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Author Topic: Magnetic Loop antenna for HF  (Read 11812 times)
N3LCW
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Posts: 150




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« Reply #15 on: September 05, 2008, 08:10:44 AM »

I believe everyone in the group already knows that a heavier conductor will obviously be more efficient.    That wasn't the point of my original post.
I modified a 40M 28 ft circumference 1" copper pipe loop by shortening it to a full 1/4 wave for 30M and changed the gamma match to a toroid matching transformer.  Moved it to the attic and it is now a  dedicated 30M loop.    More efficient than the 80M loop?  Obviously.  For my purposes re-plumbing the 10ft x 22ft wire loop to 1" copper would not be worth the potential gain nor practical at this location.

More amateurs would try magnetic loops if construction was  easier, regardless of the trade offs.   I doubted G0CWT's claims at first but it took me less than 2 hours to build and install his 80M loop.  The on air results convinced me of its relevance.  
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VK4AMZ
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Posts: 7




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« Reply #16 on: September 07, 2008, 10:18:59 PM »

I agree that when making a loop aerial a 1/4 wave in length that the effort and trouble in using copper over wire would not really be noticeable in on air tests. It depends on (I think we both agree) what you want your loop to do. A 1/4 wave wire loop would be a great way for someone to try a loop and see for his or her self that a small loop (less that a 1/3 wavelength) does really work.  If they then want a loop smaller than a 1/4 wave or a multiband loop then they can follow on from there knowing that these things are not the dummy loads that others portray them as. As you and I have shown, they can give fantastic results.
Mike    
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N3LCW
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Posts: 150




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« Reply #17 on: September 16, 2009, 05:56:41 PM »

Mike,

It's been a year and how fast that went!  BTW, I am MOST impressed with your website and 160M loop construction and performance details.  

This past year I dared to work top band from my postage stamp QTH and modified the 80M loop feedpoint toroid matching transformer.  The loop is 1/8 wave at 160M and by matching the 50ohm feedline to the 2ohm impedance and adding a 150pf HV doorknob cap across the remote tune capacitor I can tune across the entire 160M band (in fact I can tune down to the 1700Khz MedFer experimenters band for receive, but that's a topic for another day).

If you lookup my call on QRZ.com you will find a photo of my loop.  As low as it is to the ground (I need to stay stealthy and below the tree tops) the performance has been spectacular confirming your findings with your loop.  Although QRO here is 100w, I hope to come up with an impedance transformer that will allow me to run my amplifier, at least 400-600watts.  The current impedance transformer (T240-43 ferrite, 24turn primary, 5 turn secondary) starts heating over 200watts.
I have not noticed any heating in the doorknob capacitor yet and frequency stability has been good.


Stations I work consistently commend my signal for such a small and low antenna.  I am consistently S9 - 10db/9 to stations over 1000 miles out or more.  Closer in my signal has been near or equal to that of those with full size dipoles installed at less than optimum heights.  Received signals are always far above the background static crashes, which max out here at S2-S3, 4 if a storm is near.  Unfortunately, others on frequency experience static levels of S9 outside of the winter months.

If I had the opportunity I would duplicate your loop design and mount it over my screened patio room, but the trees need to grow taller a bit more before I can do that and safely hide the loop.

Kudos to you and your efforts, if others tried these loops I believe 160M would start to become much more crowded than it is!

All the best!

Andy N3LCW
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PA3GMP
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Posts: 75




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« Reply #18 on: October 25, 2009, 04:25:50 AM »

I have long been in exactly the same situation as you describe. Having been an apartment dweller for decades, with little or no space available, draconian restrictions imposed by a body corporate somewhere, and atrocious levels of QRM, it has been Ham Hell.

I have found the magloop to be the best solution for those particular situations. Not only is it small yet more effective than any other small antenna of the same size I have tried, but also it is much less susceptible to local QRM, and since it is so narrow in bandwidth most of the QRM nonsense gets filtered out even before it reaches the front end of your rig.

However it is NOT a miracle antenna. It is less efficient than a dipole, for example. That said, it is infinitely better than a dipole that you cannot put up due to QTH-restrictions!

Magloop antenna's can be bought commercially (MFJ have one and there are other suppliers as well) but they are not cheap.

Building it involves a fair amount of mechanical work. If you cannot obtain a good variable capacitor (a vacuum cap is ideal but a conventional butterfly model will do as well for lower power levels) you have to make the capacitor yourself. If you can get a tuning cap off the shelf, you still need to make the actuator (motor, reducing gear box, drive electronics etc.) that enables you to tune the antenna remotely. So be prepared to get mechanical here.

The breakdown voltage of your tuning cap will limit the maximum power that your antenna can handle. Also keep in mind that close to the antenna there will be a strong magnetic (!) RF field, which is not too healthy, and can induce QRM in nearby conductors, so if your magloop is closely surrounded by your neighbours' TV sets, you'll want to keep the power down to a reasonable levels.

Keeping these limitations in mind, a magloop will get you on the air and pulling in stations better than any other small antenna I have tried. So if I were you, I'd go for it!

Good luck,

73 de Frank ZS6TMV / PA3GMP
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W8JI
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Posts: 9296


WWW

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« Reply #19 on: October 25, 2009, 03:07:31 PM »

<<This loop is up to 30db better than a base loaded 40ft Vert on 80m. I had NO room for any other aerial until I tried this loop. The 40ft Vert had 3 x 33m radials and was mounted on top of a 6m x 9m shed. I had a local carrier on 80m at 20 over 9.>>

Your vertical must have had terrible efficiency for a loop to be 30 db better. Perhaps .03% or less unless you compared the peak of the loop with the pattern null of the vertical.

Tom
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SV9SK
Member

Posts: 1




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« Reply #20 on: November 25, 2009, 10:25:47 AM »

By the way, me have also constructed the Ben G0CWT antennas on 80m, 40m, and 20m bands...

   I have to say that they perform really very well...
For example, i had a 1-hour solid copy ragchewing ssb qso on 20m band with 4 greek-speaking stations from USA, Germany  and Canada, one year before, using my FT-857 on 50W....It is still difficult to believe it myself yet today, but the hand-written logbook is still in the shack....
   Same good results on 40m band, working in night-time stations from JA, VK and YB0 that same period...

   Note, that i have constructed and used conventional copper magloops, and have understood and enjoy their advantages....

73 to all of you
Sincerelly yours
Alex, SV9SK
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VK4AMZ
Member

Posts: 7




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« Reply #21 on: July 05, 2010, 04:29:35 PM »

<<This loop is up to 30db better than a base loaded 40ft Vert on 80m. I had NO room for any other aerial until I tried this loop. The 40ft Vert had 3 x 33m radials and was mounted on top of a 6m x 9m shed. I had a local carrier on 80m at 20 over 9.>>

Your vertical must have had terrible efficiency for a loop to be 30 db better. Perhaps .03% or less unless you compared the peak of the loop with the pattern null of the vertical.

Tom

Hi Tom and apology for the late reply. Yes sometimes the meaning get lost along the way, there are two lines further on that read:

“By as much as 30db on RX and TX, local and interstate.
The skip zone due to radiation angle, polarization and efficiency all come into play in various ways”
And:  “stronger on transmit and receive to horizontally polarised and high angle signals and is always at the very least 6db stronger on transmit and receive”

I agree totally that the efficiency of that vertical was compromised: Short radials could be run mainly in one direction only due to the aerials location at the rear corner of the block – the only location it could be mounted and I have very poor ground here. I will go even further and state that if I could get a dipole up at a good height the SN ratio of the dipole would beat the loop, after all the loop is down low and close to all this noise. However one would go a long way to find something that could null the +20db noise from various power polls and unknown sources into my S6 background noise and put out the signal on 160m that I can within the limitations imposed on me.

It is so unfortunate (sad) that most Hams, at least over here, are being confined to every decreasing sized allotments ever increasing noise pollution and more height and size  restrictions  from our “all controlling” local and state authorities. It’s two years on and the performance of the loop has not changed. Smiley
Mike
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