Return-Path: X-Spam-DCC: paranoid 1290; Body=2 Fuz1=6 Fuz2=2 X-Spam-Checker-Version: SpamAssassin 3.1.3 (2006-06-01) on lipkowski.org X-Spam-Level: X-Spam-Status: No, score=-7.9 required=5.0 tests=BAYES_00,DNS_FROM_AHBL_RHSBL, HTML_40_50,HTML_MESSAGE,USER_IN_WHITELIST_TO autolearn=no version=3.1.3 Received: from post.thorcom.com (post.thorcom.com [195.171.43.25]) by paranoid.lipkowski.org (8.13.7/8.13.7) with ESMTP id t6QN0YPx025777 for ; Mon, 27 Jul 2015 01:00:34 +0200 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1ZJUmZ-0007yM-3g for rs_out_1@blacksheep.org; Sun, 26 Jul 2015 23:52:35 +0100 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1ZJUmY-0007yD-A7 for rsgb_lf_group@blacksheep.org; Sun, 26 Jul 2015 23:52:34 +0100 Received: from omr-m09.mx.aol.com ([64.12.143.82]) by relay1.thorcom.net with esmtps (TLSv1:DHE-RSA-AES256-SHA:256) (Exim 4.85) (envelope-from ) id 1ZJUlN-0006ry-Dc for rsgb_lf_group@blacksheep.org; Sun, 26 Jul 2015 23:52:33 +0100 Received: from mtaout-aaf01.mx.aol.com (mtaout-aaf01.mx.aol.com [172.26.127.97]) by omr-m09.mx.aol.com (Outbound Mail Relay) with ESMTP id 1B0247000009B; Sun, 26 Jul 2015 18:51:04 -0400 (EDT) Received: from White (ipb21bee4a.dynamic.kabel-deutschland.de [178.27.238.74]) by mtaout-aaf01.mx.aol.com (MUA/Third Party Client Interface) with ESMTPA id D937C38000087; Sun, 26 Jul 2015 18:51:00 -0400 (EDT) Message-ID: <94D7BD8F8ABB4AB798759379C79927F3@White> From: "Markus Vester" To: , , Date: Mon, 27 Jul 2015 00:50:59 +0200 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal Importance: Normal X-Mailer: Microsoft Windows Live Mail 12.0.1606 X-MimeOLE: Produced By Microsoft MimeOLE V12.0.1606 x-aol-global-disposition: G DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mx.aol.com; s=20150623; t=1437951064; bh=9b2+h6ma6Tz/UsjLELC2JIYeiITiVzZAJAmxuPxxyIg=; h=From:To:Subject:Message-ID:Date:MIME-Version:Content-Type; b=MBIn0xY6+vDm26ERFy1dNxSD5dBsuDzp4L0wW2RKKMUtZsivZ0AaXYRl1jAE7hWHn 6x0MqYzwW0jn7tEqaOetDMJEsOY0c2yfoKmM2uCdN6nQCHn7a0xZwvSzwwgkxoaRQl MT72VmlAboiMjE0g1AmGv3f7ya0hReUC9CbPC6/4= x-aol-sid: 3039ac1a7f6155b5645479bc X-AOL-IP: 178.27.238.74 X-Scan-Signature: dd7342f7a67c58184f15e70b9d6d7f8e Subject: Re: VLF: Tweek mode resonances Content-Type: multipart/alternative; boundary="----=_NextPart_000_0017_01D0C806.4DB04120" X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-Scanned-By: MIMEDefang 2.56 on 10.1.3.10 Status: O X-Status: X-Keywords: X-UID: 3707 Dies ist eine mehrteilige Nachricht im MIME-Format. ------=_NextPart_000_0017_01D0C806.4DB04120 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Friday night (July 24/25) was stunning. Coming home from a beer after = local midnight, I noticed spectacular VLF spherics resonances on the = DK7FC remote garden grabber (screenshot = http://df6nm.bplaced.net/VLF/spherics/dk7fc_VLF_150725_0704.jpg from the = morning). I decided to restart the dual-loop directional spectrogram, = the one which sometimes feeds my VLF wideband grabber.=20 After a couple of minutes, the first spectrum appeared, and I was = literally swept away: Fat resonance peaks at all multiples of 1.62 kHz = up to the tenth harmonic, and up to 20 dB above the noise: = http://df6nm.bplaced.net/VLF/spherics/vlfwide_150725_0700_cursor-2347.png= . This was really surprising, as my suburban loops are usually very = prone to local interference, and I had never seen or heard something = like that before. The colour direction finder displayed the resonances = in grey, which means "no direction", just as expected for a = vertical-incidence circular polarized wave.=20 http://df6nm.bplaced.net/VLF/spherics/tweek_resonances_dk7fc_df6nm_Blitzo= rtung_150725_0700.png is a combined image showing two screenshots from = Stefan in Heidelberg (top) and myself in Nuernberg (bottom), stacked = above one another with identical timescales. The resonances in = Heidelberg were strongest between 22 and 23 UT, became significantly = weaker at 23:20, and died out about 0:10 UT. Resonances in Nuernberg = came about an hour later, strongest at 23:40, and fadeout between 0:30 = and 1:10 UT. Interestingly, the resonance frequency goes through a flat = minimum at the time of maximum amplitude and sharpness, which is = different for the two locations. The spectrograms can be compared to a Blitzortung screenshot taken at = 0:40 UT, inserted at the bottom left of the image. Blitzortung sorts = displayed flashes in 20 minute bins, indicated by colours from red = (oldest, 22:40 - 23 UT), through shades of orange, to white (last 20 = minutes, i.e. 0:20 to 0:40 UT). For clarity, I repeated the Blitzortung = colours in rectangles corresponding to the respective parts of the = spectrogram timescale.=20 The position of the stormfront at the time of fadeout allows us to = estimate the lateral range across which resonance amplification could be = used to enhance communication. We find that tweek resonances were strong = up to about 90 km away, and became very weak at about 180 km (indicated = by blue circles around Heidelberg). Unfortunately this means that the = distance between DK7FC and DF6NM (180 km) is probably already too large = to benefit much from such a vertical resonance mode. But on the other hand, it is known that tweeks have often been heard = from great distances, up to 3000 or 6000 km over seawater. So why are = the resonances restricted to a couple of 100 km? I think the answer for = this is dispersion - the same effect that creates the characteristic = chirp sound in distant tweeks. For a purely vertical wave, the time = interval between successive hops is always 2h/c, about 0.6 ms with h =3D = 90 km ionosphere height- This is the case if the source is close to the = receiver. However at increased ranges, the path is initially at lower = angle, and becomes steeper and steeper for later hops. Thus the time = interval between successive pulses is initially smaller, and grows = towards later (more vertical) hops. Source distance can be calculated = from a tweek's knee in a spectrogram - far away sources have less = rectangular, more rounded knees. In the resonance spectrum, this means = that energy from for further away sources will be more spread out, and = the maximum shifted to higher frequencies. This seems to be exactly what = we have observed in the slow spectrograms: the sharpest resonances at = the lowest frequencies occur when the storm passes just overhead. At times, the resonances even appeared to split up. Look at the zoomed = section around the fourth harmonic at the bottom: During the maximum in = Nuernberg (23:45 UT), there are two distinct peaks, one at 6430 Hz and = another at 6518 Hz. In the correponding orange Blitzortung crosses, we = indeed find two separate sections of lightning activity, a smaller and = nearer one passing south of Nuernberg, and another broader one passing = north. Let's take another look in time domain. = http://df6nm.bplaced.net/VLF/spherics/dualloops_tweeks_scope.png are = SpecLab scope shots from a couple of tweeks from recordings. For these, = the frequency range had been restricted to 4 - 16 kHz to eliminate = interference from 3 kHz railway noise and military MSK stations, and = SpecLab's autonotch filter was engaged to suppress residual mains and = railway harmonics. The two traces correspond to the two loops, red is = the North-South loop (actually pointing 330=B0 azimuth), and blue is = East-West (60=B0). Tweek #3 shows a direct groundwave received only by = the red channel, and a subsequent train of echoes which are received by = both, with slightly increasing echo spacing. We find that undulations in = the blue trace preceed those on the red one, showing that the circular = polarized magnetic field vector rotates counterclockwise, i.e. from east = to north. There's also a couple of wav and ogg recordings in = http://df6nm.bplaced.net/VLF/spherics/ - these nearby tweeks sound = rather musical, like plucking on a short tight piece of guitar string. All the best, Markus (DF6NM) PS: I started this thread on the rsgb LF reflector, but it may be = appropriate to post to the VLF / Natural radio group guess I'll just do = both. From: Markus Vester=20 Sent: Saturday, July 18, 2015 6:50 AM To: rsgb_lf_group@blacksheep.org ; rsgb_lf_group@yahoogroups.co.uk=20 Subject: VLF: Tweek mode resonances Waking up early this morning, I took a look at Stefan's garden grabber = http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_remote_Grabber.html = and was greeted by a fascinating display on his VLF panel.=20 The screenshot = http://df6nm.bplaced.net/VLF/spherics/dk7fc_VLF_150718_1326.jpg shows a = number of narrow tweek-mode resonances at multiples of 1.72 kHz. These = are obviously spherics from nearby lightnings, bouncing multiple times = vertically between the ionosphere and ground (much the same as clapping = your hands between two parallel brick walls). The resonances are rather = sharp, indicating a high Q-number (ie. around 100 bounces until decay). = They are visible up to about 20 kHz, showing unusually small damping of = vertical incidence reflections at these frequencies. There is a small = variation of resonance frequency over time, reflecting the variable = height of the ionospheric ceiling. The fundamental resonance at 1.7 kHz = is probably not visible due to the frequency response of the loop and = receiver. A (somewhat late) screenshot from Blitzortung = http://df6nm.bplaced.net/VLF/spherics/image_b_de_150718_0324.png shows = the last red crosses between Wiesbaden and Stuttgart passing over = Heidelberg at around 1:30. The tweek resonances were received on the loop antenna in the garden but = not on the E-field antenna of the (somewhat whitened out) city grabber = http://df6nm.bplaced.net/VLF/spherics/dk7fc_wideband_150718_0330.jpg. = This corroberates the notion of near vertical incidence and horizontal = H-field polarisation. According to the literature, tweek tails are = usually circular polarized as only one sense of rotation exhibits a high = reflection coefficient. They are predominately excited by horizontal = current components in intra-cloud lightnings. Of course the resonances will also be there in quiet nights without = spherics, so they could probably be employed to enhance fieldstrength = (up to a factor of Q) for medium-range VLF communication experiments = using magnetic transmit and receive antennas. When Stefan still had his = earth dipole, we already discussed a 2 kHz tweek-mode experiment, which = for various reasons hasn't taken place yet. Now it looks like one could = even employ the fifth mode near 8.5 kHz... All the best, Markus (DF6NM) ------=_NextPart_000_0017_01D0C806.4DB04120 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Friday night (July 24/25) was stunning. = Coming home=20 from a beer after local midnight, I noticed spectacular VLF spherics = resonances=20 on the DK7FC remote garden grabber (screenshot = http://df6nm.bplaced.net/VLF/spherics/dk7fc_VLF_150725_0704.jpg = from=20 the morning). I decided to restart the dual-loop directional = spectrogram,=20 the one which sometimes feeds my VLF wideband grabber. =
 
After a couple of minutes, the = first spectrum=20 appeared, and I was literally swept away: Fat resonance = peaks at all=20 multiples of 1.62 kHz up to the tenth harmonic, and up to 20 dB above = the noise:=20 http://df6nm.bplaced.net/VLF/spherics/vlfwide_150725_0700_curso= r-2347.png.=20 This was really surprising, as my suburban loops are usually very prone = to local=20 interference, and I had never seen or heard something like=20 that before. The colour direction finder displayed the=20 resonances in grey, which means "no direction", just as = expected=20 for a vertical-incidence circular polarized wave. 
 
http://df6nm.bplaced.net/VLF/spherics/twee= k_resonances_dk7fc_df6nm_Blitzortung_150725_0700.png is=20 a combined image showing two screenshots from Stefan in Heidelberg (top) = and=20 myself in Nuernberg (bottom), stacked above one another with=20 identical timescales. The resonances in Heidelberg were strongest = between=20 22 and 23 UT, became significantly weaker at 23:20, and died out about = 0:10 UT.=20 Resonances in Nuernberg came about an hour later, strongest at=20 23:40, and fadeout between 0:30 and 1:10 UT. Interestingly, = the=20 resonance frequency goes through a flat minimum at the=20 time of maximum amplitude and sharpness, which is = different for=20 the two locations.
 
The spectrograms can be compared = to a=20 Blitzortung screenshot taken at 0:40 UT, inserted at the bottom left of = the=20 image. Blitzortung sorts = displayed=20 flashes in 20 minute bins, indicated by colours from red (oldest, = 22:40 -=20 23 UT), through shades of orange, to white (last 20 minutes, i.e. = 0:20 to=20 0:40 UT). For clarity, I repeated the Blitzortung colours in=20 rectangles corresponding to the respective parts of the spectrogram = timescale.=20
 
The position of the stormfront at the = time of=20 fadeout allows us to estimate the lateral range across which=20 resonance amplification could be used to enhance communication.=20 We find that tweek = resonances were strong up=20 to about 90 km away, and became very weak at about 180 km = (indicated by=20 blue circles around Heidelberg). Unfortunately this means that the = distance=20 between DK7FC and DF6NM (180 km) is probably already too = large to=20 benefit much from such a vertical resonance mode.
 
But on the other hand, it is known that = tweeks have often been heard from great distances, up=20 to 3000 or 6000 km over seawater. So why are = the resonances=20 restricted to a couple of 100 km? I think=20 the answer for this is dispersion - the same effect that creates = the=20 characteristic chirp sound in distant tweeks. For a purely vertical = wave,=20 the time interval between successive hops is always 2h/c, about 0.6 = ms with=20 h =3D 90 km ionosphere height- This is the case if the source is close = to the=20 receiver. However at increased = ranges, the=20 path is initially at lower angle, and becomes steeper and steeper = for later=20 hops. Thus the time interval between successive pulses is initially = smaller, and=20 grows towards later (more vertical) hops. Source distance can be = calculated from=20 a tweek's knee in a spectrogram - far away sources have less=20 rectangular, more rounded knees. In the=20 resonance spectrum, this means that energy from for further away = sources=20 will be more spread out, and the maximum shifted to higher frequencies. = This=20 seems to be exactly what we have observed in the slow=20 spectrograms: the sharpest resonances at the lowest frequencies = occur when=20 the storm passes just overhead.
 
At times, the resonances even = appeared=20 to split up. Look at the zoomed section around the fourth = harmonic at=20 the bottom: During the maximum in Nuernberg (23:45 UT), there are two = distinct=20 peaks, one at  6430 Hz and another at 6518 Hz. In the = correponding=20 orange Blitzortung crosses, we indeed find two separate = sections of=20 lightning activity, a smaller and nearer one passing south of Nuernberg, = and another broader one passing north.
 
Let's take another look in time domain. = http://df6nm.bplaced.net/VLF/spherics/dualloops_tweeks_scope.png&nbs= p;are=20 SpecLab scope shots from a couple of tweeks from recordings. For these, = the=20 frequency range had been restricted to 4 - 16 kHz to=20 eliminate interference from 3 kHz railway noise and military MSK=20 stations, and SpecLab's autonotch filter was engaged to=20 suppress residual mains and railway harmonics. The two traces=20 correspond to the two loops, red is the North-South loop (actually = pointing 330=B0=20 azimuth), and blue is East-West (60=B0). Tweek #3 shows a direct = groundwave=20 received only by the red channel, and a subsequent train of echoes which = are=20 received by both, with slightly increasing echo spacing. We find that=20 undulations in the blue trace preceed those on the red one, showing = that=20 the circular polarized magnetic field vector rotates=20 counterclockwise, i.e. from east to north.
 
There's also a couple of wav and ogg = recordings in=20 http://df6nm.bplaced.net/= VLF/spherics/ -=20 these nearby tweeks sound rather musical, like plucking on a short tight = piece=20 of guitar string.
 
All the best,
Markus (DF6NM)
 
PS: I started this thread on the rsgb = LF reflector,=20 but it may be appropriate to post to the VLF / Natural radio = group =20 guess I'll just do both.
 
 
From: Markus Vester
Sent: Saturday, July 18, 2015 6:50 AM
To: rsgb_lf_group@blacksheep.org= ; rsgb_lf_group@yahoogroups= .co.uk=20
Subject: VLF: Tweek mode resonances

Waking up early this morning, I took a = look at=20 Stefan's garden grabber http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_remote_Grabbe= r.html and=20 was greeted by a fascinating display on his VLF panel. =
 
The screenshot = http://df6nm.bplaced.net/VLF/spherics/dk7fc_VLF_150718_1326.jpg = shows a = number of narrow=20 tweek-mode resonances at multiples of 1.72 kHz. These are obviously = spherics=20 from nearby lightnings, bouncing multiple times vertically between the=20 ionosphere and ground (much the same as clapping your hands between two = parallel=20 brick walls). The resonances are = rather sharp,=20 indicating a high Q-number (ie. around 100 bounces until decay).=20 They are visible up to about 20=20 kHz, showing unusually small damping of vertical incidence = reflections at=20 these frequencies. There is a small variation of resonance frequency = over time,=20 reflecting the variable height of the ionospheric ceiling. The = fundamental=20 resonance at 1.7 kHz is probably not visible due to the frequency = response of=20 the loop and receiver.
 
A (somewhat late) screenshot from=20 Blitzortung http://df6nm.bplaced.net/VLF/spherics/image_b_de_150718_0324.png&nbs= p;shows=20 the last red crosses between Wiesbaden and Stuttgart passing = over=20 Heidelberg at around 1:30.
 
The tweek resonances were received on the loop antenna in the=20 garden but not on the E-field antenna of the (somewhat = whitened out)=20 city grabber http://df6nm.bplaced.net/VLF/spherics/dk7fc_wideband_150718_0330.jpg= .=20 This corroberates the notion of near vertical incidence and horizontal = H-field=20 polarisation. According to the literature, tweek tails are usually = circular=20 polarized as only one sense of rotation exhibits a high reflection = coefficient.=20 They are predominately excited by horizontal current components in = intra-cloud=20 lightnings.
 
Of course the resonances will also be there in quiet nights = without=20 spherics, so they could probably be employed to = enhance fieldstrength (up=20 to a factor of Q) for medium-range VLF communication experiments using = magnetic=20 transmit and receive antennas. When Stefan still had his earth dipole,=20 we already discussed a 2 kHz tweek-mode experiment, which for = various=20 reasons hasn't taken place yet. Now it looks like one could even = employ the=20 fifth mode near 8.5 kHz...
 
All the best,
Markus (DF6NM)
 
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