Topband: An optical analog

[Robert Brown]

Friends in Radio Land -
 
        Earlier, I sent some 80 meter power coupling data to Bill,
W4ZV, and he was puzzled by the small change from 160 meter data,
thinking it would have been greater.  So let me offer a few words
of explanation.
 
     I think the "expectation" of a larger magnitude of the change,
as I'd call it, is due to previous experiences with propagation where
the ionospheric effects vary roughly with the square of the wavelength
- say signal absorption and refraction (or skewing).  Those factors
come out of the complex index of refraction for wave propagation,
refraction from the real part and absorption from the imaginary part.
In basic magneto-ionic theory, both functions have what are termed
"resonance denominators" that result in large effects for certain
directions of propagation relative to the magnetic field.  One
well-known aspect is that RF power used to generate X-waves is
essentially wasted as heat because of their high rate of absorption
(see Davies, 1989).
 
        Power coupling is more geometrical in nature, looking at
the polarization match between the limiting polarization for waves
in the geomagnetic field, going in and out of the lower ionosphere,
and that of the antenna on the ground.   The limiting polarization
is a linear function of the ratio of the local electron gyro-frequency
and the operating frequency, but modified by trig functions of the
direction of propagation relative to the field.  So the change in
insertion loss from power coupling by reducing the wavelength from
160 meters to 80 meters is not a factor of 1/4, as one would expect
for one-half the wavelength with a typical ionospheric function, but
only 1/2 or more, depending on the direction of propagation relative 
to the field.  Expressing the results in logarithmic terms only adds
to the confusion.
 
        It should be noted that this ionospheric problem has an analog
in the field of optics - like an optical bench with a source of
polarized light, a polarizing filter and an analyzer.  In the present
instance, the source is a distant transmitting antenna, the polarizing
filter is the geomagnetic field at the bottom of the ionosphere and the
analyzer is the receiving antenna, with either horizontal or vertical
polarization.  The output through the whole system depends on the
intensity variation across the angular distribution of the transmitting
antenna, the angular response of the filter and the orientation of the
analyzer, whether parallel or perpendicular to the polarization of
greatest transmission by the filter.
 
        The polarizing filter has a narrow angular range where its
transmission is greatest - the direction where the E-field of the
incoming radiation is parallel to the geomagnetic field.  At that angle,
the transmission is also least when the E-field is perpendicular to the
geomagnetic field.  But with a polarizing filter having fixed properties,
the signal strength at the output depends on the signal strength
variation across the incoming angular distribution and the properties
of the analyzer; and it may be quite significant even for the least
favorable angles for the filter.  So it is for low-band propagation
using dipoles, the polarization features of the ionosphere remaining
the same but the sensitivity and output of the system differing for
high and low dipoles.
                                      
     Now it should also be noted that the ideas of power coupling are
applicable for every transit of the lower ionosphere along a path.
If one neglects X-waves, their RF power lost in heat during long-haul
contacts, then the limiting O-wave polarization for waves that leave
the lower ionosphere must be evaluated for each transit.  That 
elliptically polarized signal has to be broken down into horizontal and
vertical components, with a phase difference between them.  Those
components are incident on the ground reflecting surface and separately
undergo reflection, with changes in amplitude and phase depending on the
surface.  The reflected signals are then compared with the limiting
polarization so as to determine the insertion loss from the ground
reflection.  Needless to say, this is a very complex process, as noted
by the BBC engineers who pioneered this subject, and they did little
more than consider power coupling at the start and end of one hop-paths
in their '65 study.  I doubt that radio amateurs are prepared to do more.
 
        Nobody said that radio propagagtion was a simple matter but
it is most complex at the bottom of the spectrum where the earth's
field makes magneto-ionic effects important.  But DXing is still fun!
 
73,
 
Bob, NM7M