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: [email protected] [mailto:[email protected]] On Behalf Of Markus Vester
Sent: Monday, January 23, 2017 5:16 PM
To: [email protected]
Subject: VLF: Three dots