Hi Gary

Yes I see Jim 'tunes' his loop after the coax feed or is it HF filtering.

My Loop is 800mm sq 50 turns and 3.6mH according to rjeloop.

I am using SpecLab and PC

73 Eddie
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On 05/03/2011 18:52, Gary - G4WGT wrote:

Hi Eddie,

Firstly, I have not detected any signals on any of the three VLF antennas during the test period.

I haven't tried my loops without a pre-amp or resonated them but I note that Jim's (M0BMU) loop is smaller than mine at 800mm x 800mm & only 30 turns (2.85mH) were mine is 1100mm x 1100mm, 46 turns & about 4mH. Jim,s pre-amp also has a tuning capacitor (fixed value) which I removed. The strange thing is that most successful loop systems seem to be 800x800 but all use a pre-amp.

Well, the results of the tests show that the noise level varies quite a lot. Using a sound card & SpecLab the noise levels are as follows, the ferrite loopstick is -140 dB, the Mini-Whip is -150 dB & the large loop is -120 dB. This surprised me as I believed the loops would be quieter. The best visibility & strength of the Alpha's was using the Mini-Whip. The Mini-Whip may be the best choice for me being omni-directional & I have never seen all four of the Alpha pulses on one RSDN before using this modified Mini-Whip. At the moment (1840utc) RSDN 1 & 2  I am seeing 3 signals & RSDN3 seeing 4 signals.

So, a worth while test result for me but disappointed not to receive any amateur signals.

BTW, are you using SpecLab on a PC or receiving on a real receiver ??

73, Gary - G4WGT.

On 5 March 2011 16:48, [email protected] <[email protected]> wrote:

Hi Gary

Thanks for the reply, first thing, for "10dB hump at 8.9kHz" read 20dB. Direct coupling of coax to the feed was killing the Q, I have added a 4 turn coupling loop. The flatness was only really noted once it was outside.

I can see the need for an amplifier on a broad band loop as with the E-Probe which is non resonant and at a few inches long hasn't got a lot going for it but not for a resonant loop which untuned produces 10dB of noise and resonant 30dB I am dubious still.

Maybe M0BMU will have a comment :-) please.

73 Eddie G3ZJO


On 05/03/2011 15:21, Gary - G4WGT wrote:

Hi Eddie,

Yes, you will need a pre-amp, don' forget that the E-Probe has a pre-amp. I advise you to build the M0BMU VLF pre-amp with 7 to 10 kHz band-pass filter, circuit attached.

73, Gary - G4WGT.

On 5 March 2011 14:22, [email protected] <[email protected]> wrote:

Hi Gary

I am testing a very similar loop to your 1.1m square, mine is 800mm sq. and uses Litz wire. Results at the same hight as my E-Probe are disappointing as yet.

I have no pre amp on it, and am wondering whether it would be advantageous as when tuned to 8.9kHz the noise level rises by 10dB. I suppose I shall end up building an amp to try but I would welcome any thoughts from the group.

73 Eddie G3ZJO

On 05/03/2011 12:35, Gary - G4WGT wrote:

Hi All,

Whilst monitoring 9090kHz & 8970kHz using SpecLab at 423uHz, I am testing 3 different VLF Rx antenna types :-

1 - An 84mH shielded ferrite loopstick (10mm x 200mm), 1 metre above ground
2 - A modified PA0RDT Mini-Whip, 8 metres above ground
3 - A 1.1 metre square loop with 46 turns of 1mm insulated wire, 1 metre above ground

Both of the loops using M0BMU VLF pre-amp.

I have had No. 1 connected since 2000utc last night until 1030utc this morning, no signals detected on the SpecLab display. I have connected No. 2 at 1030utc & notice a fall in noise level of about 10dB & a greater visibility of the Alpha signals. Later today I will connect the large loop & report my findings.

73,

Gary - G4WGT - IO83qo.