The Thin Skinned Magnetic Loop
As part of a larger experiment two members of the Paisley Amateur Radio Club, Stephen GM0FFB and Alan GM4FLX have been exploring and building a new form of magnetic loop.
Steve & Alan
Stephen is a World class CW specialist able to compete at 60wpm and has built many scores of magnetic loops using copper tube. Alan is an inventor and trouble shooter who had never built a copper tube loop to be a radio aerial but had done a lot of copper tube work in all sizes up to 1.5 inch diameter and has the necessary bending tools and springs
Because the thinking in October 2012 was that they would need two loops each and that the maximum power used would be 3 Watts, they decided to look at the loop itself as a QRP project. Three years earlier Alan had formed solar tunnel support frames by heat bending PVC tubing and many years earlier had used copper foil when wiring a doll’s house for his granddaughter. Thinking about how the electrons flow at the skin of the tube led the two men to set about layering copper foil left over from the doll’s house onto curved PVC tube. This proved easy to do and they bought supplies of 20 millimetres wide copper foil normally for slug control at about £1 a metre plus P&P.
Stephen by using eBay, bought four 90 centimetres Hula-hoops and these have been formed into magnetic loops for 20, 30 and 2 x 40 metre frequency bands by sticking copper foil onto them as one would gold leaf.
The options were :
To wind the ¼”foil like an inductance all the way round the loop and vary the pitch.
To wind 20 mms wide tape in the same way.
To lay the 20mms tape parallel to the circumference outside and inside and the fill the edge gaps with the ¼” tape.
To use Faraday loops, Gamma matches and Delta matches
And to wind coils of one or more turns of single or multi-strand wire inside or outside the plastic tube in any desired way.
Since October 2012 All of these process have been tried out by Stephen on air and all of them have proved worthy of future exploration and development. All but the one described here, take the work out of the safe description of a magnetic loop as understood by most amateurs.
The team has chosen to describe this simple basic apparatus in this article because it incorporates only one major new feature, that of replacing the thick walled copper tube by 2.5 thousandths of an inch thickness of copper foil laid onto a hollow plastic ring. The size and shape of any antenna formed in this way is limited only by the method used to form the basic shape. This could be by purchasing ready made plastic hoops, by forming hoops out of plastic tube or by making hoops by GRP methods around formers such as bicycle tyres, car tyres or tractor tyres, all of which Alan has done at various times or for example entirely within a computer with a 3D printer and or by electro-plating.
At the present time the team is content to say this antenna has been made on a table with the minimum workshop back up of a workmate and hand tools. Tin/lead solder and an ordinary soldering iron have been used throughout. There are no soldered joints between the two ends. There are five connection points. These are: two coax connections, one to the loop and one to the Gamma match, one from the gamma match to the loop and one each from the capacitor to each end of the loop.
The concept is offered for use as an inexpensive an experimental tool that, provided you can source one good quality transmitting capacitor, the loop can be made for less than ten pounds and that, once you get the idea you can change this into a wide variety of other loop antennas, not necessarily to be called magnetic loops but of a generally circular shape. The team remind you that we are in the business of communication and if you take the accepted magnetic loop into a fresh range of successes then that is progress.
Because you are no longer limited to stock diameters of copper tube you can use any cross section you can handle or make. Hula-hoops come in a wide variety of sections and diameter. We suggest to you look at those of 30 mms cross section and 1.25 metres in diameter because that is where we are likely to go next. The loop shown can, by choice of capacitors be made to resonate at 40, 30, and 20 metres with ease. Having access to an MFJ 269 Antenna Analyser saves time but is not essential.
To get the copper tape onto the loop having bought more than enough to go round the perimeter of your chosen tube as many times as is necessary one whole piece at a time. You then must decide where your top point will be and mark the start for the tape about one centimetre away.
Lay the loop flat on a table. Put the reel of tape on its edge and adjust the height of the loop so that the touching face is in the middle of the tape and plan to only unreal about three inches at a time pressing the centre of the tape onto the loop about two inches away from the reel. Do not rush. Allow about fifteen minutes to get right round. Press the tape down as you go inch by inch. You will get little creases. Just accept them and keep going keeping the tape parallel to the tube all the way round. Cut the protection tape off frequently and push it aside so that it does not interfere with what you are doing. Don’t be afraid to stop and relax.
When you get round to your starting point cut your finishing end to leave a 2 centimetre gap. This will be widened later. Using the handle of a knife or something similar and aiming to start on the centre line pick up the loop very carefully and go all round it rubbing the foil to make it lay smoothly and as evenly as possible right round the ring. You will not produce a perfect job. Do the best you can in short steps. When you have been all the way round and the tape is reasonably well placed then go round again using more speed and more pressure and you should be able to squash most of the creases out. The end result will still not be perfect. Now repeat the process on the inside edge of the tube and you will end up with a gap on both sides of about 5 mms plus or minus 2 mms. With luck some quarter inch tape used on both sides will fill the gap or lay down on top of the edges of the other two strips. How many strips you will need will depend on which tube you purchased.
The beginning and the ends will be close together and if you have not broken the tape you will have no soldered joints in your loop. The worries about soldered joints will not arise. We decided on a gap of 40 millimetres between the two ends. If you have better knowledge then by all means use it.
Where ever you decide to end your copper tube cut it neatly and then wrap at least one layer of tape round the plastic tube over the top of your layers about a couple of millimetres back from the edges of the layers so that you can solder both together to form a joint all the way round all of the edges. Then solder the other side of that ring of tape. The leads from your capacitor will be soldered to these two rings.
We have mounted our variable capacitors 30 millimetres below the gap as you can see in the pictures and 30 millimetres above we have placed a choco-block connector for adding other capacitors. We tried two kinds of ceramic HV capacitors and found they got very hot indicating that we were converting radio energy into heat which was not the idea. Our final capacitors are one fixed one of five lengths of high quality coax with solid insulation and single wire cores surrounded by a closely woven copper screen. Each 300 millimetre length gave us 32 picofarads of capacity. Across this capacitor we have a high quality High Voltage transmitting capacitor with a range of 19 to 150 pfs. We used knitting needles sharply bent at right angles to keep our hands away from the capacitor and to get the necessary leverage to turn the blades. We finished off the arrangement with a knob, a school protractor and a handbag mirror to see where the vanes were for recording purposes. In our case roughly one Pico farad per degree. But it is the numbers relating to the frequency that count. EG 104 degrees = 7.028 MHz. But only on this set up. Yours will definitely be different by at least one degree and all the variations in cut lengths of coax. Don’t worry about it.
There are lots of ways of making gamma matches. We used the inner wire of some good quality coax and its insulating cover to get a good stiff self-supporting link. But, and this is very important, we found that by moving it relative to the main tube we caused a change in the SWR and by winding one turn of it round the edge of the loop as shown we got the SWR lower than in any other place so we kept the arrangement and commend it to you as being both useful and stable.
The positioning of the gamma match was found by trial and error starting with a lead long enough to reach from the coax socket to about 2 O’clock. We found our best position was within a little of four o’clock by patiently watching the SWR meter and keying down for a dash. To determine the length of the gamma match we made it half Pi D where D was the straight line distance from the start of the gamma match to the loop. Then once we had some idea we folded the tin base of a cocoa tin into a narrow Vee and trimmed it with the kitchen scissors to follow the shape of the loop and soldered it on both sides to give the powerful clip a good anchor point that would not tear. We later added a piece of 16g unvarnished wire along the folded edge to improve the grip of our bulldog clip.
When you are satisfied with it, you may decide to wrap your thin copper skin with PVC tape. Our loops are for use indoors and all the work we have done with them has been from indoors either downstairs on the backs of doors or hanging from the dining room ceiling or upstairs in a different house alongside the chimney stack. To get the loop through the trap door, squeeze the sides carefully and pass the loop through the diagonal of the opening.
Obviously these loops require setting by hand and eye to one frequency at a time and are very basic but infinitely adjustable. For other work we have successfully built up gear motor capacitor drives and remote controls and used rotators and we or you might fit any of these tools to your loops. However our aim this time has been to produce a basic working tool for single frequency operation in a bigger project and we saw no point in waiting any longer before telling you.
Alan GM4FLX is well known to members of RAIBC as the narrator of Radcom for the Reading Rattle from 1976 until about 2001 and thanks RAIBC for the variable capacitors used in the project.
Photo 1. A view of the Copper foil Magnetic loop.
Photo 2 The tuning capacitor end of the loop.
Photo 3 The Tuning Capacitor lever arm and read-off scale and its sighting mirror
Photo 4 The feeder connection point and gamma match flying lead showing how it is wrapped around the main loop to bring the SWR down
Photo 5 The Gamma Match tuning arrangement that provides a gripping surface for the clip.
Photo 6. Lengths of high quality coax cable formed into a high Voltage Fixed capacitor to bring the working range of the variable capacitor into the desired frequency range.
( I have copied this from a document sent to me, but as I have no knowledge of magnetic loops etc, I only hope I have managed to put the pictures in the correct order. Also, many thanks to the boys for sharing this info with us all, a lot of time and effort went into the article and even more so the antenna. Good work lads. 2m0iob )