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Re: [VLF_Group] Ghost bands created by VLF ionospheric heating

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  • Paul
    ... That would be a remarkable kind of 2nd order Luxemburg effect. One minor flaw in this suggestion is that the heating effect would be at 2 * 1.65kHz since
    Message 1 of 8 , Feb 10, 2013
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      Markus wrote:

      > DHO38 on 23.4 kHz and HWU ... 21.75 kHz

      > difference frequency of 1.65 kHz,

      > beat frequency between the two transmitters
      > modulates the electron temperature

      > creates crossmodulation on the carriers of HGA22
      > (135.43 + 1.65 kHz) and DCF39 (138.83 - 1.65 kHz).

      That would be a remarkable kind of 2nd order Luxemburg
      effect.

      One minor flaw in this suggestion is that the heating
      effect would be at 2 * 1.65kHz since the heating
      doesn't care what polarity the signal has.

      Perhaps it is simply the LF receiver itself picking up
      a bit of DHO and HWU and mixing them.

      I am inspired to feed a little bit of signal from prx-e1
      into an LF scanner or my BT878 card that samples at
      448k/sec.
      --
      Paul Nicholson
      --
    • Markus Vester
      Hi Paul, VLF, ... Like classic Luxembourg effect, the crossmodulation amplitude scales with the power of the heater(s) (ie 2nd order), and also linear with the
      Message 2 of 8 , Feb 10, 2013
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        Hi Paul, VLF,

        >second order...
        Like classic Luxembourg effect, the crossmodulation amplitude scales with the power of the heater(s) (ie 2nd order), and also linear with the victim. So overall it's a third order effect. In this case we look at the product appearing at f1 - f2 + f3 (ie. DHO - HWU + HGA). The same relation holds for classic AM heating, where f1 and f2 correspond to the carrier and an AM sideband of the heater.
        > 2 * 1.65kHz...
        No. Heater power density scales with (E1+E2)^2, and the modulation comes only from the mixed term 2*E1*E2', which is purely sinusoidal with the difference frequency.

        I am fascinated by this effect, and have been pursuing it for some time. I am quite sure that it's not an instrumental artefact, for several reasons:
        - the ICM product appears only at night, and only when there is also Luxembourg effect from other AM broadcasters,
        - I've observed it on two different passive resonant LF antennas. It is also picked up by the crossed loop DF-channel and lines up with the expected direction,
        - I do not see other possible combinations with high difference frequencies. These are suppressed by thermal inertia, but would be present in case of rx or antenna nonlinearities. Nothing is observed when HWU is on its alternate frequency 18.3 kHz,
        - the impressed depth of modulation is approximately right for the heater power and distance,
        - the ICM bands have sometimes appeared on other receivers as well (DK7FC LF-DX grabber, Twente WebSDR),
        - the relative phase of the crossmodulation is not constant but varies with propagation (see below).

        To study the effect further, I have set up a correlation experiment, using SpecLab to simultaneously receive the heaters, the victims and the ICM product.

        The right stereo channel receives VLF directly from two series-connected loopstick antennas, which are resonated and pointed to HWU 21.75 kHz and DHO 23.4 kHz. After some software filtering, and a small delay to compensate for path differences, the VLF channel is AM-demodulated in software to generate the 1.65 kHz difference frequency. This is further processed by an AGC and an FFT filter.

        On the left channel, I process the audio from my LF SSB receiver which is tuned to 135.5 kHz USB. After noise blanking, the signal passes a bandfilter for the ICM bands around 137.1 kHz. Note that to this stage the channels are completely independent, and the noise blanker knows nothing of the VLF signal.

        Finally the two outputs are multiplied. This removes the MSK spread from the LF ICM band, and transfers it back to the original victim frequency. The multiplier output feeds the primary channel of a phase-senstitive colour spectrogram, with display slots centered on the DCF39 (3330.6 Hz) and HGA (-70 Hz) idle carriers. The secondary channel is taken from the LF input before the bandpass, resulting in a colour which encodes the phase lag for the ICM path, relative to the direct paths from the VLF and LF transmitters.

        To visualize the procedure and results, I have uploaded some files to
        http://www.df6nm.bplaced.net/LF/vlf_lf_icm_2013/
        - df6nm-grabber_090221_0100_annotated.png is an older colour-DF spectrogram from Feb 2009, which nicely shows the two "ghost bands" in different colours,
        - HWU_DHO38_DCF39_HGA22_DF6NM.jpg shows the geographical locations of the heaters, victims and receiver site,
        - Twente_LF_VLF_ICM_130210_0010_annotated.png shows the ICM products as seen in Holland yesterday night. Note that there are also 1.3 kHz bands which are presumably caused by ICM from DHO (23.4) and GQD (22.1 kHz),
        - VLF_loopsticks.jpg shows the loopsticks for DHO and HWU,
        - icm_130210_0200.jpg is an example of despread output from last night,
        - icm_plot_130210_0808.png is a level plot for last night. Despread ICM products are shown bright red (137.08 kHz) and bright purple (137.18 kHz). Strong red and purple are the direct levels of the LF victims, HGA22 and DCF39. The blue ones are the direct waves from the heaters, DHO38 and HWU,
        - SpecLab_settings_vlf-lf-icm_130210.ini is a SpecLab configuration file for the correlation experiment, for anyone wishing to try it at home.

        Best 73,
        Markus (DF6NM)


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      • john heath
        Hi Markus     For a true beat frequency , heterodyne , there would have to be some kind of nonlinear gain yes / no ? Nonlinear effects are possible in the
        Message 3 of 8 , Feb 10, 2013
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          Hi Markus
           
           
          For a true beat frequency , heterodyne , there would have to be some kind of nonlinear gain yes / no ? Nonlinear effects are possible in the atmosphere but I find it more likely to happen in a slightly nonlinear RF stage of a receiver front end . Is it possible your RF receiver is just slightly nonlinear ? It will always be non linear to some degree so the question is how much . For broad bandwidth with different dynamic ranges the problem increases . Day and night will have an effect on a RF stage as the AGC setting will change . I assume it is not fixed . Leaving this aside --- 

           
           
          >electron temperature in the D-layer, and the variable absorption creates crossmodulation on the carriers of HGA22 (135.43 + 1.65 kHz) and DCF39 (138.83 - 1.65 kHz).

           
          A variable impedance , temp coefficient , does not make for cross modulation in the strictest theoretical sense . However saturation does . This is a long shot but can you see anything in the D-layer that would put it on a saturation bubble . Bubble is not a technical term . I will rephrase . At a give temperature a voltage gradient per meter hits a saturation point in the D- layer thus opening the door for non linearity , cross modulation .
          Probably not but I wanted to seed the thought in case. 
           
           
          John H
           
            
           
           
          --- On Sat, 2/9/13, Markus Vester <markusvester@...> wrote:


          From: Markus Vester <markusvester@...>
          Subject: [VLF_Group] Ghost bands created by VLF ionospheric heating
          To: VLF_Group@yahoogroups.com
          Received: Saturday, February 9, 2013, 4:18 PM



           



          On my LF grabber http://www.df6nm.de/grabber/Grabber.htm there is again a ~ 200 Hz wide red noise band centered on 137.08 kHz. It has been observed several times before but has been absent for a few months. Sometimes it is accompanied by a weaker "brother", centered on 137.18 kHz with purple (ie. northerly) colour. The structure is reminiscent of an MSK signal ("the ghost of CFH"). The bands were seen regularly but are stronger in nights with strong Luxemboug effect.

          After a lot of head scratching, a while ago I have found a really astonishing explanation for this effect: It is ionospheric cross modulation, with a pair of strong VLF transmitters acting together as a modulated heater.

          DHO38 on 23.4 kHz and HWU (which has recently reappeared on 21.75 kHz) have a difference frequency of 1.65 kHz, with the sum of the two FSK shifts. Similar to an AM demodulator, the beat frequency between the two transmitters modulates the electron temperature in the D-layer, and the variable absorption creates crossmodulation on the carriers of HGA22 (135.43 + 1.65 kHz) and DCF39 (138.83 - 1.65 kHz). This constellation is rather unusual not only in that the heating is done by VLF signals. What is even more peculiar is that unlike a simple AM transmitter, the two non-colocated heaters must produce an inhomogeneous and fast moving heating pattern.

          If you look at the Twente WebSDR http://websdr.ewi.utwente.nl:8901/ now, you can currently see and hear prominent 1.65 kHz FSK sidebands around DCF39 and also DCF49 (128.93 kHz). However HGA seems less affected at Twente, presumably because the easterly reflection area is not so well illuminated for that path.

          Best 73,
          Markus (DF6NM)

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          [Non-text portions of this message have been removed]
        • Markus Vester
          Hi John, ... An incandescent lamp inserted series to an RF line makes a nice analogy to the ionospheric nonlinearity. Imagine two independent transmitters
          Message 4 of 8 , Feb 10, 2013
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            Hi John,

            > A variable impedance , temp coefficient , does not make for cross modulation in the strictest theoretical sense ...
            > John H

            An incandescent lamp inserted series to an RF line makes a nice analogy to the ionospheric nonlinearity. Imagine two independent transmitters sending their signals down the line, a weak unmodulated probe signal (the "victim") on one frequency, and a strong AM modulated signal (the "heater") on another. Each time the heater ramps up, the filament becomes warmer, thereby increasing its resistance. This will increase the attenuation for the probe signal, thus impressing a negative carbon copy of the heater AM on it. This is pretty much how Luxembourg effect happens up there.

            Best 73,
            Markus (DF6NM)


            [Non-text portions of this message have been removed]
          • john heath
            Hi Markus     Cool , I like the light bulb analogy , variable impedance with variable current . So impedance now becomes dynamic depending on EM strength of
            Message 5 of 8 , Feb 10, 2013
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              Hi Markus
               
               
              Cool , I like the light bulb analogy , variable impedance with variable current . So impedance now becomes dynamic depending on EM strength of a strong carrier increasing electron hits , temperature . Wiki does not cover  Luxemburg-Gorky effect well but freedictionary.com does.
               
               
              http://encyclopedia2.thefreedictionary.com/Luxembourg+Effect

               
               
              No matter what ones take is on this Russian radios did received cross modulation from other stations so it falls under empirical evidence regardless of the reason why. In short your notion of upper atmosphere cross modulation has historical support provided there is reasonably strong EM carrier there to modulate with. You can see from my phrasing I am still squirming on this one . It takes a while to get used to a new idea :<) .   
               
               
              John H
               
                  
               

              --- On Sun, 2/10/13, Markus Vester <markusvester@...> wrote:


              From: Markus Vester <markusvester@...>
              Subject: [VLF_Group] Re: Ghost bands created by VLF ionospheric heating
              To: VLF_Group@yahoogroups.com
              Received: Sunday, February 10, 2013, 3:48 PM



               



              Hi John,

              > A variable impedance , temp coefficient , does not make for cross modulation in the strictest theoretical sense ...
              > John H

              An incandescent lamp inserted series to an RF line makes a nice analogy to the ionospheric nonlinearity. Imagine two independent transmitters sending their signals down the line, a weak unmodulated probe signal (the "victim") on one frequency, and a strong AM modulated signal (the "heater") on another. Each time the heater ramps up, the filament becomes warmer, thereby increasing its resistance. This will increase the attenuation for the probe signal, thus impressing a negative carbon copy of the heater AM on it. This is pretty much how Luxembourg effect happens up there.

              Best 73,
              Markus (DF6NM)

              [Non-text portions of this message have been removed]








              [Non-text portions of this message have been removed]
            • Paul
              Thanks for taking the time to explain this in a bit more detail. I understand your suggestion better now. In your first email I thought that DHO and HWU were
              Message 6 of 8 , Feb 11, 2013
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                Thanks for taking the time to explain this in a bit more detail.
                I understand your suggestion better now. In your first email
                I thought that DHO and HWU were mixing to produce 1.65kHz
                and then 1.65 went on to do the heating. Now I understand it
                is the linear sum of DHO and HWU which is modulating the electron
                temperature.

                This seems more likely. (If pairs of MSK transmitters could
                mix to produce signals at their difference frequency, we would
                hear them regularly in the lower few kHz of the VLF band.)

                I will try to look for this effect but maybe at other frequencies.
                In Todmorden I am not so well placed with respect to the heating
                zones.

                Tested recording raw the entire spectrum 0-224kHz at 448k
                samples/sec, this worked well but last night reception was not
                good - lots of direct intermod due to rx overload. Strong winds
                were flapping the antenna and causing some problems. The very
                low frequency response of the rx is a nuisance here.

                --
                Paul Nicholson
                --
              • Markus Vester
                Three days ago (May 22), the powerful French Navy station HWU shifted its operating frequency from 21.75 to 22.6 kHz. This is closer to 23.4 kHz from German
                Message 7 of 8 , May 25, 2013
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                  Three days ago (May 22), the powerful French Navy station HWU shifted its operating frequency from 21.75 to 22.6 kHz. This is closer to 23.4 kHz from German DHO, and consequently the intermodulation bands created by joint ionospheric heating have moved and become more prominent due to the lower audio beat (0.8 kHz).
                  http://df6nm.bplaced.net/LF/vlf_lf_icm_2013/test1305242330.jpg shows crossmodulation impressed on HGA22, a fat red band centered on 136.33 kHz (ie. 135.43 + 23.4 - 22.6). There is also a weaker band visible on 136.73 kHz (135.43 + 23.4 - 22.1) from DHO with GQD, which appears much more rarely here. Observers in Western Europe might also see crossmodulation from other VLF combinations, eg. GQD - HWU = 0.5 kHz.

                  There are some more explanations in this thread:
                  http://tech.groups.yahoo.com/group/VLF_Group/message/20598

                  Best 73,
                  Markus (DF6NM)

                  From: Markus Vester
                  Sent: Sunday, February 10, 2013 2:07 AM
                  To: rsgb_lf_group@... ; rsgb_lf_group@...
                  Subject: Ghost bands created by VLF ionospheric heating


                  On my LF grabber http://www.df6nm.de/grabber/Grabber.htm there is again a ~ 200 Hz wide red noise band centered on 137.08 kHz. It has been observed several times before but has been absent for a few months. Sometimes it is accompanied by a weaker "brother", centered on 137.18 kHz with purple (ie. northerly) colour. The structure is reminiscent of an MSK signal ("the ghost of CFH"). The bands were seen regularly but are stronger in nights with strong Luxemboug effect.

                  After a lot of head scratching, a while ago I have found a really astonishing explanation for this effect: It is ionospheric cross modulation, with a pair of strong VLF transmitters acting together as a modulated heater.

                  DHO38 on 23.4 kHz and HWU (which has recently reappeared on 21.75 kHz) have a difference frequency of 1.65 kHz, with the sum of the two FSK shifts. Similar to an AM demodulator, the beat frequency between the two transmitters modulates the electron temperature in the D-layer, and the variable absorption creates crossmodulation on the carriers of HGA22 (135.43 + 1.65 kHz) and DCF39 (138.83 - 1.65 kHz). This constellation is rather unusual not only in that the heating is done by VLF signals. What is even more peculiar is that unlike a simple AM transmitter, the two non-colocated heaters must produce an inhomogeneous and fast moving heating pattern.

                  If you look at the Twente WebSDR http://websdr.ewi.utwente.nl:8901/ now, you can currently see and hear prominent 1.65 kHz FSK sidebands around DCF39 and also DCF49 (128.93 kHz). However HGA seems less affected at Twente, presumably because the easterly reflection area is not so well illuminated for that path.

                  Best 73,
                  Markus (DF6NM)


                  [Non-text portions of this message have been removed]
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