Topband: Power coupling

[Robert Brown]

Friends in Radio Land -
 
In his earlier message to K9LA and me, Bill, W4ZV, tried to
summarize the power coupling issue in three points, saying first, it is
primarily an issue for stations located within 20 degrees of the equator.
(I think he meant the geomagnetic dip equator but I won't quibble at
this point.)
 
Second, the local attenuation peak is on paths between 20N and 20S
of the E-W direction.  (In geomagnetic dip coordinates?)
 
Third, power coupling is not significant unless the path transits
the equatorial area.
 
My view of power coupling would be summarized somewhat differently
so let me take a crack at it and share it with you.
 
First, the ionosphere and geomagnetic field are everywhere, with
the result that signals on 1.8 MHz, close to the range of electron
gyro-frequencies in the field, are elliptically polarized as they
are propagated.  Thus, they may be linearly, circularly or
elliptically polarized at a given instant, depending where they
are in the field and their direction of propagation relative to
the field.
 
Second, power coupling comes into the discussion whenever those signals
enter or leave the lower ionosphere and power coupling is derived
from the details of the geometry of that limiting magneto-ionic
polarization and the polarization of the antenna on the ground,   
typically vertical or horizontal.  The loss from each and every
transit of the lower ionosphere, no matter what the latitude and
longitude, must be added to other losses incurred in the course 
of ionospheric propagation.
 
Third, power coupling, is local in nature for each individual
operator as it is determined by the local properties of the
the ionosphere, the earth's magnetic field and the antenna
installation.  That suggests a vertical antenna works best in an
environment where the local field is at a high inclination (the
Continental USA and Europe), and further, it shows horizontal
polarization is poorer when radiation goes perpendicular to the
local field (to the north in the USA and Europe) and better when
it goes parallel to the local field (E-W at low magnetic dip
latitudes).
 
Fourth, in any 160 meter DX contact, there are two operators
involved - the local one and the distant one and each is affected
by effects from the geomagnetic field: local O-wave power coupling
and the X-wave loss of up to 50% of the effective radiated power
for long-haul contacts for the signals leaving an antenna.  Each
operator, within practical limits, should choose antenna systems
with polarization and radiation patterns (or TOAs) which best serve 
the making contacts with the population of interest.
 
Fifth, as another practical matter, for any operation at a low
latitude, the International Geomagnetic Reference Field should be
consulted to determine its location relative to the geomagnetic
dip equator as the dip equator can be as much as 10 degrees above
or 15 degrees below the geographic equator!  ( For details, see
the global ionospheric map option in the PropLab Pro program.
Lacking that, I should add that R. Fricker, a propagation engineer
retired from the BBC, has a simple BASIC algorithm for calculating the
location of the dip equator in geographic coordinates or the 
"dip latitude" for any low latitude site.)
 
For what it is worth, that is how I think about power coupling
in low-band DX propagation, a little more general and a bit less
practical than W4ZV but covering essentially the same points.
 
73,
 
Bob, NM7M