Magnetic Loop Aerial BVWS article

[Radio Fixer]

Thought I would post this article: a few may be interested.   After all the seed was sewn here.  It appeared in the BVWS Bulletin a couple of issues ago so I'm not taking any limelight from that publication which I don't like to do.  Hopefully it will be of more interest than the last article I posted.


  TheBulletin_Magnetic Loop article.pdf (Size: 4.98 MB / Downloads: 308)

Gary

[radioman]

Thanks Gary, a very useful and well developed project.
Also, you should be congratulated for such a well presented and professional article.



Andy

[Radio Fixer]

Thanks Andy

I put a lot of time into it and did enjoy it;  success is always encouraging.

Haven't heard of anyone making one yet but ignoring the article caveat,  IMHO its the best relatively low cost improvement to old radio listening that can be made.

The "My comments" should have come after David's evaluation and "he" should be more polite and use his name.  Being a Proof there may be a few more typos in there but the gist of it is OK.  Other than that I was very happy with the article presentation and thank Carl for that and all the many excellent other efforts he has made on my behalf.

If anyone does decide to make one, with hindsight, I would go with a 4m loop as David did and mount it remote away from household mains.  I wouldn't bother to make the matching transformer (or its attempt to match) as it works just fine for me without it.  Simply removes another box behind the radio.

Gary

[Murphyv310]

Hi.
Of course when I did the original conversion to my Wellbrook loop some years ago the main reason was the cost of Wellbrook's replacement amplifier that now stands at around £100.
Even with the two transistor balanced amplifier it performed as good if not slightly better than the Wellbrook did originally.
Eventually I had to make a more watertight solution and improve the circuits rejection of strong signals (I have an amateur 100 meters away) so as the loop would work or more importantly be reliable.
The two transistors need to be driven fairly hard and I eventually found the 2N2222A to be beefy enough for the job. Although I don't particularly like diodes back to back across the loop I decided in my case the advantages were greater than the disadvantages and that was the route taken, it has helped the intermodulation when the neighbour is transmitting, we now though work together in times on and off which suits us both fine.
TBH the first stage of mine & Gary's amp has been floating around the NET for many years, mine is a bitty different with different matching, values and transistors and seemed to be working better than the original circuit with gain up to 100mhz, and even this year it has given me some Band1 Spe reception form Russia and other countries on analogue, it's dying though.

[uvs300]

I'm looking forward to reading the article..

[BusyBee]

I think I have seen this one before when researching loops, but have concentrated on internal multi-turn frame aerials so put it to one side. It does look well put together though and worth a more detailed look. Thank you for the post Gary.

Tracy

[Murphyv310]

Hi.
Well I have to say my modified Wellbrook just goes on and on.
I have recently bought an SDR Play and I'm going to do some noise tests on the loop, it's actually looking good and plus the fact I have usable results up to 100MHz which is to my mind excellent.
I have thought about adding in emitter followers to the original design but although matching could improve there is always the possibility of crossmod and noise.

[Crackle]

Hi Gary
Are there any problems with directional nulls, where some stations may be in a blind spot, or is this as good as a high and remote long wire for receiving stations from all round.
thanks
Mike

[Murphyv310]

Hi Mike.
The directional nulls should be very sharp, even a few degrees of rotation should bring back a fair increase in signal level.
Here I can receive Radio Caroline on 648KHz quite easily but the adjacent frequency can cause a lot of problems. Now I have two ways of removing side-band splatter.
1 is to carefully rotate the loop to null the Spanish station, this works well with only a small drop in the wanted signal.
2 is to use the SdrPlay receiver and trim off the offending side-band from the Spanish station.

The whole idea with a magnetic loop is the use of the balanced amp, without it being balanced there would be no way of rejecting local noise and the nulls would go or are least be much reduced.
It is important to use close tolerance components in the build, a neat layout and if possible transistors that are as closely matched as possible. Toroid transformers must also be carefully wound with matching turns and gaps, its not hard but attention to detail really can improve performance.
I firstly fitted my two transistor amp about 10 years ago and improved it around three years ago with a better sealed box and intermodulation protection.

[Murphyv310]

Gary's article is very professional indeed. One thing I did consider is a balancing pot on the emitters, whether this would bring down noise rejection is debatable but perhaps worth a try.