Return-Path: X-Spam-DCC: paranoid 1170; Body=2 Fuz1=2 Fuz2=2 X-Spam-Checker-Version: SpamAssassin 3.1.3 (2006-06-01) on lipkowski.org X-Spam-Level: X-Spam-Status: No, score=-0.3 required=5.0 tests=BAYES_00,DNS_FROM_AHBL_RHSBL, HTML_30_40,HTML_MESSAGE,RATWARE_GECKO_BUILD 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 t6MML9Sf015561 for ; Thu, 23 Jul 2015 00:21:09 +0200 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1ZI2FQ-0007qJ-TZ for rs_out_1@blacksheep.org; Wed, 22 Jul 2015 23:12:20 +0100 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1ZI2FQ-0007qA-89 for rsgb_lf_group@blacksheep.org; Wed, 22 Jul 2015 23:12:20 +0100 Received: from mx02.posteo.de ([89.146.194.165]) by relay1.thorcom.net with esmtps (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256) (Exim 4.85) (envelope-from ) id 1ZI2EL-0000lH-RT for rsgb_lf_group@blacksheep.org; Wed, 22 Jul 2015 23:12:19 +0100 Received: from dovecot04.posteo.de (unknown [185.67.36.27]) by mx02.posteo.de (Postfix) with ESMTPS id 4A6B925ACC27 for ; Thu, 23 Jul 2015 00:10:57 +0200 (CEST) Received: from mail.posteo.de (localhost [127.0.0.1]) by dovecot04.posteo.de (Postfix) with ESMTPSA id 3mc9wc6kyjzFpW3 for ; Thu, 23 Jul 2015 00:10:56 +0200 (CEST) Message-ID: <55B014EF.3050601@posteo.de> Date: Thu, 23 Jul 2015 00:10:55 +0200 From: DK7FC User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; de; rv:1.9.1.8) Gecko/20100227 Thunderbird/3.0.3 MIME-Version: 1.0 To: rsgb_lf_group@blacksheep.org References: <3EEE0BD431264721BAD95B0598D0A459@White> <55AA13A5.5090300@posteo.de> <55AEA269.9020900@posteo.de> <55AEBA43.3040006@posteo.de> In-Reply-To: <55AEBA43.3040006@posteo.de> X-Scan-Signature: ea6d85a53e31aad43b971253a5c6867b Subject: Re: VLF: Tweek mode resonances Content-Type: multipart/alternative; boundary="------------070306050802030802000909" 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: 3692 This is a multi-part message in MIME format. --------------070306050802030802000909 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit Hi VLF, Today I've built a transformer to match the antenna to the PA. This is not the ideal solution for QRO / highest antenna currents but i like to idea of a galvanically decoupled antenna and TX. Maybe 5 A is possible with that arrangement. I took 7 ferrite cores in parallel to build that transformer. 39 turns primary, 7 secondary. So 1 Ohm is transformed to 31 Ohm. The transformer handles (no saturation) 1V per turn at 6 kHz, at least. This should give a first detectable signal in the garden loop, i hope. https://dl.dropboxusercontent.com/u/19882028/VLF/20150722_134348.jpg 73, Stefan Am 21.07.2015 23:31, schrieb DK7FC: > Hi VLF, > > Now i continued with measurements of the new loop. I took 3 different > capacitors in series and searched for the maximum antenna current for > a fundamental frequency. I had and have no idea about the L of that loop. > Measurement: > https://dl.dropboxusercontent.com/u/19882028/VLF/20150721_225451.jpg > > C = 100 nF in series => f(res) = 42.1 kHz > C = 1 uF => 12.35 kHz > C = 4 uF => 5.48 kHz (not a sharp resonance peak) > > Hmm, the VFO has a sine output so i assume the measurement should be > accurate, reasonably. Currents have been in the range of 50...70 mA. I > need to repeat the tests with more power and an analog RF current > measurement. This could be done tomorrow. > Actually f(res, 1uF) should be f(res, 100nF)/3.16 but it isn't! The > factor is 3.4. Maybe this has to do with ground conductivity??? No > idea. I don't even know which conductors take part to the loop. But > the loops L seems to be about 170 uH ? > > Maybe it is not even a pure loop! I'm thinking about Rogers "utility > assisted earth mode". Maybe there are a few more resonance peaks for > different antenna configurations, like a lazy vertical with gamma > match or different loops switched in parallel? Complex! > > 73, Stefan > > Am 21.07.2015 21:50, schrieb DK7FC: >> Hello Markus, VLF, >> >> A few minutes ago i've grounded the far end of my antenna with a >> massive wire. This would handle 100 A :-) >> See https://dl.dropboxusercontent.com/u/19882028/VLF/20150721_211722.jpg >> >> I've then determined the loop resistance, which is R = 0.857 Ohm (at >> 1 A DC) >> A = 2000 m^2 is a good estimation. >> >> I'm curious to find the necessary C to resonate the loop on VLF now. >> That's the time to decide which frequency i want to use for a first >> test... >> >> 73, Stefan >> >> Am 18.07.2015 15:20, schrieb Markus Vester: >>> Hi Stefan, LF, >>> we're not just playing around... I would rather consider this an >>> original scientific experiment, perhaps worth proposing as an >>> research project in environmental physics ;-) >>> Frequency stability will not be much of an issue. Throughout the >>> night, the 5th order resonance moved around by two or three pixels, >>> approximately 100 Hz or twice its own width. So after having found >>> the right frequency in the first place, we could just let the >>> transmitter sit on the exact QRG for many hours without loosing much >>> signal (even more so for 1.7 kHz). >>> Here are some back-of-the-envelope calculations for your institute >>> antenna, grounded at the other end: Assuming a loop area of around >>> 2000 m^2, radiation resistance at 8.5 kHz would be 0.17 >>> microohms. Given the cross section of your horizontal wire and >>> the lightning protection system, you could perhaps run up to 100 A >>> loop current (maybe too optimistic?), which would give 1.7 >>> mW radiated power - comparable to your first kite experiments and >>> surely enough to be received here easily even without any resonance >>> enhancement. At 1.7 kHz, radiated power would be 625 times smaller >>> (2.6 uW) but probably still detectable under quiet conditions. >>> Estimated inductance would be around 110 uH or 5.9 j ohms at 8.5 kHz. >>> Depending on the quality of the grounding installation you will >>> probably require some kilowatts drive power, and a suitable array of >>> parallel FKP's to cancel inductance. Alternatively, the company I >>> work for produces some cool fat MRI gradient power amplifiers, >>> designed to deliver hundreds of amps into inductive loads ;-) >>> On the receive side here, my biggest problem at low frequendcies is >>> railway interference - - this is one reason to start at higher >>> frequency before going to 1.7 kHz. As the incident wave will be >>> circularly polarized I could try to rotate the receive loop to >>> minimize local QRM. An interesting and favourable point is that your >>> TX loop will be pointing north-south, which will minimize >>> "contamination" by direct groundwave to my direction. However I do >>> not know what the lateral extent of the resonance mode is, in other >>> words how much enhancement we will still get at 180 km distance or >>> 45° elevation. >>> Alternatively you could try to detect the tweek resonance using >>> your garden receiver, close enough to provide true vertical skywave >>> incidence and full cavity resonance enhancement. To be able to see >>> the skywave, you will only have to turn the receive loop carefully >>> to null direct nearfield inductive coupling. >>> One last point for dreamers: The counterrotating polarized component >>> which is not reflected will penetrate to the magnetosphere and may >>> propagate as a whistler wave. This could theoretically be received >>> at the magnetic conjugate point, somewhere near Madagascar... >>> 73, Markus (DF6NM) >>> >>> *From:* DK7FC >>> *Sent:* Saturday, July 18, 2015 10:51 AM >>> *To:* rsgb_lf_group@blacksheep.org >>> >>> *Subject:* Re: VLF: Tweek mode resonances >>> >>> Hello Markus, >>> >>> Thanks for your observation and article! :-) >>> Most interesting. >>> I can indeed confirm there were a number of cloud-cloud lightnings >>> last night, up to 20 per minute were visible at times. >>> My VLF loop in the garden is a vertical hula hup circle with 80 >>> turns of wire inside. About 80 cm diameter. The receiver is the >>> stereo soundcard, making the stream for MF+VLF. >>> >>> This observation reminds me (of course) on our idea to try to >>> transmit (clap our hands!) on about 2 kHz, the 150 km band :-) with >>> a vertical loop and receive with a vertical loop! I should check if >>> it is possible to reach the roof of the other building, where my >>> TX-antenna is mounted, to ground the wire, i.e. to build a vertical >>> loop! >>> >>> BTW, yesterday i officially requested a permission by the chief of >>> the local forest district to put some electronic equipment on a high >>> tree in the forest!! Solar, modules, batteries, WLAN-antenna and a >>> box including electronic equipment. The location is 3 km distant >>> from the institute, much more distant to man made noise sources then >>> my garden and still in a direct view to arrange the WLAN-link! No >>> answer yet... >>> >>> The 5th mode near 8.5 kHz? Well, that resonance isn't really stable >>> over the frequency, so it is not possible to try modes like >>> DFCW-6000 or even 600. So the possible distances would be rather >>> small. But woth to try playing, obviously :-) >>> >>> 73, Stefan >>> >>> Am 18.07.2015 06:50, schrieb Markus Vester: >>> 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. >>> 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) --------------070306050802030802000909 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Hi VLF,

Today I've built a transformer to match the antenna to the PA. This is not the ideal solution for QRO / highest antenna currents but i like to idea of a galvanically decoupled antenna and TX. Maybe 5 A is possible with that arrangement.
I took 7 ferrite cores in parallel to build that transformer. 39 turns primary, 7 secondary. So 1 Ohm is transformed to 31 Ohm. The transformer handles (no saturation) 1V per turn at 6 kHz, at least. This should give a first detectable signal in the garden loop, i hope.
https://dl.dropboxusercontent.com/u/19882028/VLF/20150722_134348.jpg

73, Stefan

Am 21.07.2015 23:31, schrieb DK7FC:
Hi VLF,

Now i continued with measurements of the new loop. I took 3 different capacitors in series and searched for the maximum antenna current for a fundamental frequency. I had and have no idea about the L of that loop.
Measurement: https://dl.dropboxusercontent.com/u/19882028/VLF/20150721_225451.jpg

C = 100 nF in series => f(res) = 42.1 kHz
C = 1 uF => 12.35 kHz
C = 4 uF => 5.48 kHz (not a sharp resonance peak)

Hmm, the VFO has a sine output so i assume the measurement should be accurate, reasonably. Currents have been in the range of 50...70 mA. I need to repeat the tests with more power and an analog RF current measurement. This could be done tomorrow.
Actually f(res, 1uF) should be f(res, 100nF)/3.16 but it isn't! The factor is 3.4. Maybe this has to do with ground conductivity??? No idea. I don't even know which conductors take part to the loop. But the loops L seems to be about 170 uH ?

Maybe it is not even a pure loop! I'm thinking about Rogers "utility assisted earth mode". Maybe there are a few more resonance peaks for different antenna configurations, like a lazy vertical with gamma match or different loops switched in parallel? Complex!

73, Stefan

Am 21.07.2015 21:50, schrieb DK7FC:
Hello Markus, VLF,

A few minutes ago i've grounded the far end of my antenna with a massive wire. This would handle 100 A :-)
See https://dl.dropboxusercontent.com/u/19882028/VLF/20150721_211722.jpg

I've then determined the loop resistance, which is R = 0.857 Ohm (at 1 A DC)
A = 2000 m^2 is a good estimation.

I'm curious to find the necessary C to resonate the loop on VLF now. That's the time to decide which frequency i want to use for a first test...

73, Stefan

Am 18.07.2015 15:20, schrieb Markus Vester:
Hi Stefan, LF,
 
we're not just playing around... I would rather consider this an original scientific experiment, perhaps worth proposing as an research project in environmental physics ;-)
 
Frequency stability will not be much of an issue. Throughout the night, the 5th order resonance moved around by two or three pixels, approximately 100 Hz or twice its own width. So after having found the right frequency in the first place, we could just let the transmitter sit on the exact QRG for many hours without loosing much signal (even more so for 1.7 kHz).
 
Here are some back-of-the-envelope calculations for your institute antenna, grounded at the other end: Assuming a loop area of around 2000 m^2, radiation resistance at 8.5 kHz would be 0.17 microohms. Given the cross section of your horizontal wire and the lightning protection system, you could perhaps run up to 100 A loop current (maybe too optimistic?), which would give 1.7 mW radiated power - comparable to your first kite experiments and surely enough to be received here easily even without any resonance enhancement. At 1.7 kHz, radiated power would be 625 times smaller (2.6 uW) but probably still detectable under quiet conditions. Estimated inductance would be around 110 uH or 5.9 j ohms at 8.5 kHz.
 
Depending on the quality of the grounding installation you will probably require some kilowatts drive power, and a suitable array of parallel FKP's to cancel inductance. Alternatively, the company I work for produces some cool fat MRI gradient power amplifiers, designed to deliver hundreds of amps into inductive loads ;-)
 
On the receive side here, my biggest problem at low frequendcies is railway interference - - this is one reason to start at higher frequency before going to 1.7 kHz. As the incident wave will be circularly polarized I could try to rotate the receive loop to minimize local QRM. An interesting and favourable point is that your TX loop will be pointing north-south, which will minimize "contamination" by direct groundwave to my direction. However I do not know what the lateral extent of the resonance mode is, in other words how much enhancement we will still get at 180 km distance or 45° elevation.
 
Alternatively you could try to detect the tweek resonance using your garden receiver, close enough to provide true vertical skywave incidence and full cavity resonance enhancement. To be able to see the skywave, you will only have to turn the receive loop carefully to null direct nearfield inductive coupling.
 
One last point for dreamers: The counterrotating polarized component which is not reflected will penetrate to the magnetosphere and may propagate as a whistler wave. This could theoretically be received at the magnetic conjugate point, somewhere near Madagascar...
 
73, Markus (DF6NM)

 
From: DK7FC
Sent: Saturday, July 18, 2015 10:51 AM
Subject: Re: VLF: Tweek mode resonances

Hello Markus,

Thanks for your observation and article! :-)
Most interesting.
I can indeed confirm there were a number of cloud-cloud lightnings last night, up to 20 per minute were visible at times.
My VLF loop in the garden is a vertical hula hup circle with 80 turns of wire inside. About 80 cm diameter. The receiver is the stereo soundcard, making the stream for MF+VLF.

This observation reminds me (of course) on our idea to try to transmit (clap our hands!) on about 2 kHz, the 150 km band :-) with a vertical loop and receive with a vertical loop! I should check if it is possible to reach the roof of the other building, where my TX-antenna is mounted, to ground the wire, i.e. to build a vertical loop!

BTW, yesterday i officially requested a permission by the chief of the local forest district to put some electronic equipment on a high tree in the forest!! Solar, modules, batteries, WLAN-antenna and a box including electronic equipment. The location is 3 km distant from the institute, much more distant to man made noise sources then my garden and still in a direct view to arrange the WLAN-link! No answer yet...

The 5th mode near 8.5 kHz? Well, that resonance isn't really stable over the frequency, so it is not possible to try modes like DFCW-6000 or even 600. So the possible distances would be rather small. But woth to try playing, obviously :-)

73, Stefan

Am 18.07.2015 06:50, schrieb Markus Vester:
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.
 
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)
 
 
 
 
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