Some thoughts on LF receiving loops by the late* John Sexton G4CNN.
Some while back I bought some of the ex-Decca thick plastic covered Litz and wound a 6 foot diameter KI0LE type loop with 36 turns in a basket-weave type construction. This was partly to augment my rx ability but also because of a growing interest in VLF and VF. I promised at the time to let you all know how it turned out.
The antenna is about 30 metres from the shack and at the highest point of the garden. The inductance of the loop was found to be about 3 mH. 30 metres of RG58CU has a capacitance of about 3000 pF, so with a pre-amp in the shack the highest frequency it could be tuned to was about 53 kHz. This is fine for VLF but not for LF, so I built a replica of the pre-amp that I had made for my G3LNP loop. First results were very disappointing, much more noise than from the LNP loop. I put this down to electrical field pick-up, which the LNP loop seems to be much more immune to, although neither is screened. Screening the new loop is not really practical because of the large area and wind resistance.
I decided to try another pre-amp, and built a design by Lloyd Butler, (VK5BR) from the old LF source book using a single op-amp the OPA111AM. I chose this design because a couple of years ago I had built his Simple Regenerative VLF-LF Receiver and Front-end, both of which work very well although with some limitations. This pre-amp turned out to work well with the new loop, about 12 dB better than the LNP loop, even though the op-amp gain was set to supposedly 1 and with an earth at the loop the electrical noise was much reduced. Curiously an earth at the shack end has the opposite effect - so not all earths are the same! However I soon noticed a "new" RTTY station on 136.25 and realised that it must be an Inter Modulation Product, but where and from what? I prepared a list of all the strong stations from 200 down to 10 kHz and using Excel calculated all of the f1 +/- f2 and 2*f1 +/- f2 products (strictly sums and differences). It turned out to be BBC Radio 4 minus a strong RTTY station on 61.75 kHz.. To confirm it, I used two receivers and feeding them both into Spectrogram could see the identical modulation (the IMP was of course upside down) at 61.25 and 136.25. So was this IMP generated in my receiver or in the pre-amp? The rx attenuator did not remove it and nor did an external attenuator, so it was obviously coming from the pre-amp.
First conclusion: Feeding a decent antenna directly into a wide-band op-amp is asking for trouble unless you really live far from civilisation. Some sort of band-pass or low pass filter is essential. By the way, the list of potential IMPs from my calculation is available on request, but depending on where you are, you may have a completely different list of strong stations.
Back to the LNP pre-amp. On checking the operation I found that the 2n3819s were operating at a fairly high Idss so that the drain voltage was nearer to 1 volt than the recommended 5 volts. None of the 6 or more 3819s in my junk box were any better, but on trying some 2N5457s I found that they were all quite close to the 5 or 6 volt level. On substituting a couple of these, the pre-amp ran much better, with the same sort of performance as the op-amp and more importantly no IMPs, at least none that I have found so far. The electrical noise is also reduced.
Second conclusion: Check your FETs and you may find as I did that the 2N5457 is a better choice than the 3819 at least in this circuit.
At one stage I had the pre-amp under test attached to the receiver with no antenna, but with just a short circuit to earth in the place of the loop and to my surprise DBF39 was still S9! I then realised that this was the result of using a plastic box. Solution line the inside of the box with baking foil - hey presto, absolute silence. Just sticking a small screwdriver into an antenna socket and removing my hand I could still get DBF39 at S7!
Third conclusion: Use metal boxes or if like me you can't face all that metal work, line your box with aluminium foil. By the way I noticed that you can buy rolls of foil ready to be stuck to whatever takes your fancy in my local DIY store. It is also cheaper than metal boxes. Another source is the thick aluminium foil used for what we used to call TV dinners, masses amounts of which just seems to get thrown away. Use some plastic foam to keep your circuit board away from the metal, but do make a good contact between the foil lining and earth and a foil lining stuck inside the lid.
The electrical field pick-up by the new loop is still there, but I have found that at times it disappears completely, furthermore I have realised it is the same noise that makes my vertical practically useless as a receive antenna. So the source is most probably a light dimmer or something similar nearby. Solution build a noise canceller. I have built the GW4ALG canceller but haven't got it to work satisfactorily yet - more later - meantime the house is becoming festooned with wires including now noise antennas, hi!
Fourth conclusion: The LNP loop is hard to beat. The KI0LE loop is very sensitive but subject to local noise pick-up - should be good out in the countryside though. When the noise is low, I am now hearing some weak stations well (enough to read them by ear) which before were only just detectable.
I have also been trying it out on VLF with a couple of Rennato Romero's designs for sensitive audio pre-amps and getting superb results. At these frequencies this type of construction is really coming into its own.
I will end with a question: The purpose of the pre-amp is partly to get a bit of gain, but primarily to tune the loop and match it to 50 ohm coax. Could one use a transformer instead to effectively reduce the inductance so that it could be tuned in the shack without being swamped by the coax capacitance?
*Sadly, John died on 21st July 2002. He will be missed by the LF community for his great enthusiasm.
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