Hello Markus, Thanks for the information, and sorry for very late reply. Yes, I have definite plans for VLF; have built many good receivers and good RX antennas; built and carefully tested many bad TX antennas and good TX antennas; have reliable robust clean 24kW PA (running now, keeping the room warm); ultra-small dynamic matching unit; have a Helikite picked out; good RX/TX sites; keep getting sidetracked, in particular too much time playing with interesting details and improving components. Trimble Thunderbolt E (E version has good clock stability) sitting on bench 12 months waiting to be hooked up. Always some reason to postpone group tests and I don’t even have a cat. 73, Jim AA5BW From: [email protected] [mailto:[email protected]] On Behalf Of Markus Vester
Sent: Wednesday, April 20, 2016 4:07 AM
To: [email protected]
Subject: Re: LF: RE: 8270.0025 Tuesday Apr 19 the transmit antenna is physically larger: The apex of the topload is at 21 m above ground or 12 m above our rooftop. But the wires are sloping downward at steep angles, and the antenna is partly shielded by surrounding houses and trees. The effective height was measured as 8.7 m at 139 kHz by comparison of received open-circuit voltage versus a small nonresonant loop. I believe that effective height may be about 20% lower at VLF due to more effective shielding from resistive objects, but this difference has not been measured so it's just a guess. Thanks Paul for the comparison and valuable details regarding spherics blanking.
Jim, any plans for VLF activities from your end? -----Ursprüngliche Mitteilung-----
Von: hvanesce <[email protected]>
An: rsgb_lf_group <[email protected]>
Verschickt: Di, 19 Apr 2016 11:22 pm
Betreff: LF: RE: 8270.0025 Tuesday Apr 19
A question perhaps for Paul or yourself follows: At ~0.22 fT (estimated from Paul’s spectrogram) I expected to see >> 10dB SNR in 278uHz. 10dB SNR is off scale so I looked at the on/off tails and noise, and I still seemed to be off by a substantial amount in the relationship of flux density and SNR in Paul’s spectrogram, but I had not factored in sferic noise and cancellation. If I assume 30fT/rt-Hz for sferics, and 20dB sferic noise cancellation in a clipper, then a 0.22fT signal (with 3.0 fT/rt-Hz sferic noise after clipping) would yield roughly 13dB SNR in 278uHz, which seems consistent with Paul’s spectrogram. Is 20dB noise reduction a reasonable estimate for the sferic clipper/blanker? I think that I had become accustomed over the winter to seeing near-preamp-noise-limited SNR on a number of VLF receivers, notwithstanding sub-fT/rt-Hz preamp noise levels, and that may explain why I was expecting >>10dB SNR for ~0.22fT and noise in 278 uHz. If Paul considers the sferic clipper/blanker noise reduction to be roughly 20dB that would explain my overestimate. Nice signal from your (9m?) antenna! After a couple of minor modifications (different soundcard, thicker piece of wire from the top of the loading coil), I ran a test carrier from 13:15 to 15:15 UT. I briefly ventured pushing the drive up to 0.42 A (= 30.0 kV rms) with no adverse effects, but then settled to 0.35 A to have a bit of margin for the long transmission. Unfortunately not much appeared in Heidelberg and Breclav in the upcoming afternoon QRN. But then, looking at Paul's spectrogram saved my day: |
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