Part I ... by Bob Brown .. NM7M

Friends in Radio Land,

I have to agree there is a lot of information out there on the Internet;
but what about understanding? Let me put out a few remarks that might 
help your understanding of propagation.

First, we depend on ionization of the upper atmosphere. That results 
from solar ultra-violet, "soft Xrays", "hard Xrays", and the influx 
of charged particles. Leaving the charged particles out of the 
discussion today, the solar photons have their origin largely in
active regions on the sun,.

Historically, active regions were first counted and tallied, then the 
next step was to measure their areas. Both methods have their
problems with weather conditions and after WW-II it was found that 
the slowly-varying component of solar radio noise at 10.7 cm was 
statistically correlated with the method using sun spot counts. 
Later, with the Space Age, it was found possible to measure the 
"hard Xray" flux coming from the sun in the 1-8 Angstrom range.

In my opinion, the 1-8 Angstrom background Xray flux is a better 
measure of solar activity, at least for our radio purposes. Let
me explain. 

First, the Xray flux has been found to come from regions more 
centrally located on the visible hemisphere of the sun; that 
means a significant fraction of their Xrays will reach our atmosphere. 
Second, it takes 10 electron-Volts (eV) of energy to ionize any
constituent in the atmosphere; the energy of 1-8 A Xray photons 
exceeds that by over a factor of 100.

The energy of 10.7 cm photons is .00001 eV, a factor of 1,000,000 
too LOW to ionize anything in our atmosphere. So the 10.7 cm flux 
only tells us about the presence of active regions on the sun, not 
directly about the state of ionization in the ionosphere. If that 
was not bad enough, it has been found that the 10.7 cm flux can come 
from the corona above regions which are behind the east and west 
limbs of the sun. Those regions are much less likely to have their 
ionizing radiation reach the ionosphere directly. So the 10.7 cm 
flux has its purpose, indicating the presence of active regions, 
and it is a mistake to think that changes in that flux are
always associated directly with the state of our ionosphere.

Having said all that, let me conclude by pointing out the 1-8 A 
Xray flux values are given by NOAA in ranges which differ by a
factors of 10, such as A 2.3, B 4.0 or C 1.5. The numbers are the 
multipliers and the letters give the category. Now I have logged 
the 1-8 A Xray flux through all of Cycle 22 and now into Cycle 23. 
The sum and substance of my experience is quite simple: the A-range 
is found around solar minimum, the B-range on the rising and falling 
parts of a cycle and the C-range during
the peak of a cycle.

So what about Cycle 23? We suddenly moved out of the A-range (with 
sporadic B-outbursts) in August of '97, hovered in the low B-range 
until March '98, were in the mid-B range to the present time when 
there were recent outbursts in the C-range. It is still too early 
to say if solar activity has moved into the C- or solar maximum phase; 
several months of data will be needed before any such estimate can 
be made.

But logging the 1-8 A Xray flux, with 4-cycle log paper, will give 
you insights as to the state of the ionosphere and recurrences in
the plot will serve to point out good/bad times for DXing. While 
spikes in the 1-8 A diagram may suggest "hot times" for DXing,
they can be brief and difficult to take advantage of. It is more 
productive to look at the broader peaks in flux in planning one's
DXing. The flares and coronal mass ejections associated with 
outbursts of activity that take place now are more likely to give 
bad propagation conditions because of all the geomagnetic activity 
that follows. For DXing, the broad peaks are more productive.

All of the above involved words, no great mathematical exercises. 
But I like to tie it together mathematically using a simple
proportion that everyone can grasp quickly:

When it comes to changes in the state of the ionosphere, Xrays 
are to solar noise as, with DXing, beam antennas are to dipoles.
OK?

73,
Bob, NM7M

Friends in Radio Land -

The discussion so far has dealt with the creation of ionization
and how various frequencies in our spectrum make out as far as
propagation and absorption are concerned. There's one problem
with that discussion, the omission of how, in the course of time,
ionization reaches the steady-state electron densities overhead.

So let's turn to that but do it as simply as possible. That means
we'll focus on electrons, positive and negative ions. The solar
UV and Xrays create those from the oxygen and nitrogen molecules in
our atmosphere. I can say it is a big, complicated ion-chemistry
lab up there but we'll stay at the generic level, nothing fancy,
just electrons and positive ions.

In simple terms, there is a competition between the production and
loss of ionization, just like your bank balance where depositing
paychecks and paying bills are in competition. So for us, there's
a certain number of electrons created per second in a cubic meter
of air in the ionosphere by the solar radiation and whatever the
number of electrons present, some are being lost by recombining
with positive ions to form neutral atoms or molecules again. If
the two, gain and loss, are equal, there is a steady-state of
ionization; otherwise, there will be a net gain or loss per second
from some cause or other.

I haven't said so but the atmosphere is only lightly ionized, say
one electron or positive ion per million neutral particles. So
electrons have a greater chance to bump into a neutral particle
(like in ionospheric absorption) than a positive ion, to recombine
to make a neutral atom or molecule. And, of course, there's a
vast difference in those rates between the lower parts of the
ionosphere, the D-region below 90 km and the F2-region above 300
km. So electrons created by solar UV would be gobbled up rapidly
in the D-region but linger on for the better part of a day up in
the F2-region.

Good illustrations of the fast processes are found nowadays, solar
flares illuminating half the earth with hard Xrays (like those in
the 1-8 Angstrom range). They penetrate to the D-region, release
electrons which rapidly transfer wave energy to the atmosphere.
As soon as a flare ends, the sudden ionospheric disturbance (SID)
or radio black-out ends as the electrons in the D-region recombine
rapidly and signal strengths return to normal.

The lingering on of electrons in the F2-region is responsible, in
part, for the fact that there's still ionization and propagation
in hours of darkness. In short, electrons at high altitude
recombine slowly after the sun sets. But there's more to the
story than that, the role of the earth's magnetic field. Let me
explain.

The earth's atmosphere is immersed in the geomagnetic field so any
charged particles, say ionization created by solar UV, will then
experience a force from their motion in the field. For electrons,
that means they will spiral around the field lines when released
by UV and not fly off in any direction to another location, higher
or lower in the ionosphere. In the propagation business, that is
called geomagnetic control, meaning that the earth's field largely
determines the distribution of electrons in the ionosphere. True,
the solar UV creates them and they are most numerous where the sun
is overhead but they are held on field lines and linger on after
dark, to our great advantage.

But the earth's field also creates problems, especially for the
low-band operator. It turns out the gyro-frequency of electrons
around field lines is about 1 MHz and comparable to frequencies in
the 160 meter band. Thus, a more general approach has to be made
in the theory of propagation at that frequency, adding the effects
of the earth's field on ionospheric electrons. The results are
quite complicated, with elliptically-polarized waves on low
frequencies where linearly-polarized waves were the story earlier
on high frequencies. That is a subject in itself and has to be
left for a rainy day. But those are not the only ways that the
earth's field enters into the propagation picture. Stay tuned.

Friends in Radio Land -

Earlier, I said there were other ways that the earth's field
enters into the propagation picture. But that's sort of getting
ahead of my development so let's backtrack a bit and look at the
historical picture.

The study of geomagnetism goes back more than 100 years, well
before the advent of radio. It was known that the occurrence of
magnetic storms was related to the solar cycle and, by the same
token, it wasn't long before it was realized that HF propagation
was related to it too. The two really came together about 70
years ago when commercial radiotelephone service was established
across the Atlantic Ocean. Then it soon became apparent that
there were disruptions in service during magnetic storms. You can
find all that discussed in the I.R.E. journals in the early '30s.

In that period it was thought that the ionosphere was the result
of solar UV, the photons reaching the earth 500 seconds after
leaving the sun. And while magnetic storms were known to disrupt
radio propagation, there was no obvious connection as experience
showed magnetic storms occurred a couple days after the flash
phase of a large flare on the sun. True, there was the idea of
solar material, electrons and protons called "plasma", approaching
the earth after a solar outburst and engulfing the geomagnetic
field, even compressing it. But the two effects from plasma and
UV seemed separable just because of differences in time-of-flight
across "empty space" that were associated with the two effects.

But all that changed with the Space Age when it was found that
solar plasma was out there all the time, the solar wind, and that
it blew past us with differents speeds, 200-1,200 km/sec, as well
as different particle densities and even carried magnetic fields
along. But for us earth-bound souls, the big surprise was that
the solar plasma distorted the earth's magnetic field, essentially
taking some field lines on the sunward side and pulling them back
behind the earth to form a magnetotail. Moreover, with the solar
plasma coming at us, it became clear that a ordered, dipole field
did not go on forever, only out to 8-12 earth-radii in the sunward
direction and even that depended on solar activity.

So what does this have to do with propagation, you ask. Well
remember I said geomagnetic control of the ionosphere means that
electrons are held on magnetic field lines, making the earth's
field something of a reservoir for ionospheric electrons. But if
field lines can be distorted, that would surely affect the density
of ionospheric electrons gyrating around them and propagation.

The worst-case scenario is when field lines are dragged way back
into the magneto-tail by an increase in solar wind pressure,
taking ionospheric electrons with them. That field configuration
is sketched crudely below where two compressed field lines are
shown in front of the earth, in the solar direction, and two
magnetotail field lines in the anti-solar direction:
Solar Wind

                                                   ( <-
Magneto-tail              *   *         * *        (
                      *  . . .   *     *    *      ( <-
                 *   .          . *   *  .   *     (
             *   .               . * * .   .  *    ( <-
        *   .                  .*       *.  . *    (
   *     .                       (Earth)    . *    ( SUN
        *   .                  .*       *.  . *    (
             *  .          .       * * .   .  *    ( <-
                 *   .          . *   *  .   *     (
                      * .  .  . *      *    *      ( <-
                          *   *         * *        (
                                                   ( <-

That would mean a depletion of electrons at F2-region heights and
drastic reductions in MUFs, affecting propagation. Fortunately,
that fate is reserved primarily for sites at high latitudes,
around the auroral zones and poleward.

What I described was what takes place during a major geomagnetic
storm. The recovery is a slow process as ionospheric electrons
have to be replaced in the usual way, by solar UV and day by day
while the sun is up. So it can take days for the bands to recover
when a strong magnetic storm reduces MUFs by a large fraction.

Now to be practical again, magnetic activity on earth is caused
by interactions of the solar wind out there at the front of the
geomagnetic field. The field region around the earth is called
the magnetosphere so we're talking about effects on high latitude
field lines that go out to the magnetopause, the dividing surface
between terrestrial and interplanetary regions. But it must be
recognized that this sort of thing is not toggled on and off; it
is going on all the time as the solar wind sweeps by. It is just
a matter of degree. But how to deal with it in DXing?

The clue comes from an interaction within the magnetosphere, local
electrons being accelerated to high energies and then spiralling
down field lines to make visible aurora and ionization at E-region
heights. Those events are triggered by solar wind interactions at
the magnetopause and accompanied by horizontal currents in the E-
region that show up in magnetic observations on the ground. It
then becomes a matter of using the strength of the local magnetic
effects at auroral latitudes, with K- and A-indices like those you
hear about on WWV, to judge the energy input from the solar wind.

To bring this to a conclusion, good propagation conditions are
found when there is a strong UV input to the ionosphere and low
magnetic indices, the 3-hour K-index less than 4 and the daily A-
index less than 25. Dreadful propagation conditions were found
recently in the magnetic storm of August 27 when K reached its
limit, 9, and the planetary average of the A-index was 112. But
it could have been worse! However, let's look at the brighter
side next time, how signals get from A to B.

73,

Bob, NM7M

Friends in Radio Land -

Let's leave a curved ionosphere to later and do some "Flat-Earth
Physics" to see how signals get from point A to point B. For that
we start with a simple model of the ionosphere in which the
electron density increases upward and peaks at about 300 km
altitude. That's something like a night-time ionosphere.

Now it may seem strange but one can draw an analogy between the
flight of a baseball and RF going up through that ionosphere. For
the baseball, high school physics teaches you how to calculate how
high a baseball would go if thrown vertically upward. In college,
the ball is thrown or hit upward at an angle. The method is the
same in both cases: the ball rises until the increase in its
potential energy in the earth's gravitational field is equal to
the kinetic energy it had from its initial vertical motion.

Neglecting friction, the baseball's path is a parabola that is
symmetrical about its highest point and the ball returns to the
ground at the same angle to the vertical as it was launched.
While not really parabolic in shape, the flight of RF through that
simple ionosphere is similar, reaching a peak altitude that is
determined by the frequency and launch angle, symmetrical about
the peak and returning to ground at the same angle. How does that
happen? Let me explain.

The flight of a baseball and the path of RF in a simple ionosphere
are determined by gradients, of the gravitational energy of the
ball in the first case and the electron density distribution in
the second one. There is a gradient of either of those quantities
if there's a change in value with altitude, say gravitational
energy or electron density greater at higher altitudes than lower
at altitudes. The gradients are responsible for the bending or
curvature of the paths in the both cases and, numerically, they
are given by the change in value per km change in altitude. OK?

In spite of all the "Home Run Fury" these days, let's leave the
baseball part of the analogy and focus on what happens to RF. So
we see that hops, with RF rising and then returning to ground, are
the result of the vertical gradient of the electron density in the
ionosphere. On reflection at ground level, angles of incidence
and reflection are equal and the path continues upward again.

But there can be horizontal gradients as well, say across the
terminator where there is more ionization on the sunlit side than
the side in darkness. So if RF signals were sent initially
parallel to the terminator, one would expect the RF to be bent
away from the sunlit side, with its higher level of ionization,
and toward the darkness. Right? That's skewing, pure and simple,
with the RF refracted away from the region of greater ionization.

The height a baseball reaches depends on its speed and direction;
for RF, that translates into frequency and launch angle. But one
sees that from different arguments. Let me add a few words there.
At any height in the ionosphere, there are electrons and positive
ions. If, by mystical powers, you could grab a handful of each
and then pull them apart, they would be attracted to each other by
the electrical forces between unlike charges and on release,
they'd swish back and forth, carrying out an oscillatory motion.
The frequency of that motion is called the plasma frequency and it
depends on the density or number of particles per unit volume, N.

For the ionosphere, where ionization increases with height, the
plasma frequency increases too. For our night-time case, the peak
electron density in the F-region might correspond to a plasma or
critical frequency of 7 MHz for the F-region. Now vertical
ionospheric sounding shows that pulses of RF below 7 MHz would be
returned to ground while any above 7 MHz would penetrate the peak
of the ionosphere and go on to Infinity.

For oblique propagation, we have to find the effective vertical
frequency of the RF, just like the vertical component of the
baseball's velocity. For RF, it's found the same way, multiplying
the frequency by the cosine of the zenith angle at launch. So, in
the "Flat Earth" approximation, 7 MHz RF launched from ground at
30 degrees above the horizon (or 60 degrees from the vertical)
would have an effective vertical frequency of 3.5 MHz. OK, the
"baseball analogy" would say that the RF going off obliquely would
rise until it reached a height where the local plasma frequency is
3.5 MHz and then return to ground. Of course, it would be on a
curved path, the RF would be moving parallel to the earth's
surface at the top of the path and returning to ground at the same
angle as when launched, just like the baseball problem.

In baseball, there's friction and that changes the flight of a
baseball. We don't put "friction" in the RF problem. Instead,
the electron density at a given height may vary along the path
direction, say become smaller. That would serve to "tilt" levels
of the ionosphere upward and weaken the density gradient. As a
result, there would be less refraction or bending after the peak
altitude than before, and that tilt serves to increase the length
of a hop and change the RF angle on return to a lower value.

In reality one would expect some change in electron density along
any path, increasing as a path goes into sunlit regions or
decreasing when going into the dark. So even if nothing else
changed, one would not expect hop lengths nor radiation angles to
always remain exactly the same all along a path.

The above approach, equivalent to mirror reflections of RF, is
Newtonian in the sense that the analogy treats a RF path like that
of a particle (baseball) and not a wave. When the Maxwellian or
wave approach is carried out, one finds that refraction is the
same except that the effects vary inversely with the square of the
wave frequency. So in a given part of the ionosphere, 80 meter RF
paths are refracted or bent much more than 10 meter RF paths,
either vertically or horizontally. OK?

Friends in Radio Land -

OK, now we have the idea of critical frequencies and hops so it is
no big deal to work out how propagation on a path may be open or
closed for DXing on a given frequency. But to do that, we need at
least map of where the RF is headed and an idea of how many hops
would be involved. Beyond that, some ionospheric details are
required, the critical frequencies along the path at the date and
time in question.

If one gets into the mathematics of all this, it turns out that
hops via the F-region may reach about 3,500 km and half that via
the lower E-region. So using those ideas, one can estimate the
hop situation, at least as long as there is not a mixture of E-
and F-hops. So consider a path from my QTH in the Northwest to
London, some 7,500 km in length. That would work out, to a first
appoximation, to 3 F-hops of 2,500 km each. Now what about the
critical frequencies at the peaks of the hops; how high are they
and what bands might be open to me, say at 1200 UTC?

To answer that question, one would need some sort of database, an
array of observations from which an estimate could be obtained by
interpolation, or a mathematic simulation of the database that
could be used to calculate the critical frequencies. Actually
both methods are used in modern propagation prediction programs
but either way, appropriate numerical values could be obtained for
the peaks of the hops. But what to do with that data?

For a one-hop path, the matter is simple; the effective vertical
frequency of the RF that is launched must be less than the
critical frequency for the path to be completed. No problem.
For two hops, the effective vertical frequency of the RF must be
less than the SMALLEST of the critical frequencies of the two
hops to have a complete path. And the operating frequency that
gives the highest effective vertical frequency that can complete
the path is called the Maximum Useable Frequency (MUF) for the
path at that time and for the corresponding solar conditions.

But the path from my QTH to London involves 3 hops; what's the
story there? Historically, the idea was handled like the 2-hop
path, using the critical frequencies at the first and last hop to
determine the MUF. The idea was that if propagation failed, it
usually would be due to conditions at one end of the path or the
other. Anyway, this is called the "control point" method and is
used in most simple propagation programs. More sophisticated
approaches would use critical frequencies at each and every hop
and the lowest would be the important one that limits propagation.

It should be noted that the control point method would be quite
satisfactory for MUF calculations so long as the critical
frequency of the middle hop is not less than those at either end
of the path. That would be the case for paths going across the
more robust ionosphere at low latitudes where the sun is more
overhead during a day. But MUF calculations using two control
points for high latitude paths, like from the Northwest to London,
can be misleading as the critical frequency for the middle hop
(over Northern Canada and Greenland for the path to G-land) could
be lower than at the end points and thus propagation not supported
across the entire path using the MUF from control points.

The MUF calculations play an important part in propagation
predictions but it must be remembered that signal strength, in
comparison with noise, is an important consideration. As noted
earlier, ionization and MUFS are more important for the higher
ends of the amateur spectrum and signal/noise considerations for
the lower end. In any event, for communication a path must be
open or available and signals must be readable and reliable.

All of the discussion up to this point has dealt with propagation
from a conventional viewpoint - determined by the ionosphere that
is overhead and, in turn, one controlled by the level of solar
activity. Obviously, propagation is a complicated process and it
may seem a bit naive but we try to make all our predictions on a
given date using using databases which rest on only a few numbers
- sunspot number and magnetic indices. It is not surprising that
predictions are not 100% reliable. Such high expectations would
deny the variability of the original data input from ionospheric
sounding and not reflect the roles of dynamic solar variables.

So, having given a brief summary of the principal points that are
involved in HF propagation, the question is how to proceed from
here. And some input is needed at this point. In particular, is
this a time best spent by pausing and dealing with some specific
questions on magnitudes and methods related to the material or
should new or more advanced topics be discussed?

What is your pleasure? Please respond privately or publicly, as
you wish.

73,

Bob, NM7M

Friends in Radio Land -

OK, having given a brief summary of the principal points that are
involved in HF propagation and not having any questions as to the
magnitudes and methods related to the material, I have to assume
that Propagation 101 has come to a close and we can move on to
Propagation 201, with new and more advanced topics. If I am wrong
in that regard, please speak up and correct me now or hold your
peace.

So far, the discussion has largely centered on words and concepts.
More advanced topics require a good deal of graphics so I will
make appeal from time to time to a figure or two in one or more of
the reference books given earlier. While figures are the best way
to convey some of the material, I will also try to put the ideas
in simple words that will carry most of the meaning.

To me, the study of ionosphere and propagation changed markedly
with the advent of the Space Age. Thus, with the International
Geophysical Year (IGY) in '57, high-altitude balloons, rockets and
satellites began to probe the regions where only radio waves had
been before. So the "Photochemical Era", where solar photons and
atmospheric processes were thought to control the dynamics of the
ionosphere, gave way to the "Plasma and Fields Era" we're in now,
where the interaction of the solar wind with the earth's field
and the atmosphere are the controlling factors for propagation.

In simple terms, hams no longer look out the window for their
local weather, determined by the day, time and season, but now
turn to the Internet to get a daily report on the Space Weather.
In a sense, propagation and DXing just became less mysterious and
even more interesting. That's what we'll be pointing toward in
Propagation 201, preparing for all the details in Propagation 301.
So go prowling around the Internet and see what you can pick up
between now and then. School starts with the first session on
October 1.

73,

Bob, NM7M

Friends in Radio Land-

Here are some books that would be helpful in regard to learning
about propagation. They are listed in ascending order of scope
and difficulty:

ARRL Handbook, 75th Edition, American Radio Relay League (1998)

Antenna Book, 18th Edition, American Radio Relay League (1997)

The New Shortwave Propagation Handbook, by Jacobs, Cohen and Rose,
CQ Communications, Inc. (1995)

The Little Pistol's Guide to HF Propagation, by Robert R. Brown,
Worldradio Books (1996)

Radio Amateurs Guide to the Ionosphere, by Leo F. McNamara,
Krieger Publishing Company (1994)

Ionospheric Radio, by Kenneth Davies, Peter Peregrinus Ltd. (1989)

73,

Bob, NM7M