Topband: 160 Meter Propagation

[Robert Brown]

Larry,

Thanks for the note.  But your question does not deal with what I was
talking about; you are asking about contacts between stations well above
the low-latitude range.  And looking at the possible paths between AZ and
JA as well as AZ and DL, W4ZV's response was the correct one: the path to
JA is sub-auroral and less subject to ionospheric absorption of auroral
origin than the path to DL.

I was talking about paths between low-latitude stations like 9V and
stations at higher latitudes, like in the USA.  At low latitudes, 160
meter propagation toward the USA is nearly along the direction of the
geomagnetic field.  That is called quasi-longitudinal propagation in the
language of magneto-ionic theory and if you work with the PropLab Pro
program, you find that ducting is a common feature for radition angles
between 10 and 25 degrees.  That gets signals into the USA with little
loss due to ground reflections or traversals of the lower, lossy part of
the ionosphere (even at night).

On the other hand, signals TOWARD 9V from the USA get their start 
where the geomagnetic field lines are 60-70 degrees from the horizontal;
that is called quasi-transverse propagation in magneto-ionic theory and if
you play with PropLab Pro, you find the probability of getting a ducted
mode is much lower, about one-quarter that for the 9V->USA path.

Now I have to say that all modes of propagation are excited when RF goes
into the ionosphere.  What you hear is probably the strongest, surviving
mode.  Close to a transmitter, multi-hops paths predominate but with
increasing distance, propagation can be by a mixture of multi-hops and
ducted portions of the path.  But, as K9LA has shown recently when looking
at the sensitivity of typical ham receivers, the "reach" of pure multi-hop
paths is about 10,000 km; after that, successful propagation depends on
low noise and some propagation through ducting.

In any event, from low latitudes, W->E paths are more likely to have
ducting than E->W paths; that makes a difference when it comes to DXing.
But certainly one does have to be aware that ionospheric absorption in the
auroral region is an important factor too, as W4ZV indicates.

73,

Bob, NM7M


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