Markus,
Yes, I agree, many temperature-related near-field and ground-effects mechanisms in VLF related to radiation efficiency of small antennas, and temperature-related ground-effects mechanisms related to long-distance path loss, are substantially different in MW and VLF. Dielectric characteristics would also have been my first example, with respect to (a) displacement currents as you mentioned; and (b) with different loss mechanisms in VLF and MF (such as in the case of ice), and also affecting the H-field pattern in soil (thus different frequency-normalized H-field path lengths in MF and VLF bands).
Regarding Siple:
Markus: “This may be good for exciting steep magnetospheric whistler modes…”
Stanford: “This antenna is to be used to inject VLF waves into the magnetosphere in order to perform wave particle interaction experiments”
Other Stanford comments:
Matching over the range 1-20 kHz
Efficiency greater than 2 percent over the approximate frequency range 5-14 kHz
1kW ERP 5-14 kHz
Maximum efficiency at approximately 10 kHz, of approximately 5 percent (2.5kW ERP)
Low voltage (reliable)
Inexpensive: the vertical monopole at North Cape, Australia, constructed at a cost of $25 million, has a calculated system efficiency of only 4 percent at 10 kHz. The Siple antenna, constructed at a small fraction of this cost, has a calculated system efficiency greater than 2 percent at 10 kHz.
Jim: in “inexpensive” above, Stanford does not mention Antarctic travel costs; this suggests to me that in California, Antarctica is considered a getaway spot
Radiation pattern not provided.
Paper on Siple Station antenna efficiency and impedance attached (I hope; otherwise: “VLF Antarctic Antenna: Impedance and Efficiency”)
73, Jim
Reviewing Bob's direct mails, he mentioned a body of evidence of improved groundwave propagation at low temperatures and frost, based mostly on mediumwave broadcast experience. That seems to contradict the notion that decreased conductivity would hinder propagation.
Maybe the clue to this riddle is the contribution of dielectric permittivity and displacement currents, which increases with frequency. For each material, there is a crossover frequency above which omega * epsilon becomes greater than sigma. In that high frequency regime, earth may be acting more as a lossy dielectric waveguide, refracting the wave around the curvature. Lower conductivity will then mean lower loss tangent and less absorption. On the other hand, at VLF frequencies displacement currents just tend to be neglegible, and we are looking at a purely conductive (quasi metallic) boundary, where ohmic losses increase with surface resistance.
Still wondering about that horizontal dipole above ice (Siple station): It may radiate effectively, but mostly upwards! This may be good for exciting steep magnetospheric whistler modes, but would it also be useful for long-distance communication?
Sent: Sunday, March 09, 2014 10:25 AM
Subject: Re: LF: Daytime 29.499 kHz
Thanks Jim for the detailed information. Bob and I had discussed the temperature effects in a private mail exchange, and hadn't quite come o the same conclusions.
My take on the subject was that with lower temperatures groundwave propagation is more (not less) attenuated. But regarding the near field, losses and E-field shunting from trees around the antenna decrease with frost, so radiation efficiency improves significantly. This effect is probably not covered by the literature, as commercial LF antennas tend to be installed above a ground screen on a clear field. With valley-spanning installations (eg. the former Haiku antenna on Oahu), it was mentioned that forestry beneath the antenna had been removed to reduce losses.
Sent: Sunday, March 09, 2014 9:36 AM
Subject: RE: LF: Daytime 29.499 kHz
Bob,
Some information below on the effects of above-freezing and below-freezing temperatures on various VLF losses, including (a) ground-wave losses, (b) near-field ground effects losses and (c) skywave (waveguide) ground effects losses.
A short preface:
The discussion below is for a VLF system with a vertical transmit antenna.
In a VLF system with a vertical transmit antenna, high surface conductivity under the antenna and in the near and far fields is desirable. (1)
In a VLF system with a horizontal antenna, low surface conductivity under the antenna is desired, and high surface conductivity is desired elsewhere in the near field, and in the far field. (1)
This makes Antarctica a great place for VLF QRP: ice under the antenna; salt water elsewhere (whence the 42 km long dipole at Siple Station: https://s3.amazonaws.com/Antarctica/AJUS/AJUSvXVIIIn5/AJUSvXVIIIn5p270.pdf http://nova.stanford.edu/~vlf/Antarctica/Siple/ )
Temperature coefficients of conductivity:
As temperature drops, non-frozen soil conductivity (mhos, siemens) decreases by roughly 1.9% per degree C. (2)
As temperature drops below freezing, soil VLF conductivity decreases rapidly; the rate of change is highly dependent on soil type, but a 2:1 change per 10 degrees C at 10 kHz (for below-freezing temperatures) is a reasonable example for some soils. (3)
Near field and ground wave losses:
As conductivity decreases (i.e. with decreasing temperature), VLF near-field and ground wave losses increase non-linearly (4)
At 30kHz, ground wave amplitude is low at 5000 km (4)
At 30kHz, ground-wave loss in non-frozen soil of poor conductivity (10^-4 mhos/m) might be 20dB greater (20dB more loss) than in non-frozen soil of fairly good conductivity (10^-2 mhos/m) (4)
At 30kHz, ground-wave loss in frozen soil would be very high (4)
All of the above might suggest a very low amplitude ground wave at TA distances in mid-winter (ground wave including direct, ground-reflected, Norton surface and trapped surface waves)
In that case the long-path part of the problem reduces to skywave losses (skywave ionospheric losses and skywave ground-effects losses):
Skywave loss (waveguide loss in this case) due to ground effects is nominally:
d_alpha_gnd = [.046 * sqrt(f)] * 1/{h * sqrt(s) * sqrt[1 – (fc/f)^2]} (in units of dB per 1000 km) (5)
where f = 29,500 (Hz), fc ~ 2100 (Hz) (day) or 1700 (Hz) (night), h ~ 70 (kilometers), .0001 < s < .01 [s for sigma (conductivity), in mhos per meter; range of values from .01 for very good (highly conductive) soil to .0001 for frozen earth; values below .0001 occur in arctic regions, but such regions tend to have snow and ice above the soil. A different formula is used for snow and ice losses at frequencies below 1 MHz]
The formula above may be sufficient for analysis of cold-weather propagation losses over distances of 5 Mm or more. Besides propagation losses, there are near-field and antenna-system losses.
Near-field and antenna-system losses:
VLF near-field and antenna-system losses associated with ground effects, increase as temperature (and therefore conductivity) decline. The increase in near-field and antenna-system losses can be between sqrt (s) and s^1.5, but the nominal value of these losses is hard to determine analytically for an electrically-short antenna. The calculation for ice/snow cover for your antenna system may be easier to determine analytically; if you’re interested I’ll send a notional formula.
If your VLF signal reports do not change appreciably after a snowfall or an ice storm, that information can be used to help generate a temperature model of your ground-effects-related near-field and antenna-system losses, in which case you might have a good starting point for a whole-system model for all temperatures. Noting what happens to signal reports after a freeze, after a snowfall and after an ice-storm can help to nail down the most difficult part of the system model for a VLF system with an electrically short antenna (antenna-system losses and near-field losses).
In summary:
A) VLF near-field and antenna-system losses associated with ground effects, increase as temperature declines. Noting what happens to signal reports after a freeze, after a snowfall and after an ice-storm can help to establish a good model for these. Such a model will be useful at all temperatures, and very helpful in the optimization of an electrically-short VLF antenna.
B) The formula above for d_alpha_gnd (relative ground-effects losses incurred by the skywave in the waveguide) gives values from about 2 dB per megameter to 6 dB per megameter at 29.5 kHz for soil conditions ranging from fairly good to frozen. This formula is useful by itself for assessing the difference in far-field path loss at various temperatures. Total path loss includes other terms that add to and subtract from* d_alpha_gnd, but those terms are not necessary for assessing changes in ground-effects-related skywave path losses (in the waveguide) over temperature.
C) Ideally, you would add (A) to (B) to obtain the sum of the predominant contributors to loss variation over temperature.
* terms such as the convergence factor, and reinforcements at discontinuities, subtract from path loss
(1) Biggs, AGARD, 1970
(2) a general approximation; Corwin 2005, Ma 2010
(3) Moore 1992
(4) Watt 3.2
(5) Watt 3.4.33
73, Jim AA5BW
Paul;
It is not only the ground but the air temp has great influence and maybe much more than the shallow depths of the ground freezing-Bob
> Date: Sat, 8 Mar 2014 17:06:09 +0000
> From: [email protected]
> To: [email protected]
> Subject: Re: LF: Daytime 29.499 kHz
>
>
> Bob wrote:
>
> > Wonder what effects soil conductivity changes has on the
> > propagation at these VLF freqs??
>
> I have no information. You would expect ground resistance
> to rise, but would that make noticeable difference to
> propagation if it is only a freezing of a shallow surface
> layer?
>
> I had to go to a narrower bandwidth to produce phase and
> amplitude plots for last night's test
>
> http://abelian.org/vlf/tmp/29499_140308a.gif
>
> Signal is down by some 5dB compared with some recent
> tests.
>
> Nothing detected this afternoon.
>
> Propagation seems normal, noise floor normal.
>
> Maybe the cold and frozen ground is affecting the tx
> efficiency, some lower Q of the loading coil - antenna -
> ground loop, or a reduction of effective height.
>
> Will be interesting to see what happens after the thaw.
>
> --
> Paul Nicholson
> --
>
