Re: LF: RE: Three dots

[Markus Vester <markusvester@aol.com>]

Hi Jim,

thanks for sharing your thoughts and experience!

First I should clarify that the "three dots" observation was on 8270, not 2970 Hz. But the fundamental physics and geometry considerations should apply top both frequencies.

The riddle is that we need to explain a radial H (eastward) component from a vertical TX antenna. I think that even with steep skywave (~ 45° in our case), we would still be left with a purely azimuthal (northward) H-field, unless we include Faraday rotation. Another hint in that direction might be that on Paul's directional spectrogram, I sometimes find that I have to rotate the compass bar a couple of notches to maximize the signal at different times. However to build up a dot in 47 uHz, the Faraday rotation would need to be more or less stable over a few hours.

On the other hand, I don't know whether the loop in the tree is really aligned accurately orthogonal to my incidence. On earlier occasions when Stefan used his other (east-west) loop, I was able to produce 15 to 20 dB SNR in that bandwidth, Thus the rejection on the current loop woud need to be less than about 10 dB, corresponding to an angular offset of 18° or more from N-S. 

Best 73,
Markus 


-----Ursprüngliche Mitteilung-----
Von: hvanesce <hvanesce@comcast.net>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Di, 24. Jan 2017 8:48
Betreff: LF: RE: Three dots

Markus,

 

Very interesting observations. I had been wondering about the orientation of the loop vs incident wave polarization for three reasons:

1) Changes in wave polarization from near field to far field: with a small loop TX antenna the magnetic field polarization changes from near field to far field in a manner that is consistent with your RX-antenna orientation example, but I confirmed via NEC 6+ with a realistic ground that this is not the case with the magnetic components from small horizontal or vertical monopoles/dipoles. I’m guessing that you and others already knew this but I didn’t have a strong intuition about the magnetic near field of a short monopole especially if inclined from vertical, so I simulated. Item (1) seems irrelevant based on the simulation.

2) Mixed skywave/groundwave polarization variation: I ran some simulations with fairly reasonable 10-minute updates to h’ and beta, showing ~ 60 degree daily phase variability at 3kHz / 170km, mostly from 1st and 2nd hop mixing with groundwave (almost 1:1 ratio). The simulated diurnal phase signature was not nearly as distinct and simple in the 2970 Hz 170 km case as in a 20 kHz 2000 km case; the (2970 Hz 170 km) results show phase changing constantly i.e. no distinct stable time of the type seen at 20kHz 2000 km; and the results show phase fairly sensitive to ionospheric variations (h’ and beta). These characteristics are perhaps qualitatively and perhaps even quantitatively reminiscent of the color changes in Stefan’s color-DF grabber, but I did not think of that particular association until I read your message below; thank you for that valuable observation. One big caveat: I am not confident about  the fidelity of the simulations at 2970 Hz; I think that the models are poorly validated between 2kHz and 4kHz (i.e. at/near modal cutoff and attenuation peak).

3) Mixed skywave/groundwave polarization bias:  based on the above, it seems unlikely that the skywave would be near-vertically polarized at such a steep angle and at such a short distance from the polarizing reflection; and with skywave/groundwave ratio ~ 1:1 at 2970 Hz 170km, it seems reasonable that the projection of the non-vertical vector onto the sensitive plane of the loop would be significant. Even a 20 degree angle from vertical could project up to almost 34% of the field onto the sensitive plane of the loop (does this seem reasonable to you?) I have seen substantially non-vertical skywaves at VLF on many occasions, and have been uncertain about fixed-orientation RX loops for that reason (a compact 3-axis ferrite antenna would dispense with that uncertainty but ferrite has its own issues). I usually angulate single axis loops in both axes for best signal, but I am often portable near infrastructure so in many cases I don’t know if the unusual polarizations are due to infrastructure or due to polarization preserved from a high-angle and therefore also short range polarized reflection. But in many cases it has been possible to confirm unexpected polarization due to modal mixing with skywaves.

 

Best,

 

Jim AA5BW  

    

 

From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Markus Vester
Sent: Monday, January 23, 2017 5:16 PM
To: rsgb_lf_group@blacksheep.org
Subject: VLF: Three dots

 

Interestingly, my on 8270.0025 Hz carrier transmissions during the last three evenings
 Jan 20, 18:00 - 23:00 UT,
 Jan 21, 15:28 - 23:00 UT,
 Jan 22, 13:23 - 23:00 UT
left three dots on Stefan's 47 uHz spectrogram http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber2.html .

This is astonishing because the tree-receiver is currently connected to a north-south oriented loop, whereas I am due east in a null. So I was wondering whether reception was made possible by skywave with rotated polarisation? Perhaps reminiscent of the playful colour changes on Stefan's MF colour-DF grabber.

Despite much lower noise background, the (shorter) daytime carrier
 Jan 22, 07:00 - 10:00 UT.
left no trace at all.

Best 73,
Markus (DF6NM)