Experience with Loop Antennas for LF and VLF at G4CNN

Antenna farm at G4CNN

G3LNP Loop                                                              KI0LE Loop

On first getting interested in LF, I built the G3LNP loop design published in the July 1999 issue of RadCom. As luck would have it I managed to do everything right and was very impressed with the results. Using a home-made receiver from a design in the first edition of the LF Source book, I was able to hear for the first time the strange sounds from the Decca navigation system (now defunct),  from the Benelux Datatrak system and from Loran ( to some it sounds like galloping horses, but to me like frying) and various other weird and wonderful transmissions beyond the end of the Long Wave broadcast band. It was like exploring a new world. I soon heard my first Amateur station on the 136 kHz band, G4GVC, followed by many others. The 2.1 kHz wide band was just a hair breadth on my scale and with the wide bandwidth of the receiver (about 5 kHz) I could not only hear the hams but a lot of other stations as well. Being a true amateur with no professional experience, I soon realised that there was no way that I could build a receiver with the requisite bandwidth and performance needed to take part in LF activity and bought an AOR 7030 receiver. Now I could see exactly where I was on the band to within 1 or 2 Hz.

With the loop in the North/South plane I could hear the UK hams well, but practically nothing from continental Europe, so the loop was turned to East/West. After that many of the call signs that had become familiar were rarely heard or very weak, but now I could hear the Dutch, Belgian and German stations and occasionally stations from further afield: Sweden, Finland and Switzerland.

The loop is as far away from the shack as possible at the highest part of the garden ( the back garden is practically vertical!) about 30 metres away. The centre of the loop is earthed immediately alongside and the pre-amp is balanced. This has turned out to be important because it considerably reduces mains borne and local noise. On LF noise of all sorts, particularly natural QRN is a major factor. QRN tends to be least in the winter and can be almost absent on occasions, but during the summer it can be almost continuous and very tiring to listen to. Being practically retired and working at home, I can have the receiver on all day long while I am working at the computer.

New Loop

After a couple of years using the G3LNP loop, I began to wonder whether I couldn't do better, especially when some UK hams were hearing Italian stations and I couldn't. Also my interest in going lower in frequency was growing.The AOR 7030 tunes continuously from 0 Hz to 32 MHz, but I could not hear anything below 10 kHz, although the Russian Alpha navigation transmitters at 11.905, 12.649 and 14.881 are clearly audible. So I started to explore the Web to see what other designs were around. From participating in the RSGB E-Mail reflector, I knew that building something too large (more than 2 or 3 metres diameter) would not be a good idea, since the gain in wanted signal would be overwhelmed by the increase in noise pick up.
In the end I settled for a loop of about 2 metres diameter based on a design by KI0LE.

The KI0LE loop is a basket weave construction on a wooden structure with 5 arms. A basket-weave winding reduces the inter winding capacitance and hence increases the Q of the loop. In order to improve the Q even further I managed to purchase some litz wire acquired from the recently decommissioned Decca stations. This has hundreds of fine enamelled strands, separated into bundles each covered with cotton, and the whole contained in a 2mm polythene sleeve. This plastic covering means it is suitable for out-door use, provided the ends are sealed against the weather. Litz wire tends to pick up damp like a wick. The left- hand picture shows the completed loop and the right- hand the detail of the construction. I drilled 18 holes every 0.5 inch (12.5 mm) on each side of each wooden spar. The holes on one side were staggered by 0.25 inches (6mm) and plastic wall plugs glued into each hole. These plugs provided the support for the windings. To ensure that the windings do not come off in a wind, the turns are lightly bound to the spar with thin nylon string - thin so that it does not hold much water when wet. The whole coil has 36 turns, with a centre tap at 18 turns, which can be earthed and is in fact earthed with the 136 pre-amp, but need not be for example for VLF.

The Litz endings are terminated in a small sealed box with external connections for pre-amps, etc. The biggest single problem I had was to keep out rain water, which if it can get in, it will. This was eventually conquered with lots of plastic coverings with wires always entering from the underside and individually sealed. A little bag of Silica Gel is sealed inside the pre-amp to ensure it remains dry. Do not attempt to cover the windings - the first strong wind will bring the whole lot down.

The antenna is about 30 metres from the shack and at the highest point of the garden. The loop was wound with 36 turns and centre tapped. The inductance 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.
The list of potential IMPs from my calculation is available on another page, 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, massive 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.

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?

My idea is not to tune the loop, but simply transform the complex impedance.
At f = 137.8 kHz, X = 2600, R = 10. To tune X out needs 445 pF.
At f = 135.7 kHz, X = 2558, R = 10. To tune X out needs 459 pF.

but these are swamped by the 3000 pF of the coax.

I have 36 turns on my loop, so if I divided them into Primary 26 turns and Secondary 10 turns, I would get a ratio of 2.6, which would transform the reactance to 381 to 375 ohms, and the resistance to 50 ohms. This would require 3032 to 3127 pF to tune, of which 3000 is supplied by the coax, so in theory only a small variable in the shack should be necessary.
The only problem I see is that both Primary and Secondary are picking up signal, which probably totally invalidates the above calculations. However a ferrite transformer in a screened box wouldn’t suffer from this problem, so it might work.

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