[Lf] Re: Loops, Pre-amps and Imps]

[Andre' Kesteloot]

john sexton wrote:

> Hi All,
>
> Warning: This is a fairly length E-mail. If you don't have time to read it,
> please just delete it.
>
> 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?
>
> 73 and apologies for what is probably an over-long E-mail.
>
> John, G4CNN
>
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