The AM Forum

I have been reading a lot about magical antennas, like magnetic loops and find them quite fascinating.
I just installed a magnetic loop 3 foot diameter antenna for listening. Got it on a TV rotator and it is a pleasure to pick something out of the grass...the noise, local noise, ma nature. I can see the noise floor go up over 10dB by switching between the conventional dipole or vertical to the loop.

I'm looking at some designs for a transmitting loop.........12 foot square, using aluminum pipe....rotating might become a problem. BTW this is probably the one antenna that becomes directional for transmit. A dipole only 40 feet off the ground on 75 or 160 is not directional. But a magnetic loop antenna is. It may not be beamed like a Yagi and have gain, but it does exhibit direction.
They can take legal limit, but the vacuum variable needs to be 30kv. A lot of hype about a magnetic loop antenna being just as effective as a dipole 1/2 wave up in the air and more efficient.
QST has an article about a Ham who built a 2 loop antenna.
Fred

Fred,

If you come up with what looks like a workable design for 160, I'd sure love to know about it. I live on a leetle, tiny city lot with not even enough room for a decent antenna garden, much less a farm. I'm using a coil-loaded dipole on 160 right now, but I need to improve the 160 antenna, so all ideas are welcome.

ldb
K5WLF

How big is your lot? Any trees or just the house/roof?  I have the same problem with our 100x100ft lot, But I have some very tall trees (65 ft or so white pine and a few oak) that make great ant supports.  One day I will mess around with actually using the Tree itself as a vertical ant on 160 meters.  It's been done and is supposed to work, tested by the Army. 

If you are very careful to minimize I²R losses, an STL (Small Transmitting Loop) can actually outperform a dipole mounted higher than the loop. Using large diameter copper pipe and a special capacitor is a must.

There's some good links and references on STLs at the bottom of http://www.w0btu.com/magnetic_loops.html.

I just received my vacuum variable (eBay, Russian military surplus) last week for my own STL project.

As for the QST article, there's a typo: it says the loop can approach the performance of a vertical, but "dipole" (more gain) should have been there instead of "vertical". Or, they could have said exceeds the performance of a vertical.

Here's one guy who did that: http://www.bulldogtrust.com/index12.htm

And I've seen others, I think they're in the links on my STL page I mentioned.

This guy has already done it.

http://amfone.net/Amforum/index.php?topic=12451.msg91347#msg91347

This guy gets on 75 with us.

http://www.qrz.com/db/K8NDS

His Signal was right there with the rest of the Dipoles on the band. 


C

I've worked Rich on 40 SSB, and he's loud. But if he runs the legal limit on AM with that loop, that's impressive. If there were a lot of losses, it would smoke.

Keep in mind that loop is only five feet off the ground! :-)

How are magnetic loops at rejecting line noise?

They were almost meant for that purpose. The rejection of a single, local RFI source at very low angles is excellent. You would probably want to use a rotor.

The null is much less pronounced at higher angles, but can be >50 dB at 1 or 2 degrees elevation. You can see that in the EZNEC 3-D model below. Notice the deep null along the X axis at ~5 degrees which decreases rapidly as you go up. (This model is a 6' diameter loop 5' high on 40 meters.)

I will probably not rotate mine (at least initially) since I have Beverages and very little line noise. But the null is quite sharp, and if you need to null line noise, etc. then a rotor is almost a necessity.

I posted this link because maybe (if interested) part of the argument, I do not know the performance of these antennas and I don't know the manufacturer/seller.

http://www.ciromazzoni.com/English/Loop%20Antenna.htm

That's the beauty of magnetic loops.....The line noise is electrical which is part of our Electromagnetic radio signal. The loop is only listening for the magnetic part. I'm not into the good tech talk using $.25 cent words. But when I switch to my magnetic loop on receive the noise floor goes down over 10dB and those little signals lost in the grass are now readable.

Thanks for the reminder link Steve.........3 inch pipe starts to be real nice. What I picked out of the QST article was losses. You need to make extremely good mechanical/electrical connections and you have to deal with distributed capacitance. That is why the Ham in QST made a 2 turn loop for his project.
And legal limit is possible. You need a 30kv vacuum variable......the only expensive part.......I'm looking at these other links......
EDIT....looking at Steve's link for one of our guys and his loop. The 3 inch aluminum pipe and big vac variables were a lot more than I thought for cost. And there were some complications to get it on the air.........
I have a nice big lot, but nothing high to hang a decent antenna for 160 or 75M.
Fred

Thinking whilst driving a school bus....
WHAT IF:
instead of using expensive metal for the loop.......make the loop FORM using 3 inch dia. Fiberglass or even thick cardboard tubes epoxied together keeping the 3 inch dia. in mind
WRAP the entire circular or octagon or rectangular Fiberglass form with overlapping 2 inch wide copper strap??
Overlapping will allow one to SOLDER the overlaps to make it like one continuous piece of copper. It would be lighter than using copper pipe or even aluminum pipe. The goal seems to be big diameter pipe and big size of the actual loop.

Please read warnings about RF exposure. These magnetic loops at legal limit can be lethal. Something about running a vacuum cleaner 200 feet from the loop antenna from the radiated RF
here's the link again: http://www.bulldogtrust.com/index12.htm

"I have a nice big lot, but nothing high to hang a decent antenna for 160 or 75M."


What do you have up for 160 now?

A mag loop is not going to give you any better signal than a dipole. An inverted-L with a good ground system may out perform the loop.

That's pretty much what K8NDS did, and I was thinking of doing the same. But when I priced all that, it cost (and likely weighed) more than 1" (actual OD is 1.25") type M copper water pipe. Two 10' sections plus the elbows is <$100 at Lowe's or Home Depot.

Having said that, perhaps your idea would work great for a ~3" OD loop.

Other ideas (which I haven't pursued) are paralleling two loops made from copper pipe, or using large Heliax.

Keep in mind that the bigger the loop diameter, the smaller pipe you can use for the same efficiency. You can see that by downloading AA5TB's loop calc spreadsheet and playing with the numbers.

I'm watching this thread with great interest.

The limitations of my current location prevent me from putting up a traditional antenna for the low bands. And I have 15KV and 20KV Jennings caps that are waiting for a good project...

My question is about doing multi-turn loops. I read the QST article this month about mag loops, and the author made it sound as if doing a multi-turn loop gives you roughly the same efficiency as a single turn, larger loop using the same amount of material. Is there any way to calculate this?

I'm curious about the multi turn loops also, and how does the feed change when the loop has more than one turn. If you're using a 1/5 size coupling loop, is that 1/5 size based on the diameter of the multiple turns, or on the effective diameter if the total circumference was converted to one turn? (my experiments suggest it's the former)

I think wrapping a flat piece of copper around a form and soldering the edges is likely to add resistive loss that you may not want, since the resistive losses play a very large role in the efficiencies of these loops. You may be ahead just using the flat material on edge as the loop material - some of the MFJ loops did that, using a band of steel for the element. I keep looking at 4-6 inch wide roof flashing as something to possibly make a low band loop with suspended inside of a small wooden cabin (or maybe wrapped around the outside). The aluminum is fairly cheap, the copper, not so much so.  :o

I've done some of my experiments using old hard line, both aluminum and some copper jacketed, as the element. Cheaper than buying and soldering up copper pipe.  ;D  Depending on the dimensions, you can use the capacitance between the inner conductor and outer shield  (about 27 pf per foot) as part of the tuned circuit. I have a 30 meter loop built that way, just needs a small trim cap to adjust the tuning across the band. That may work up to around 100 watts, for QRO you'll need the vacuum variable for sure.

I would still like to get my hand on one of those QMAC mobile loops that tune down to 3 Mhz and are supposed to be 10 db more effective than a loaded whip in that frequency range.

Fred,

Which loop did you install?  I have seen several commercial ones for sale.  Years ago, I built one designed by Doug Demaw W1FB (sk).  It was for 160 M and was 5 ft per side.  The loop was parallel resonated with a mica trimmer cap.  It was just used on receive.

Joe, W3GMS

Hi Joe
this was the PIXEL 3 foot diameter receiving loop with pre-amp and mount and control that will shut down the pre-amp and prevent your TX RF from coupling into your receiver. Their price jumped up from last year, to $400.........but very good sturdy construction.
http://www.pixelsatradio.com/product/shortwave-magnetic-loop-antenna/

Mike W0BTU, the helical winding has a purpose. It is not to reduce the weight. Please read on below.
If you QRZed K8NDS you will see links to his website and the youtube vids.
K8NDS, Rich, is using copper flashing 3 inch wide and helically winding as opposed to one long piece of pipe. This winding creates inductance and you need less C for tuning. It also lowers the HV. A 15KV vac variable and you're there legal limit.
The multi turn loop in QST was to try to distribute the capacitance. Rich thought that there were short comings to that design. He was leaving on a business trip and will return NOV 1st.
Here is his website for the helically wound loop and some YouTube videos of his antennas and actual on air QSO's.
http://www.hlmagneticloopantennas.com/

Fred

My idea was this:

Get a Childs hoola hoop at Walmart. 
Cut a 3 inch section out of the hoola hoop on one side. 
Buy some Tinned copper braid.
Slip braid over and around hoola hoop pull the braid tight. (Like chinese hand cuffs)

Mount to board or plexi.  Feed with Coax and use a simple Gamma with a 30Kv air cap.

You can get the braid at home depot or at Georgea copper:

http://www.gacopper.com/Braid.html

Cheap TV rotator, I have a homeade 15 ft tower I can Drag around the yard for experiments like this.  If only it was not 100 outside still.
 

C

Ed,

My lot is about 100' (E/W) X 70' (N/S). No trees that'd do any good and the roof of the 24' X 36' house has the Hustler 5-BTV in the geometric center for 80-10. The homebrew coil-loaded 160 dipole is about 20 feet off the deck along the south fence. It has lousy bandwidth, but I made S-9 into IA with it a while back on 100 watts SSB. I'm planning on getting an amp and I need something that'll take the power and give some decent bandwidth and coverage from a fairly low elevation.

ldb

I got kind of excited about that, too, until I saw that the two-turn loop in the 11/11 QST actually had significantly more loss than the single-turn one. But maybe I misread that.

Multi-turn loops with conductors too close together have the "proximity effect" issue. The conductor in the front may have a certain current value at a given point on that conductor; but the turn behind it next to that point may have a current that is quite different. This causes increased current in a given area. You can spread the turns out to reduce this effect, but the bottom line is that losses can be much higher than in single-turn loops.

I realize that, Fred. I've studied K8NDS's material about his loops extensively, and have his info printed and bound in a 3-ring binder along with a lot of other mag loop info. I know what he's claiming. I was just saying that I added up the cost of the copper and the pipe, and for a reasonable efficiency, Type M copper water pipe was not only cheaper, it was easier to build.

There's a lot of controversy about Rich's design. He wrote an article about his helical loop on eHam, if you're interested. The comments to his article there points out that his helical design may have additional losses, but I think that's a little overblown.

That would be fine for a receiving loop, but it would have way too much resistance for a transmitting loop. Small loops like this have been made from coax like RG-8, and they get extremely hot at 1500 watts. Almost all the power is lost in heating the copper braid. (Depending on the loop diameter, the loop current can exceed 100 amps.)

The reason coax braid is not a good choice for an STL is that the radiation resistance (Rrad) of an STL like this is in the milliohms, and the resistance of the braid can exceed Rrad of the loop itself.

That's why we have to use what might seem to be a ridiculously large conductor to have a loop with any kind of reasonable efficiency.

Even people who have made small receiving loops from, say, #16 copper wire find out that the signal level is waaaay down from one made from a much larger conductor. The Rrad is so small that the losses in the wire are enormous.

There was a good discussion on this at http://forums.qrz.com/showthread.php?311422-HF-magnetic-Loop-antenna-questions.

Interesting.  I can see why Rich used that thick copper strap!

C

Try this! ! !

I have worked Ted - K1QAR a number of times on 160 meters while he was using his TX/RX loop. His signal was comparable to other stations running a dipole. It's really amazing when you think about it.

Fred, I have been looking at that Pixel loop and wondering how much signal it captures on 160m and 75m. In spite of the improvement in SNR it often seems that small loops don't grab enough signal to establish much quieting with our mode unless the station you are receiving is quite strapping. Still, it might be fun to mess around with one on a rotator installed away from the house. I've played around with several homebrew tuned loops, but the preamplified and broadband nature of the Pixel makes it very interesting. Any chance you could make a video showing it in action as you tune a few bands?

Rob W1AEX

Thanks Fred for the response.  I was kind of guessing that is what you were using.  Like Rob, W1AEX said in a later posting, it would be good to hear some more of your analysis on its performance especially on 160 and 80M.  Maybe a separate thread is in order.  The video was pretty impressive but the signal in the video was very strong.  It would be nice to find a weak signal on the "wire antenna" and then switch to the loop and see what the overall perceived signal to noise ratio improvement was

I have never messed with an un-tuned receiving loop.  I guess for the lack of resonance he is just making it up with sheer gain from the low noise preamp.
 
It seems like as time goes on my receiving conditions just worsen.  I can remember moving in some 35 years ago and the S meter would be around an S-1 or so.  That certainly is not the case anymore and one could make a full time job of finding the noise sources.  We are out in the county and one would think that would help but not the case anymore.  Our little country general store installed some modern computer gear to scan prices as you checkout along with new computer terminals.  Ever since then, I get "buzzies" on 75 meters about every 15 KHz or so.  That issue alone even with diplomacy dial in could take major time and effort to correct.

I have established a good working relationship with our utility company.  I find the noise and let the guys whose job it is where the spots are.   

Joe, W3GMS       

I would think that resistive and or corrosive losses in the joints of a loop built of copper or aluminum tubing could be eliminated by welding aluminum tubing at the joints and silversoldering (silfos) copper tubing.....the way that these antennas use skin effect would seem to mandate low resistance joints...

Is aluminum Good enough?  I found aluminum loops.  I could have the fabricator at work cut one down and Tig weld fittings for me.  This would be lightweight, Cheap and easy to mount.

C

OK, I've been there and done that.  Living in an apt here in Greece I felt this might be a nice antenna to try.  I built one out of 2 10' sticks of 1" copper pipe, silver soldered at the top.  I used a motorized vacuum variable out of a Collins 180L aircraft antenner tuner to tune the loop.  It worked, but...

One problem is, if you use a sufficiently small loop, copper, and robust connections the Q is really high.  So, the bandwidth is extremely narrow; my 3:1 SWR points were about 10 KHz on 40.

Which brings up the second problem.  Because the Q is so high and the BW so narrow, the tuning was REALLY touchy.  The gear ratio on the 180L capacitor was way too fast, and reducing the DC voltage on the motor didn't help much either.  The thing would heat up a bit and shift its tuning, or the gear would slip a bit due to backlash, and I had to shuttle back and forth trying to find the sweet spot again.

Feeding it was another PITA.  Most recommend a smaller loop, connected to the transmitter and mounted inside the diameter of the antenna for a feed.  I dicked with that thing for the better part of a day with the network analyzer trying to find the best coupling, and then the feed loop would spring back or something and it would go to hell again.

Finally the thing was pretty top-heavy and kept tipping over.  I finally gave up on it.

I'm not sure aluminum would work all that well, Clark.  The resistivity of Al is quite a bit more than Cu and the difference might be a substantial portion of the total resistance so you'll end up giving up quite a bit in efficiency.  If you do build one out of Al, make it as as big as you can, given your target frequency.  If you make it too big, however, you won't be able to tune it.

Good suggestion Rob,
I got our little still digital camera and it makes very nice youtube videos, the audio will be a trick with its built-in mic. I'll post something soon. Thanks
Fred

That would be excellent Fred! I wouldn't worry too much about the audio as speaker to microphone will still give a very good indication of how it works. Thanks for taking the time!

Rob

Absolutely they need low resistance joints! :-)

However, I'm not convinced that silver solder would make any noticeable difference vs. ordinary tin/lead solder. Reason being, the difference in resistance between the best silver solder and regular solder is only about 15%. The very best silver solder I found had a resistance in the 20% range (copper being 100% and pure silver being ~102%).

I think the main thing is to have as little solder exposed at the joints as possible (think skin effect). If you get a lot on the outside of the copper pipe, remove it with emery cloth or a small abrasive wheel made specifically for removing solder.

It's not as good as copper, but it can be usable. There are various calculators for small magnetic loop antennas and some of them allow you to select copper or aluminum to see the difference in predicted performance. As I recall, aluminum ends up being about 40% as good as copper - mainly because of it's higher resistive losses in a circuit where milliohms count a lot. You can make up for the resistivity by using a larger size of aluminum or make the loop bigger.

For a legal limit antenna on the lower bands, probably not the best choice - but it may be a lot more available than copper. It may be that you can build a bigger aluminum loop than you can afford with copper and improve the efficiency that way. I've got a 30 foot chunk of aluminum 75 ohm hard line that is earmarked for a fixed 75 meter loop for a qrp setup at a remote location. The hardline was free, and long enough, and should be reasonably efficient because it's a larger loop. If I had to buy copper, I'd probably make it smaller to keep the cost down and lose efficiency that way.

There are a number of smaller loops (1 meter) used for portable use on the higher bands that are made from aluminum and apparently have respectable results. There are also some larger loops used on the lower frequencies, see http://www.standpipe.com/w2bri/80meter.htm

If you have some aluminum material available to experiment with, I'd say go ahead and get your feet wet. Then look into upgrading material later if needed.

Sounds good. That's exactly what I have in mind. Once I see how that works, I may build one that works on 80 and 160.

If your bandwidth was that narrow, that means you built it right, with very little ohmic resistance. I have a 50:1 gear motor that really slows down when you reduce the voltage; hopefully, I won't run into the problem you did. EDIT: If you look at K8NDS's YouTube videos, you can see that he did it right.

Anyway, how high did you have your loop, and were you able to compare it with another antenna?

The gear motor in the 180L tuner will go really slow too, but if you reduce the voltage enough there's not enough scrote to overcome the tension in the capacitor bellows.  Were I to do this again I'd put a relatively large fixed vacuum cap in there and use a smaller value vacuum.  Or HB a trombone cap, or a book cap, or something.

The loop is wholly unsuitable to tuning around the band, because you need to tune the antenna to receive anything at all if you move more than a few KHz.

I made a few contacts on it, but never compared it side-by-side to anything else.  It was mounted on my balcony, about 3' from the wall, on the 4th (top) floor of a cast-concrete apt building.

However my working condx aren't everyone else's, and this is a really simple and potentially quite effective antenna.  I certainly don't want to discourage anyone from trying one; just want to give everyone the benefit of my experiences.

http://amfone.net/Amforum/index.php?topic=12451.msg91347#msg91347


So has this one  ;D
http://amfone.net/Amforum/index.php?topic=4601.msg35203#msg35203 (http://amfone.net/Amforum/index.php?topic=4601.msg35203#msg35203)
Don't know what happened to my pictures (from 2005) though, will try and re-load them.

Found 'em  ;)

Wall-mounted 8' loop for 40m/75m:

(http://i1181.photobucket.com/albums/x426/DrCharlesMorris/MVC-253F.jpg)
(http://i1181.photobucket.com/albums/x426/DrCharlesMorris/MVC-255F.jpg)

This is the little (4') loop for 20m... the coax stub capacitor is fine at 100w but when I put the SB-200 to it, the dielectric caught fire  :o

(http://i1181.photobucket.com/albums/x426/DrCharlesMorris/MVC-254F.jpg)

Holy smoke! You ran an SB-200 into that thing inside the house? I don't think that's a best idea in the world.

Having said that, I once ran an SB-220 into one not any farther away from my operating position than you have there. :-)

This is all very interesting!

I was at Lowes yesterday, and found a roll of copper roof flashing. 3oz stuff. I was thinking that I could maybe make a loop out of PVC or even spa-flex, and wrap it up with a continuous piece of the flashing. The stuff isn't cheap at around $30 for a 10"X20' roll, but it's a heck of a lot more affordable than copper pipe around here. A 10' section of 1.5" copper pipe was over $100...  :o :o :o

LOL!

Magnetic loop Transmitting in the house.........Like Jerry said, not goodness. RF exposure is a LOT more than other antennas we typically use.
One of the links mentioned here said that he could operate hjs vacuum cleaner 200 feet from his loop...legal limit.
And Tim thanks for the heads up on the flashing....I'm looking more for a 20-10M loop now. My Yagi is a mess.
Fred

As discussed in the 2005 thread, there is a fairly deep null (20-30 db IIRC) perpendicular to the plane of the loop and I'm looking right through it... didn't seem to hurt me any, what were we talking about, oh look a chicken!  ;D

Why would exposure be more? It has less gain than a dipole, and the radiation pattern is NOT a narrow beam with concentration of the energy in any given small area (such as a microwave horn or parabolic dish antenna).  ???


I call BS on that one.  ::)

Don't try to jump through an energized loop! Remember the "Guardian of Forever"?

Dirty Copper!

I am having lunch today with kW2S Spence, who has his own antenna design company AntennaSYS. http://www.antennasys.com/design-gallery/

He apparently has a lot of experience on HF loop technology - I will squeeze him for info. Hee Hee.

That UHF Kraus helical design to the top right was one that I had him do for my company. Very nice.

Mike WU2D

I run loaded delta loops for RX 30 foot base 15 foot apex about 6 feet above ground. Load is 1K, feed side a 4:1 BB transformer. I have 1 going East and the other West. They give a good front to back up to about 15MHz so much wider frequency range than a K9AY. Each one needs a feed line so just have a selector switch in the shack.
TX loops are high Q so a pita to QSY. Also the field is not safe at high power levels. Much better off with a larger antenna if possible. 

"Why would exposure be more? It has less gain than a dipole, and the radiation pattern is NOT a narrow beam with concentration of the energy in any given small area (such as a microwave horn or parabolic dish antenna)."

An antenna produces a "near field ", that is non radiating, and which decays exponentially with distance from the antenna. Of course, it also (hopefully) produces a "far field" which propagates away from the antenna... and whose propagating E and H fields decay as 1/r (where r is the distance from the antenna).

In an electrically short antenna, the ratio of near field to far field is much higher than it is in a dipole.

Therefore, for a given radiated power level, the nearby fields (both E and H) produced by any electrically short antenna are much larger than the nearby fields of a dipole.

The reason it is hard to make an efficient, electrically short antenna, is that the relatively high near field levels produce much more heating of the surroundings (ground, trees, buildings, etc.) than a dipole does. Therefore a much larger fraction of the total power that leaves the antenna is converted to heat in the nearby surrounding objects.

Stu

Hi Stu
This copy/paste was from a link Steve provided from one of our AM guys who, built and is using his loop on 160M. This was his thoughts and technical response from the ARRL about magnetic loop radiation. And his actual experience. Running legal limit carrier.
back to the original link from Steve

http://www.bulldogtrust.com/index12.htm

DEATHRAY

Work with the ARRL's engineer in charge of public RF exposure revealed that this particular magnetic loop's danger zone is 30 times larger than a dipole's, or about 30 feet at legal limit power. A wrought iron table 60 feet from the antenna will pick up enough energy to make a visible arc to ground; At 200 feet, field strength spontaneously starts and runs our vacuum cleaner at a speed proportional to the carrier power. You don't want to even think about what it would do to a pacemaker.

  Ok, Good discussion. The strong H field is something to respect, and I can see some transformer action going on between the loop and other metal things that could masquerade as a transformer secondary. Still, I am having trouble with that vacuum cleaner comment. These use motors that are universal like an electric drill where the brush wiring is in series with a field coil. To complete the circuit wouldn't the vacuum power switch need to be ON, and the power cord terminals shorted out? Then to make DC we need rectification somewhere in the loop. Unmodulated RF at 160M or 80M shouldn't do anything to a motor like that.

   I am sure I must be missing something. Help me out guys. How does the RF field couple power into that vacuum cleaner motor? Maybe some of the new vacuum cleaners use an electronic controller where rectification from RF is possible? This is like Tesla lighting incandescent lamps from far away.

Jim
WD5JKO

He's one of us. The guy is an Am'er and some here have worked him. I do not know his call...may even be part of AMFONE.
The vacuum cleaner might be intersting.

This guy in the UK makes a 1/4 wave wire loop and feeds it with a transformer, He even patented it. I may mount something like this on my trailer for a camping antenna:

http://www.g0cwt.co.uk/magloops/new_page_6.htm

haven't tried it yet myself, but it's on my todo list

Dave, KF7JAF

Darn I saw the antenna that Spence talked about and all I can say is wow. He swore me to secrecy. He wants to write it up so we will have to wait. I can say that it solves several problems including remote tuning and uses no commercial parts other than a small motor - it is all plumbers delight construction.

Mike WU2D

I just looked at this topic again.  What's up with all the interest in teeny loops and QRO on low bands?

This is like mounting a jet engine on an old British Mini and using it to try towing a house.

Welp Rob,
The stopping point for a loop was taking QRO. Buy a big 15KV vac variable and problem solved.
For the guys on small lots, the loop might put something on the air that would be pretty respectable, besides the Ham op trying to squeeze loaded antennas in a small space.
There is an AMer who uses his magnetic loop (K1QAR) and signal reports from those on this BB have said that it is a very respectable signal for a small loop.
My tri-bander Yagi just crapped out on 20-10 and I am going to build a magnetic loop to take legal limit for those bands.
I cannot  afford to keep hiring a rigor for $600 to take the antenna down for me to re-re-inspect and re-measure the traps and elements. The boom is probably full of water. The last time I had it taken down there was a waterfall from the boom after I removed the end caps. Shudda left them off and mounted anti bug screens instead.
PLUS on receive Magnetic loop antennas are a much quieter.
Hams in Europe use magnetic loops a lot.

Fred

Okay Fred FB; sorry for the misunderstanding.  I had some idea you were focused on 160, and 75.  High bands completely different.

OK Rob,
80 and 160 will suffer with very small bandwidth. But for the guy who is in a bind to get something on the air on 160M would be better off with a magnetic loop. There are costs involved for motorized tuning for the vac variable and for legal limit AM he/she would need an expensive vac variable capable of 25KV and 100 amps.

I use to work Ted, K1QAR on 160 several years back.  I believe he changed to a different QTH and I have not heard him on since.  When I worked him, he was using a small loop for transmitting.  It was up in a tree and worked pretty well.  He was never super strong down here but most times a good Q-5 copy. 
Joe, W3GMS