Friends in Radio Land,
In connection with the remarks of Frank, W3LPL, about 160 meter
propagation, let me add a few of my own. These are related to
a long, two-part article, "160-Meter DXing, Some Remarks on Top
Band Propagation and DXing" that I published in the May/June
and July/August '96 issues of the DX Magazine. In case you do
not subscribe to that magazine, you might want find a friend
who does and to look into those articles as they bear on what
we're talking about.
First, the critical frequency of the E-region is relatively
insensitive to solar conditions, rising much more slowly with
increasing sunspot numbers than the F-region frequencies (see
p. 136 of Davies' "Ionospheric Radio", Peter Peregrinus Press,
1990). In addition, at sunrise and sunset it is about 1 MHz
at solar minimum (see p. 132 of Davies). After sunset, the
critical frequency of the E-region falls to low values, the
asymptotic limit being 0.4-0.6 MHz, depending on which of the
various ionospheric models (CCIR, URSI, IRI) are consulted.
After sunset, the critical frequency ABOVE the E-region falls
too , but not to zero. In fact, it is supported weakly by
galactic cosmic rays, UV in starlight and solar Lyman Alpha
radiation scattered by the earth's hydrogen corona. So, since
the F-region critical frequency declines too, but not to zero,
a minimum or electron density valley develops above the E-region.
If you work out the penetration of Top Band RF (1.8 MHz),
it turns out all but the very lowest angles of radiation from
an antenna penetrate the weak E-region peak when the critical
frequency is well below 1 MHz and go into the lower F-region.
That advance of the RF continues until it reaches a region
where the effective vertical frequency (p. 173-174 in Davies)
equals the local plasma frequency. (That is the condition
for reflection, i.e., peak altitude of the Top Band RF in
the F-region.)
So the RF pentrates the E-peak and is "reflected" somewhere
in the lower F-region, much like the first drawing W3LPL
showed. As a result, when taken together with the low-angle
rays, you have two possibilities for 1.8 MHz signals -
short E-hops for low-angle rays or longer hops from rays
going into the valley above the E-region.
But for the "valley RF", when it comes down to lower
altitudes, around thousand km or so from the transmitter,
it may not find as low a critical frequency there (as when
it penetrated the E-region initially) and will be "reflected"
up again, toward the lower F-region. That is how ionospheric
ducting occurs, RF bobbing up and down between the E-peak and
lower F-region and with little loss as the electron-neutral
collision frequency there is much lower than below the
E-region peak.
So everything that can happen, will happen - short E-hops,
longer E-F hops, ducting, etc. What is heard or not heard
depends on which circumstances existed along the path.
Myself, I think there are significant meteorological factors
which can affect Top Band propagation. In particular, it
would seem that internal gravity waves of the atmosphere can
affect the electron density at E-region heights. (They are
like acoustic waves but at much lower frequencies, the
oscillations depending on buoyant and gravitational effects
on parcels of air instead of its thermodynamic compressibility.)
Now I bring that up as the E-region rests atop the turbopause,
sort of the upper boundary for complete mixing of atmospheric
constituents. So it is not out of the question for internal
gravity waves to burble up from below and pass through that
region, affecting the neutral density and electron density
distribution during the oscillations. Indeed, there is good
evidence for gravity waves at much higher altitudes.
And there are other meteorological effects on MF propagation.
There is the nitric oxide problem that gives rise to increased
ionospheric absorption at mid-latitudes during DAYLIGHT HOURS
in the winter. That is probably what is termed the STRATWARM
effect on propagation but again, it is for DAYLIGHT HOURS, and
most certainly NOT when Top Band Operators are out plying
their trade in the dark.
As I see it now, whether ducting really works (and gives good
signals on Top Band) depends on whether gravity waves are
present along a path and affect the E-region electron density
to the extent that a signal is refracted upward again instead
of penetrating the E-region and losing signal strength on going
down to a ground reflection. Of course, at sunrise and sunset,
the atmospheric turbulence from cooling or heating would affect
the electron density distribution and signals would enter or
spill out of the ducting region.
So in the absence of magnetic disturbance, I would lay blame
for bad conditions on the atmosphere, not enough in the way
of gravity waves to make ducting possible along a path. And
without them, all you have is classic earth-ionosphere hops,
those weak, ground-reflected signals moving along, for
better or worse.
By the way, the E-F region electron density valley is
well-known in ionospheric circles, shows up on all the
ionospheric profiles from incoherent scatter radars and is
portrayed in the computer models in Cory Oler's very fine
PropLab Pro program.
This is surely more than you wanted to read but it is
probably closer to the situations which exist during
poor conditions on 160 M.
73,
Bob, NM7M
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