Friends in Radio Land,
I have been in the background, following the discussion about wave
polarization and angles of arrival. The discussion has been
largely experimental in nature and I cannot add anything there of
my own experience. But Bill Tippett's most recent posting, "Angle
vs Polarity" on 05 August, is something that I feel I should
respond to. So for what it's worth to you, let me make some
observations about his posting and then on to the general matter.
Bill's remarks about the F2 layer are more appropriate to signals
in the HF range (3-30 MHz) and they are not applicable to Top Band
propagation because of the lower frequency (1.8 MHz) involved as
well as the presence of the geomagnetic field. This is not to say
I am disputing what he wrote about round-the-world signals and
such. He had the right answer but to the wrong question. So you
can learn all about that, from the German work during WW-II, in an
1948 I.R.E. article by H.A. Hess. Our discussion is about 1.8
MHz, not 18 MHz or higher. But Bill's ideas related to Pedersen
ray propagation do apply to Top Band propagation. Let me explain.
Pedersen ray propagation takes place at the transition from one
ionospheric region to another, the lowest being between the top of
the E-region and the bottom of the F1-region while the highest at
the F2-peak which divides the bottom and topsides of the F-region.
This form of propagation happens when a ray which came up from
below the transition region has been refracted such that it is
finally moving parallel to the earth's surface at the ionospheric
boundary and continues that way for some distance.
Pedersen ray propagation results in long hops for signals in the
HF part of the spectrum but any change or gradient in the electron
density in the top of the F-region, an increase or decrease, will
refract the signal away from that direction, down to ground or up
to Infinity from the F-peak. In practice, this mode is rather
unstable and appears briefly (see p. 181 of Davies' recent book
for experience on North Atlantic paths.)
Turning to 1.8 MHz signals, something resembling Pedersen ray
propagation can take place at the top of the night-time E-region.
In case you have missed it, I have written on this topic several
times, pointing out that long E-F hops can take place, covering
distances up to 3,000 km instead of the more familiar 1,000 km
E-hops or 2,000 km F-hops. Such E-F hops are expected on
theoretical grounds because of the nature of the electron density
distribution, a deep valley above the E-peak developing after
sunset. Experimentally, a valley distribution is well-documented
and has been seen for years, by incoherent scatter radars from
Puerto Rico to Northern Norway, leaving no room for any dispute.
Beyond the theoretical idea, one can explore the computational
side of long E-F hops by means of ray-tracing of paths across the
ionosphere. Mainframe computer programs developed in Boulder by
the Department of Commerce radiophysicists in the 60's and 70's
allowed such studies, even including the effects of the earth's
magnetic field. Those programs have now been brought down to the
PC level in the PropLab Pro program, available for use in amateur
circles. (Let me recommend it to you!)
Briefly, such ray-tracings show that E-hops on a path are found
at low radiation angles incident on the bottom of the E-region.
Then, by advancing the radiation angle in small steps, one finds
the E-hops become longer and longer til they go over to E-F modes
where the path rises to a peak in the F1-region and then retraces
itself down through the E-region to ground:
E-F hop F1-region
*
* *
* *
- - - - - - * * * * * * * * - - - E-peak
* *
* *
* *
* *
_______________________________________________________________
ground
Finally, at slightly higher radiation angles, signals are
propagated by F-hops instead of E- or E-F hops:
F hop F1-region
*
* *
* * -
- - - - - - - - - - * * - - - - - - - - - E-peak
* *
* *
* *
* *
_______________________________________________________________
ground
The fact that ray-tracings show 1.8 MHz paths are limited to the
lower ionosphere, rising no more than about 200 km instead of to the
300+ km F-peak, stems from a fundamental result of ionospheric
theory. Briefly put, only a fraction of the 1.8 MHz operating
frequency is effective in carrying Top Band signals obliquely up
into the ionosphere. That fraction is found by multiplying the
QRG by the cosine of the angle by which the signal goes into the
ionospheric region. Thus, for a 30 degree take-off angle, RF
approaches the ionospheric layer with a 60 degree angle from the
vertical and the effective vertical frequency would be 0.9 MHz.
So signals would peak at an altitude where the local plasma
frequency is 0.9 MHz. Of course, that would depend on latitude
but would not come close to the height of the F2-peak where plasma
frequencies are always greater than 3 MHz, even at solar minimum.
Now everything that I have said above can be verified by
using the PropLab Pro program in 2-dimensions. Just pick a path
in darkness, start at a low radiation angle, say 10 degrees, and
slowly increase the angle, 0.1-0.2 degree at a time. First you
will see E-hops, then E-F hops and finally F-hops. Nothing to it!
Those simple paths would apply to Top Band propagation except for
the presence of the geomagnetic field. The theory is much more
involved now but the results are much the same, at least as far as
how high Top Band signals rise in the ionosphere. With PropLab
Pro, you can use the same incremental technique and explore paths.
I have done it thousands of times!
What you see when you take the program to 3-dimensions is about
like before: first, E-hops at low radiation angles, then E-F hops,
then either F-hops at a slightly higher angle or a DUCTED PATH,
slightly skewed, where the ducting amounts to a string of E-F
hops in series. But that is Pandora's Box (in more ways than one)
and I will have to stop right here.
This is obviously a complicated matter and I won't belabor the
subject further. Simply let me say that HF ideas are quite out
of place when it comes to Top Band propagation and a whole new
approach is required if you still think that way. But this is
not news; it has been around propagation circles for 50 years!
If you're interested in these matters, I have an article in the
Spring '98 issue of Communications Quarterly as well as a shorter
one in the Top Band Anthology published recently by the Western
Washington DX Club. Background material may be found in issues of
The DX Magazine, 1996 and onward. I had hoped these ideas, old as
they are, would not continue to remain well-kept secrets but that
seems to be the case.
Finally, I hope I haven't offended anybody. I didn't invent
these ideas; I am the "messenger" and am just trying to tell
it like it is, REALLY IS! With that I will hold my peace.
73,
Bob, NM7M
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