Magnetic Loop Antenna

Am interested in making a magnetic loop antenna as an experiment to see if it helps with local RF noise.

Anyone else used or made one?

Magloops are still susceptible to interference, albeit not as bad.

The biggest downside with a mag loop is how narrow banded they are. Literally 20-30kHz so every time you change more than a couple of channels you'll need to retune the antenna.

Here you go mate,

You may want to take a look at this video from Mike-M0MSN.

I have made a couple of this guys antenna projects and they work well.

https://youtu.be/GvqqkE82cBg

This is a link to the parts list.

https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm


Regards

Swadeyboy

Here are some construction photos of my Mag Loop stealth antenna for the attic.

1) A choke balun in the feeder cable prevents RF from flowing down the outer surface of the co-ax screen.
    12 turns of RG58U wound onto an Amidon 240-61 ferrite ring.

2)  The feeder loop is one fifth of the diameter of the main loop. It's made from heavy duty stranded wire and is held firm by some stiff foam blocks and hot melt glue.               
      
 
3)  A plywood bending jig is marked out with a piece of wire and a nail.
      

2 cuts are made in the plywood on the intended inner and outer diameters of the loop.

The soft annealed half inch copper pipe is gradually bent into a circle with a sash cramp. The outside diameter is 70 CMS, a nice manageable size with good efficiency.

A supporting plank of wood and a receiver are made from scrap timber.

A heavy wooden base is made from scrap timber.

A cheap Chinese geared motor turning at 3 revs/min does the remote fine-tuning adjustment.

The main fixed 9pF capacitor is fastened to the ends of the copper loop, and bolted to the posts of the Eddystone 2pF variable cap. It's a split-stator construction, with the two sections wired in series for increased voltage rating. The capacitor and motor setup is mounted on a polypropylene chopping board. Cable ties are used to hold the loop/cap/motor assembly in place.

Fine adjustments are made to the capacitance of the fixed 3mm hard copper plate capacitor by bending it over bit by bit with a block of wood and a hammer in a vise. It was made purposely too big to tune it slightly lower than 27 MHz. This part of the job was very troublesome and time-consuming.

The second shot is a side view of the connections between the loop, the fixed capacitor and the Eddystone variable. The spacing of the caps is 4mm for the fixed plates, and 2+2 for the variable. The connection between the loop and the fixed copper plates was eventually soldered. The voltage rating at 4mm spacing comes out as 7200 V peak on the ARRL handbook calculator.

And the last photo, below shows the loop in situ in the attic with the feeder loop held in place by some duct tape until a final shape adjustment is done. By adjusting the shape it's possible to get the VSWR right down to 1.0 over the entire 11-Metre band.

Excellent work Max!

I do like to see a bit of homebrew construction
Your photos through the process are great as well as your accompanying text. A big thanks to you for sharing your work.

I've played around myself with magnetic loops in the past and surprised with their excellent performance but found myself frustrated at the small bandwidth. However, recently utilising FT8 digital modes on HF has made me rethink them as they would fit the bill perfectly.
Your photos and this posting has given me the incentive to dig out that big circle of copper pipe I bent to try it out again!
(I used soft annealed thin pipe and slowly bent it by hand but love your plywood clamp method.)


Just a quick question.....How are you tuning it?

I only ask out of interest as I went through the process of tuning through a painful process. First was the antenna analyser method, (PITA), then SWR method, (a rude & crude method!), then using a RF noise bridge, (again a PITA method), before realising that they have such a tight performance that they can be simply tuned by "ear"!

It'd be great to hear your methods.


Again, awesome work. Well done mate.

All the best,
Victor

@Victor

Thanks for your appreciation Victor. I was hoping it would inspire some members to have a go. To answer your question about tuning:  First things first. I built a small variable 12 Volt power unit that goes from 12 to 3 Volts, which enables me to turn the Eddystone cap from 3 revs per minute down to an incredibly slow crawl. The PSU output has a double pole two-way switch which reverses the supply to the motor. This gives very precise tuning when I come close to resonance. My tuning indicator is a RigExpert AA-200. I leave it switched on and set to it read VSWR (key no. 7), and when I want to QSY, I switch the antenna over to it through a co-axial switch and quickly change the test frequency to the new one, and then operate the variable PSU, first at about 8 Volts, and then slow it down with about 4 Volts which goes up to the attic on a long length of audio cable. I've practised a lot and can do the whole operation in 10 seconds. The secret behind the tuning speed is that I'm only adjusting a 2 pF variable cap, which covers just 1.5 MHz, enough for the 2 main CB bands.

I eventually replaced the broken ceramic end-plate of the Eddystone cap. I cracked and broke it by overtightening the copper-plate mounting bolts.

I'm happy to answer any other questions that members may have.  Cheers, Max.

Cheers for that Max, much appreciated. Some great info for all members reading.

I used small coax lengths as high voltage fixed capacitors, (trim to suit), and played around with "book" style plates for the variable capacitance. I may try a worm-wheel motor out inspired by your setup. In the mean time getting one working on a fixed frequency for FT8 work would be just the ticket for me.

Well done again.

73
Victor

hi i have built a receiving loop for am medium wave reception with a tuning capacitor  it works well for its size thinking of building a long wave frame/loop along the same lines but bigger but not to big for the living room thanks for looking have a great christmas stay safe do radio 73s paul m0kbd

rabbi wrote:

hi i have built a receiving loop for am medium wave reception with a tuning capacitor  it works well for its size thinking of building a long wave frame/loop along the same lines but bigger but not to big for the living room thanks for looking have a great christmas stay safe do radio 73s paul m0kbd

Yes, making a frame loop would be good for size reduction, but many turns for Long Wave. The Magnetic Loop that most would be familiar with is the ferrite rod antenna in Medium and Long wave transistor radios. It was (is) usually wound with Litz wire to reduce the resistance at RF due to Skin Effect.

Thanks for taking the time to write that @Ethergoddle, great write up and some good ideas in there, will post mine when I get round to making it.

Regarding the capacitor, I saw on a video that the person used a 40pf variable capacitor.  I'm planning on a 2m loop and I already have a capacitor that is 25-75pf apparently.  However it sounds like you have less capacitance in yours.  Is there a handy calculator for this?

@cube

Hi Quentin. I presume you are making one for operation on 11 Metres. A 2M diameter loop is far too big for that band. Here is a calculator from the ARRL which works fine for diameter calculations, but has been proven to be inaccurate for all other parameters.  

https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx

Here is a comparison of all the available calculators:

https://owenduffy.net/blog/?p=1693

As you will see the RJELOOP1 seems to be the best, and I've found it to agree with my own experiments. The problem with it is that it is a DOS programme, and needs a 32 bit Windows computer to work. 

Did you see this article by VK5KLT?

https://www.on6yc.eu/pluginAppObj_312/article-antenna-mag-loop-2.pdf

I'll answer any other questions you may have about loops in general or my own construction. Max.

Thanks @Ethergoddle.  Yep, this is for 11m.  Just to clarify, am planning on a loop with 2m circumference as opposed to diameter, a bit under 1/4 wavelength. Will take a look at RJELOOP1, will try running under Windows command prompt.  Thanks again,

hi i built  with the help of sarah my wife a frame antenna with 100 meters of thin insulated wire wound on a square frame  sarah did the winding with a variable capacitor it works great on lw and with a datong vlf converter excelent  directional receive  stay safe do radio paul m0kbd 73s

Just picking up on this now I've had a bit of time to make the loop.  And passed my Foundation exam (yay) so can try out 10m and 12m too when I get this sorted out.

Btw @Swadeyboy, thanks re your post above, I missed it originally. I separately found Mike M0MSN, he's been doing some great stuff with mag loops.

So I've just been testing for receiving so far and don't have a choke in place so it's not usable at the moment for transmitting (v high SWR, guessing as a result of no choke).

Still plenty of noise (arggg).  But surprised I've received anyone at all from inside my shed with loads of stuff around the mag loop, which I have!  Including someone who I think is about 30 miles away.

I saw above that Max used a 61 mix ferrite ring for his choke, but from what I've read a 43 mix is more suitable.  Looks like Max has left the forum.  Anyone got a view on that?

Thanks.

what the different between Copper pipe loop and Coax cable 
the perform is it better performance then all the best