Re: LF: Sub 9kHz antenna tests.

[Gary - G4WGT <g4wgt@tiscali.co.uk>]

Hi Jim,

Thank you for the very informative description, I now more fully understand your loop system design criteria. It seems that I worsened the receive capabilities in one way another by my twiddling with the loop etc. At least now that I understand the system I can return the loop & pre-amp to it's original state & note the difference.

What did take me by surprise that my two loops produced a much higher level of noise than the Mini-Whip which I modified for VLF but since reading your e-mail I can see the probable reason for that.

I will return my loop system to original & report my findings to the LF group.

Regards, Gary - G4WGT.

On 5 March 2011 22:47, James Moritz <james.moritz@btopenworld.com> wrote:

Dear Eddie, Gary, LF Group,

The loop is tuned, but is very low Q, and has more of a low-pass response really. It was done this way partly in order to obtain a wide bandwidth around 9kHz, which allows effective use of noise blanking or clipping (implemented using the DSP facilities of Spec Lab) to reduce the impact of QRN impulses. It also avoids the need for re-tuning, which is very convenient. At the same time, it provides a decent amount of attenuation of signals above the VLF range, which would otherwise tend to overload either the preamp or the sound card input. The frequency response is defined by the loop inductance, the shunt capacitor at the input, and the input resistance of the preamp stage (about 300ohms, due to the series/shunt negative feedback). The low Q means the loop voltage is small, so a fairly low-noise preamp is needed. The band noise has always been >10dB above the internal preamp noise in practice, so this is satisfactory. The overall gain of the preamp is about 60dB at 9kHz, and this was chosen as a compromise - it is large enough so that the band noise swamps the internal noise and QRM in the sound card input, but small enough so that the utilities and QRN do not overload the sound card.

You can get much higher signal voltage out of the same loop antenna by changing it to a high Q resonant design. I guess with the relatively high impedance load of the sound card input (several kohm), you might get a Q between 10 and 100. This would give a voltage gain of 20 - 40dB, more if you made a really low-loss loop and optimised a tapping point for the sound card input connection. But the bandwidth would also be greatly reduced, probably to only a few hundred Hz or less, and this is a bad thing. The narrow bandwidth "stretches" the QRN impulses making clipping / blanking less effective. Also, accurate tuning is then required.

I suppose one motive for having such a high Q loop would be to eliminate the preamp. This might work, depending on how noisy the sound card codec chip and the PC it is attached to are. My experience has been that this varies a lot between different PCs and frequencies, and which audio input you use. E.g. the laptop I usually use has a "Mic" input with considerably higher gain than the "line" input. But for some reason, the SNR is worse even though the signal level is higher when using this mic input. Also, the input noise level may vary depending on the source impedance of the antenna connected to the input. Since the preamp components only cost pennies, and allow you to get ample SNR as well as several dB improvement through the use of noise clipping and "no-tune" operation, this seems a better and simpler solution.

When comparing noise levels, it is important to check what the noise actually is, especially since PCs and associated stuff are potentially powerful noise sources. Listening to the output around 9kHz using the SpecLab VLF SDR function is useful - you are hoping to hear the band QRN and probably assorted mains-borne noises, rather than a lot of digital hash from the PC.

Cheers, Jim Moritz
73 de M0BMU