Friday, 1 February 2019

Homemade HF magnetic loop for SWL / Receiving Inductive vs Transformer Coupling

fig 1


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fig 5
I can't believe my last post was almost a year ago! Anyway, I wanted to share a new design skill I have learnt for coupling my small magnetic loops antennas. As those of you who have read my previous posts might know, I have been using these homemade passive tuned loops for years now, as they are consistently the best type of aerial for urban dwellers like myself who can not or don't want to put up elaborate outdoor aerials or ground rods for earthing them. They are easy to make, can be built for less than $10, and are much more enjoyable and satisfying to use than any commercial aerial I've tried.

Up until now, the main 35-40cm receiving loops have been coupled to the radios inductively. This means I have made a second loop from a piece of coaxial cable or insulated wire about 1/5 the diameter, which is then connected to a coaxial feed cable, very close to the main loop but not physically connected to it (this is inductive coupling) and away you go. (see fig 1)

Recently I have decided I want to try to amplify the loop a little, and I have bought a 9-12v low niose amplifier (ready-made), and small variable resistor (to adjust the gain). But in looking at commercial loops like those from PK Loops in Australia, and AOR, I have noticed they do not have these somewhat unsightly secondary loops in their design, and I have wanted to trhy a different coupling method to smarten things up a bit.

I'm not heavily into technical jargon as it overwhelms me and puts me off starting new projects when the instructions full of it. Usually what these articles are saying is very easy and straightforward, but the complicated language lowers my confidence and makes me think it is too technically demanding.
But as I have got older my confidence is improving and if you look around carefully you will come across one or two articles that are easier to understand and ebncourage you to have a bash. Hence my reason for blogging about loop aerials for so long.

Anyway, to cut a long story short, I have ditched the ugly second loop and gone over to transformer coupling. It is way easier and give the finished loop a cleaner, more professional look.

All you need to do is buy a small ferrite ring (toroid). A T130-2 is 1.3inches in diameter and perfect for the job. I bought two from Bowood Electronics for just over £5 incl postage. Personally I would have thought that for receiving only it wouldn't make too much difference what toroid you used, just so long as you can get half a dozen or so turns or wire and your main loop through the centre.

A word about toroid sizes: a T130-2 is 1.3 inches diameter, a T30-2 is 0.3 inches diameter. The first number relates to the diameter in inches. I can't remember the second one, but again for SWL I don't think it matter enormously. You just need a ring of ferrite.

I searched high and low for information about this as I wanted to know how many turns I needed, what type of wire, how the turns should be wound etc etc In the end I got the gist of it, and learned through reading and trial and error that for max efficiency it works best when the turns are evenly spread around the whole circumference of the toroid. I tried two or three and could tell that compared to the inductively coupled loop certain frequencies wouldn't peak up properly. I increased up to 15 and it was worse! Finally, through a long process of trial and error, I found that for my small 1 foot loops I needed 7 turns of wire (I used the blue wire from standard twin-core 5 amp electrical cable).

One end of the wire goes to the coax braid, the other end to the centre core, and then just plug the other end of the coax feed into the radio.

This new form of coupling seems to produce results just as good as the inductively-coupled loops. Better to my ears in fact as there seems to be less noise. Now, if I were to test the SWR, I could'nt guarantee the transfomer coupled loops were any better. They may even be worse. But I'm a SWL not a radio ham, Im not obsessed with measurable figures and standing wave ratios,'and my loops are for receive only, and I have listened to extremely weak sideband signals on the same loop, swapping from one coupling method to the other, and honestly, I couldn't tell any difference.


Anyway, that's enough for this afternoon. Let me know if you have any comments, questions, suggestions etc I love hearing from anyone interested. If you have never made any stuff yourself, take my word for it: IT IS TERRIFIC FUN. You'll save hundred of pounds, the finished product can be made to look pretty decent with only easily obtained bits and pieces, and using it will give you a buzz no commercial product ever will.☺

I realise I have not put any construction details for the loop so I am attaching a png from Mike at Merseyradar. who kindly did a diagram for me.

The main receiving loop is 35cm diameter and has two windings with a switch to select whether you choose one or both of them. It is tuned with a small variable capacitor that I bought on eBay and the two gangs of the capacitor have been joined together to give what I think is 760 PF capacitance but it maybe 1000pf as I'm afraid I bought it years ago and don't have the purchase details anymore.

If you were to use only a single loop winding and omit the switch then you would get a tuning range of approximately 5 to 30 mHz. But with solar activity so low over recent years I really needed it to tune down to the shanwick Night-Time frequencies in the nighttime frequencies in the 2, 3 and 4 MHz band. Oh and I also like listening to the maritime calling frequency of 2182 kilohertz, so 2 MHz seems an appropriate place to stop for my requirements. 

I also want to update you about the plans to amplify the loop as these have unfortunately been a huge waste of time! Hours of work and waiting for components to arrive from China just resulted in a whole lot of noise being pumped into the system and the whole loop became completely unusable. I am slowly coming to the conclusion that if you are turning to a loop because an outdoor antenna will be too noisy than the last thing you want is amplification. 

It may be different for a professionally constructed loop like those from aor or PK in Australia, but I strongly suspect after years of having tinkered around with aerials for shortwave listening that any performance improvement as a result of amplification would be marginal at best. So my advice would be to forget about any expensive commercial product and have a bash at building one of your own. It's so much more rewarding in every way. 

And if you do have any stories to share about your experiences with loop designs please feel free to comment.