----- Original
Message -----
Sent: Sunday, March
09, 2014 12:13 PM
Subject: RE: LF: Daytime
29.499 kHz
Markus,
Thank you for the
feedback. I have a feeling that with discussion, the group might make
particularly quick progress on electrically short VLF antennas for
long-range paths. It?s an area that may need to be led by experiment,
something in which this group excels and industry, with all respect, may
not.
You mentioned:
?My take on the subject was that with lower temperatures groundwave
propagation is more (not less) attenuated?
Yes, I tried to
say the same thing; did I say it backwards? It sounds like we agree:
groundwave is more attenuated at lower temperatures, as you stated
above.
For similar
reasons, the skywave is more attenuated by the ground effects it encounters
in the waveguide, at lower temperatures.
In both cases,
the increase in attenuation at sub-freezing temperatures is substantial.
You mentioned:
?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.?
Yes, I agree
100%.
My comments did
not consider trees.
Before leaving
this point, I?d like to include a thought for other (treeless) cases that
might arise (especially as amateur LF antennas evolve in pursuit of better
radiation efficiencies), with apologies if this is well-worn
ground:
Without trees (in
an open, flat rural area for example) a significant portion of the losses in
a small VLF vertical antenna system might be from reactive-field current
returning to the ground radials or ground screen through soil. A lot of
current flows in the reactive field, and a VLF reactive field?s footprint is
closer to the size of an old VLF transmitter ground radial system than to my
backyard ground radial or ground screen system, so reactive field return
current would flow through soil, which has higher resistance at sub-freezing
temperatures. This is one of the losses that the old commercial VLF systems
worked to minimize, but I wonder if, in a treeless environment this might be
a more substantial concern for amateur VLF vertical antennas than for the
old commercial VLF antennas. This might not apply to antennas for 137 kHz,
but I wonder about amateur VLF antennas.
Thank you for the
feedback. I?m looking forward to whatever we might learn about radiation
efficiency of short VLF transmitting antennas, in the presence of
infrastructure. As you mentioned, the literature reflects commercial
scenarios, and that leaves a substantial gap that work in this group might
help to close.
73, Jim
AA5BW
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
>
--
>
