB down from that of a balanced antenna (that requires no radials).

< Empirically, there is no evidence that making those radials longer improves anything other than the profit of the local wire supplier. >
From what I have read, the point of diminishing marginal returns comes once the radials are as long as the antenna is tall.  That's because the longer the vertical element is, the more widely distributed the ground currents are.  A shorter antenna will concentrate the ground currents closer to the antenna and reduce ground losses, but a loaded antenna will also have less efficiency due to the low radiation resistance.

< -Also, a "full sized monopole" that is elevated above ground to lose ground dependency works best with sloping radials. A 45 degree slope below the horizon will compress the area of the circle they occupy, and actually improve antenna match by raising feedpoint Z. >
Agreed, but this requires more vertical space.
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WB2WIK
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« Reply #5 on: March 06, 2002, 03:13:51 PM »

To answer the original question, "Why aren't full-sized HF loop antennas more popular?" my response is:

They are.  Very popular.  I've been a ham 37 years and have lived in 14 houses since first licensed, and have had a full-wave loop installed and operational at 12 of them.  And so have most of the active hams I know, who use HF.

WB2WIK/6
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WB6BYU
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« Reply #6 on: March 06, 2002, 04:13:00 PM »

I agree with Steve - they work well, and they have been popular
on my antenna farms over the years.

Perhaps part of the problem is that a loop occupies two dimensions
while a dipole is one-dimensional.  This translates into needing
more supports for a horizontal loop, and having them be in about
the right place.  If the loop is vertical, you don't gain much at low
heights because the bottom wire is so close to the ground (which
can also be a hazard to people.)

Meanwhile, many hams have trouble finding space for even a
dipole on the lower bands.  With the ends bent, a dipole takes
rougly half  the area of a loop and uses half the wire.  Although a
40m horizontal loop will fit on the roof of many houses, a loop for
80m takes over a tenth of an acre, which is impractical on many
urban lots.

If you check on  www.cebik.com you can see his plots.  Vertical
loops are not as good for multi-band operation, because the
pattern tends to split up into high-angle lobes on the higher bands.
However, single-band horizontal loops have a lot of advantages.
I like the delta shape with the point down - takes less space than
a dipole at the same top height, and uses a shorter feedline for the
same antenna height (when fed at the bottom corner.)

But most of the ham antenna books tend to feature the dipole as the
basic antenna, and many hams never get any further than that.  But
for those with a desire to experiment, the loop provides lots of
opportunities.
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G4HZV
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« Reply #7 on: March 06, 2002, 04:24:16 PM »

Good question. They are excellent antennas, work well on harmonic bands, benign feed impedance (much easier to multi-band tune than doublets). I used a 40 metre loop on all bands above 40 for years, with considerable success. Unfortunately our latest house doesn't have the necessary, strategically placed trees, so I now use verticals, but give me a big loop any day.
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RobertKoernerExAE7G
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« Reply #8 on: March 06, 2002, 04:51:48 PM »

I've heard one ham, who designs antennas for a living, assert that the best use for a vertical support structure is to hoist up the delta loop in ON4UN's book: fed 1/4 wave length from the top.

For the short time I've played around with EZNEC, I haven't found a better all around performing wire antenna (for one band).

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K3YD
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« Reply #9 on: March 06, 2002, 05:43:36 PM »

The best performing H.F. antenna I've managed to have at my current (small suburban lot) QTH was a 40 meter Delta loop, fed 1/4 wavelength below the apex with ladder line.  The antenna was a superb performer on 40 into Europe and VK/ZL from Pennsylvania.  Those directions were broadside to the plane of the loop.  On 20, 15 & 10 the antenna, increasingly, favored the directions of the plane of the antenna.  

The problem, and reason that I don't have the antenna up today is that the [Honey Locust] tree which supported it kept wearing through the wire.  I never managed to keep it in the air longer than 15 months.  One windstorm brought it down in 9 days!  

When 10 meters begins to decline I intend to take down my 10 M. half-square & try this antenna again--with more durable wire.
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KA4WJA
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« Reply #10 on: March 06, 2002, 07:14:07 PM »

Jason,
There are really 2 answers to your question....
Depending on your definition of "popular"...

Either, #1)  They really are popular!!!
I use 2 different full-wave loops on 80m....
And I hear many others on the air that use them as well...
And I agree with most of the others here....They are GREAT!!!!!

Or, #2) The reason they aren't MORE popular.....
is that there really aren't very many "commercially made" full-wave loops (RadioWorks, etc.) AND many hams, especially new hams, won't build their own antennas if they perceive them as "exotic" or even just "unusual" or "atypical"......
I believe that human nature is responsible for much of what you perceive as a lack of popularity.....

Many hams, again especially many new hams, find it hard to go out on a limb with something that is less than "typical".... AND as such you end up with a "self-fulfilling" tendency of following the crowd..... rather than trying something different...

And further, I believe that this is why you'll find many low band dx'ers using full-wave loops.....(dx'ers
tend to look for that last db or two of advantage....)

Check out www.cebik.com
L.B.  has a LOT of great info on loops of all kinds...
Especially their use as Vertical polarized, low-angle DX antennas....

These are just my opinions.....

73,
John,  KA4WJA

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WB9WHE
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« Reply #11 on: March 07, 2002, 10:16:07 AM »

The problem with loops is that unless they are high enough, they become "cloud warmers". This is a term used to describe an antenna that has a very HIGH angle of radiation. HIGH is great if you want local ragchew, but terrible if you want to work DX.

Loops also require at least 4 supports, if maximum enclosed area is to be achieved.  Dipoles require only 2, or three supports if you use an inverted V configuration.

Compairing a loop and a dipole at the same height, I'll take the lower angle of radiation of a dipole any day. The down side, is that a dipole (unless you have traps or multi-elements) is pretty much a single band antenna, whereas a loop works on resonant multiples.

That's where traps and multi-element dipoles come in. Traps can be lossy. Multi-element dipoles can cover 4 bands with three wires, ie 40/15, 20, 15 Mhz. Since they don't have traps, they are very efficent.
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NQ4S
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« Reply #12 on: March 07, 2002, 03:21:51 PM »

For my situation, a horizontal loop was about the only option available.  I live in a townhome with a rather small attic crawlspace.  Even hamstick dipoles are crowded up there.  The best solution I was able to come up with was to run a loop of wire all the way around the perimiter of the attic (about 58 feet of length) and finish it off with an SGC SG-239 autotuner to make it multiband.

With the random length of wire and the fact that the whole mess was indoor, I didn't expect much out of it.  Boy was that a wrong assumption.  That simple loop has talked over most of the world (including Australia from the US East Coast) on 10m, 15m, and 20m.  It will also talk a little bit on 40m and 75m.

The autotuner added a big chunk of cost (about $250), but the cost of a multiband shortened dipole (using two mobile antennas) would have actually been more.  For cost, ease of use, and effectiveness, I couldn't recommend a better solution than the loop.

73, de NQ4S (Delbert)
http://www.qsl.net/nq4s
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KD5QPF
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« Reply #13 on: March 17, 2002, 10:46:40 PM »

NQ4A (et al),

I'm in a similar situation (a townhome condo with antenna restrictions). I currently have a 'slinky dipole' (junk, to be relegated to SWL listening) and a 20 mtr Isotron (OK but single band) in the attic.

I've been seriously thinking of putting a horiz loop antenna around the perimeter of my attic. My attic (more like a crawlspace as well) is approx 25 ft per side with 1/4 of it somewhat taken up by an 'intrusion' from having a cathederal ceiling in the master bedroom. Man is it going to be a PITA to install this but a horiz loop appears to be my best shot at really being able to get out.

Couple questions:

Think it might be more advantageous to create a full wavelength loop by employing a multiple loop configuration (e.g. like a large coil)

Is it better to place the tuner at the feedpoint as N4QS did vs at the xceiver (e.g. at the beginning of the feedline)?

Do horiz loops seem to like twinlead (or ladderline) or coax better?
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