In rec.radio.amateur.antenna Jeff Liebermann wrote:
On Mon, 7 Dec 2015 07:22:53 -0000, wrote:
In rec.radio.amateur.antenna Jeff Liebermann wrote:
On Mon, 7 Dec 2015 01:17:34 -0000, wrote:
The term is "skywave" and it is not known to happen below 500 kHz.
I beg to differ. The current issue of QEX has an article on a WWVB 60
KHz frequency standard:
http://www.arrl.org/files/file/QEX_Next_Issue/2015/Nov-Dec_2015/Magliacane.pdf
On Pg 15 is the section on 60 KHz propagation:
LF radio propagation is substantially different from that
which exists at higher frequencies. Its remarkable stability
and reliability have often led to the belief that 60 kHz
signals propagate great distances over ground wave paths alone.
In reality, a combination of surface wave and D-layer
ionospheric paths are responsible for WWVB signal propagation.
At night, cosmic background radiation supports a level of
D-layer ionization that is sufficient for propagating LF (and
lower frequency) radio signals over long distances. Greater
D-layer efficiencies and increased effective height with
decreased ionization levels contribute to greater signal
coverage during the nighttime hours. etc...
Note that nowhere in there is the term skywave used.
True. That would have been too easy.
https://en.wikipedia.org/wiki/Skywave
In radio communication, skywave or skip refers to the
propagation of radio waves reflected or refracted back
toward Earth from the ionosphere, an electrically charged
layer of the upper atmosphere.
In other words, anything that bounces at least once off the ionosphere
or earth is a skywave. The D-layer mechanism is described is most
cerainly a skywave.
I would disagree with that.
So, here we have propagation via the ionospheric D-Layer which I
believe is considered a skywave. Note that the author talks about
measuring broadcast band frequencies to an accuracy 312 micro-hertz,
where such things as varying path lengths are important.
The wavelength of 60 KHz is about 5 km. The height of the D layer
varies from 60 to 90 km or about 12 to 18 wavelengths at 60 KHz.
That's too big for a waveguide structure, which suggests that the
dominant mode of propagation is skywave, not ground wave.
We will have to disagree here.
That's why I posted my rant. The idea that VLF signal will reflect or
refract off the D-layer is new to me. I had always assume that
everything with a wavelength longer than the BCB is ground wave.
Apparently not.
Well, there is line of sight, but irrelevant here.
If it were reflections from the D-layer, one would expect to see a
"hole" in the signal level starting at around a few hundred miles or so
extending to around a thousand miles or so where the level would come
back up.
Such "holes" in signal strength from skywave propagation are easily
seen in the plots you can get from the pskreporter web site.
There are no holes in the WWVB signal strength.
The wavelength of WWVB is 5,000 meters and the D-layer is 12-18
wavelengths high. If I draw a scaled model of the earth, D-layer, and
the width of such a signal, there would be considerable overlap of the
incident and reflected signals, causing significant smearing of any
holes in the signal levels. You see those holes at 80m and up because
these shorter wavelengths don't have much overlap. That's not the
case at VLF, where longer wavelengths produce less distinct holes.
Anyway, if there really were holes in the WWVB coverage pattern, it
would have been noted in the propagation predictions and by numerous
users complaining of a loss of signal in specific areas.
The wavelength doesn't have any effect on where the reflections would
appear.
Redraw your scaled model using rays.
The holes appear from two mechanisms:
The first hole will appear between the farthest reaches of ground wave
and the nearest a reflected wave can hit the Earth.
Subsequent holes appear between the regions where reflection can hit
the Earth.
I've been watching propagation using pskreporter for a few years
now. Right now propagation is so poor all you see is the first hole
in the skip propagation with a little propagation into the second
"skip" region. About a year ago when the bands were in really good shape,
you could see the hole between the second and third "skip" region.
Also, if propagation at 60 KHz were a mixture of sky and ground waves,
then the ground wave would also smear any gaps and holes. There might
be some change in signal levels near the alleged holes, but I don't
think you'll see anything under all the constantly changing
atmospheric noise.
Ground wave only goes so far. Yes the distance varies with conditions,
but ground wave has no holes in coverage. Skywave doesn't normally
exist until well after ground wave has petered out.
Something else is at play here but I'm not sure what to call it, but
I would not call it skywave.
Good point. Nothing new can be discussed without first assigning a
name. Liebermann-wave does sound rather catchy. D-layer-ground-wave
is ugly. Duct-wave might work. Incidentally, it's likely that the
ground reflects, while the D-layer refracts at 60 KHz, adding
additional confusion.
https://en.wikipedia.org/wiki/Very_low_frequency#Propagation_characteristics
Here it is refered to as "ducting".
Sounds like a better term to me.
Ducting or Duct-wave are probably sufficiently vague. I had always
assumed that ducting operates exactly the same as in a waveguide (two
parallel quarter wave stubs elongated into a tube) with no internal
reflections involved. In this situation, it seems like anything that
can propagate between two conductive layers, either bouncing or
directly, can apply. Yeah, I guess that's vague enough.
A waveguide would have very little loss, while ducting would have a
lot of loss.
--
Jim Pennino