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Author Topic: Magnetic Loop antenna for HF  (Read 10985 times)
2E0BSS
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Posts: 85




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« on: February 03, 2008, 03:38:17 PM »

I've been reading through the forums regarding antenna styles for restrictive areas. I have one of the worst I have no garden as I live in a block or semidetached flat on the ground floor. My council are really restrictive too as they are working on CB regulations with complete avoidance of Ham users.
Currently I use a half size fixed G5RV with the ends pointing downwards to fit on the buildings roof so my transmitting is limited to say the least the antenna sides are North/North East with both ends pointing to ground

Would a magnetic loop be of benefit? also is there anyway to make on multi band without getting massive? I've found one for 80 and 40

thanks
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KD0AFK
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Posts: 245




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« Reply #1 on: February 04, 2008, 09:50:02 PM »

Actually a magloop is by definition less than a 1/4 wave so it can be as small as you want to make it. Realistically though it performs the best when it is at 1/4 wave which means making an 80 meter loop 20 meters in circumference. 6 meters in diameter would be the size. I have seen them all different sizes. Are you running QRP or higher wattage? The advantages of a magloop is that although it doesn't perform better gain-wise than say a dipole, it has a better takeoff angle at lower elevations so you don't have to hoist it as high as a dipole. also, it is quieter because it only picks up the electromagnetic portion of the radio spectrum, effectively filtering out the noise made by car ignition systems, street light and such.
I hope this helps. I am also looking in to making one of these and all of the info I gave you is from memory so please anyone, correct me if I am wrong on any points.
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KD0AFK
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« Reply #2 on: February 04, 2008, 09:51:23 PM »

have you thought about a vertical dipole hidden in a flagpole?
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N7HTS
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« Reply #3 on: February 07, 2008, 11:45:54 AM »

I recently built one.  It is octagon shaped and about 132 centimeters in diameter.  I'm using coax cable as a capacitor.  So far, I've been able to tune it on 20, 30 and 17 meters using different length of coax.  I am feeding it with a Faraday loop.  It's very narrow banded but, it seems to work quite well.  I will soon be homebrewing a butterfly capacitor for it.  I have limited space in my backyard and I have vertical antennas but, they are very noisy.  The reason I built one was to help lower the noise level and it does.  I've compared the signal strength on receive with my verticals and sometimes it is much stronger with the loop and other times it is not as good.  I have it about 45 centimeters above the ground on a PVC stand.  I'm going to try to elevate even higher.  I would say that it is well worth the effort to build especially if you are in an apartment.  The only drawback is that it is very narrow banded.      
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KL7AJ
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« Reply #4 on: February 08, 2008, 12:09:31 PM »

Magnetic loops have to be built VERY robust to work effectively, since you will have massive circulating currents.  You need a tuning capacitor BUILT for this...i.e. a butterfly cap, and FAT tubing.  But, it's certainly worth a shot.

Eric
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JDWYER
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« Reply #5 on: February 09, 2008, 04:14:59 PM »

I build a magnetic loop and it was a dog. Total waste of money. Unless you have an expensive capacitor there are high losses. They are hard to tune and must be retuned every few kilohertz. They receive better than they transmit. The one I build needed about 20 watts out to make QSOs on PSK-31. I can do the same with 5 watts using a dipole or compromised vertical.

John
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JDWYER
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« Reply #6 on: February 09, 2008, 04:19:03 PM »

Also you are exposed to a lot of radiation if you use it indoors. Things to think about.
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N3OX
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« Reply #7 on: February 10, 2008, 09:55:02 AM »

"I build a magnetic loop and it was a dog. Total waste of money. Unless you have an expensive capacitor there are high losses"

You gotta do it all the way, mine actually works great for something its size but does have a $200 vacuum variable in it.

If you don't have space for anything bigger it's a GREAT antenna, I think, if you spend the money for the copper tubing and good capacitor.  If you do have space you should do something bigger and more tolerant of construction practices!

Max Gain Systems has the best prices on vacuum variables that I've seen, by the way.

I'm sporadically working on some ultra-cheap magloop ideas but it hasn't quite come to fruition.  For a fixed frequency one I think using a single piece of aluminum flashing to make the loop and a parallel plate capacitor has some promise, but it's not easy to retune.  That's what I'm working on, but not very hard. ;-)

http://www.n3ox.net/projects/magloop/magloop1_lg.jpg is a picture of my copper + vac. variable magloop

73,
Dan



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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KB7NRK
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Posts: 6




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« Reply #8 on: February 12, 2008, 08:46:06 AM »

In the early '90s '73 Magazine had an article for an indoor loop antenna for QRP. It looked like a vintage spiral antenna loop. The guy hung it from the ceiling. It was very simple, just the loop, a variable cap and lead with alligator clip to short the loop for different bands.

A compromise?... You bet but it did work. I only wish I could remember the month and year.. I thinkit was 1991.

John
KB7NRN
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KL7AJ
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« Reply #9 on: February 25, 2008, 11:25:28 AM »

Dan:

   Same deal with the DDRR.  If you look at the ORIGINAL plans, it uses muffler pipe for the ring...and if you DO that, the antenna actually sort of works...kind of....a little bit.

   A DDRR made of WIRE is worse than a dummy load, though.  At least on 160 meters...HI..

eric
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VK4AMZ
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« Reply #10 on: August 10, 2008, 03:19:04 AM »

Hi, if you look up VK4AMZ on QRZ there is a link to my small website with a bit of info on my Loop for 80m and 40m. 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.
The loop nulled this into the noise. A good TX loop is not easy to build, this is why they don't get good reports. People THINK that they are simple to build, they are NOT. The construction and tuning setup are critical to qet LOW loss on Tx.
 
Tiny loops are just that, tiny aerials. A 13ft Dia loop is something else. It rocks on 80m and 40m. It also works on 160m (Very quite RX)  
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N3LCW
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Posts: 133




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« Reply #11 on: August 19, 2008, 08:40:13 AM »

Contrary to popular belief you do not need to build a bulky and expensive copper loop IF you make the loop 1/4 wave length long and feed the loop NEXT to the tuning capacitor.  The impedance then becomes manageable, around 22 ohms, and you can use regular materials.  I recommend you visit G0CWT's web site ( http://g0cwt.co.uk/magloops/ ) for loop description and follow it allowing for customization to fit your available space.

I built the 80M version using a square loop with 10ft vertical and 22ft horizontal sides, vertically mounted using camouflage tent poles as supports.  I built the 3:2 matching transformer as he described and I used an 11 inch (approximate) length of RG8 coax as a tuning stub capacitor, with an extra 1/2 inch braid removed and coated with liquid tape to prevent RF arc overs.  The antenna 1:1 SWR at my Saturday morning net frequency and everyone gives me excellent signal reports with my 100 watts of power.  If I can hear them I can work them, very effectively.

Review the loops on Ben G0CWT's web page and use that to start with.  I have since built similar smaller loops for 40 and 20M with the same amazing results, using only 14 gauge wire.  Again, since the loops are 1/4 in circumference there is no need to worry about losses as you would with a compact loop.

I highly recommend using an antenna analyzer like the MFJ 259B, it makes tuning and matching much easier.

If space is very limiting you can make the 80M loop half the size but make it a two winding loop spacing the turns 4 inches apart.  You will also need to add a turn or two to the primary winding on the matching transformer and trim the tuning capacitor.   In this case you would have a 5ft by 11ft loop that would fit any most any small space.

BTW, these loops will outperform any mobile or coil loaded antenna since your losses are at a minimum.

Andrew
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VK4AMZ
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Posts: 7




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« Reply #12 on: September 04, 2008, 09:14:52 PM »

It has been known since forever that you can build a loop a 1/4 wave in length and not suffer the same loss as a 1/8 small TX loop. However the loss will be more in a wire loop. A copper loop at 1/4 wave is around 90% efficient so dropping the efficiency to 50% by adding loss resistance will hardly make an S-point difference.

It is also known and stated that the way to increase bandwidth is to add resistance which lowers the Q of the loop. This changes the ratio of radiation resistance to loop "LOSS" resistance. Matching impedance is not an issue even in a small loop. I had no trouble getting a 1:1 match in about 20 seconds using a gamma match. There is no extra loss by feeding or matching at a low impedance point.

However there is a change for the worse in radiation angle by feeding at the cap.

Q. How do you rotate a wire loop?
A. You build a frame, almost the same frame as a copper loop.

The need to null out local noise is one of the main reasons a lot of hams try a loop, so making it rotate is important.

I built a copper loop because I wanted one loop to work on 3 bands, 40m, 80m and 160m.

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N3LCW
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Posts: 133




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« Reply #13 on: September 04, 2008, 09:38:37 PM »

For a 1/4 wave loop fed next to the capacitor the feedpoint resistance is near 22 ohms, much closer to the coax cable 50ohms.  This is verifiable by use of an antenna analyzer.  Since you are dealing with 22ohms vs .01 ohms you can most definitely get away with using wire instead of copper tubing and not worry about solder welding connections.  The efficiency will be much greater than that achieved using a 'compact' magnetic loop, which is the point of my original post.

I have built many of both and yes, a compact loop has its place especially for multiband use in severely antenna restricted situations a 1/4 wave loop will outperform a compact loop in all cases based on efficiency and aperture alone.  I have confirmed this with over-the-air A-B switch test cases this past year and my findings backup those of Ben G0CWT.  

I am willing to sacrifice convenient multiband single loop convenience for a simple to construct, easy to erect mono band magnetic loop that will deliver maximum signal.  Even I am amazed how well my 10ft x 22ft 80M loop performs from my postage stamp back yard.  I have gone from one of the weakest stations on on my 75M net to having one of the best signals, and I have made believers of magnetic loop skeptics.

I still use compact magnetic loops for 20M and higher, but for the lower HF bands the 1/4 wave wire loops have been superior.

Andy N3LCW
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VK4AMZ
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Posts: 7




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« Reply #14 on: September 04, 2008, 10:42:13 PM »

Just because you feed the loop at a Hi voltage/ high impedance point does not mean that you can dismiss the current flowing through the loop, it is still the same. If you move the feed point to the opposite side of your wire loop are you saying that the current through the loop is now less for the same output? More current through the loop = more signal. Regardless of what you make the loop with. I was comparing a 1/4 wave loop of copper compared to a wire loop, not a 1/4 wire compared to a 1/8 wave length copper loop. Do you think that using wire with more resistance in an aerial gives you more efficiency?  A loop is no different to other aerials when it comes to antenna theory.
Mike.
 
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