TopBand: A bit more on propagtion

[Robert Brown]

Friends in Radio Land
 
I would like to add a few remarks to the posting I made yesterday
in connection with Top Band Propagation.  That, you will recall,
had to do with signal refraction more than anything else.
 
But signal loss should be considered also and thus I would like to
point out that Top Band signals suffer the most of all in the ham
bands.  That is the case as in going from the HF part of the
spectrum down to 1.8 MHz, plasma frequency considerations (i.e.,
like ionospheric sounding) increasingly limit the penetration of
signals into the ionosphere.  As a result, for Top Band signals,
 
     1) hop lengths are shorter for a given radiation angle,
     2) more hops are required to cover a given distance,
     3) a greater part of the signal paths lie at low altitudes
        where ionospheric absorption is the greatest, and
     4) there will be greater losses from more ground reflections.
 
As a result, if one averages signal power across all the angles of
an antenna pattern, Top Band signals reaching a location appear
weak in comparison with what would be found on higher bands.
 
But, if one examines the antenna pattern in detail, by means of
ray-tracing, it is found that there are angles where propagation
is much more efficient than average.  They represent the angles
for long E-F hops and signal ducting which result from Pedersen-
like refraction at the top of the E-region.  Thus, those signals
go on longer hops, have fewer ground reflections and less of the
total path length is at low altitudes where signal losses from
ionospheric electrons are the greatest.
 
Except for their length, long E-F hops are not much different than
ordinary F-hops.  Ducted paths are different in that once signals
are ducted, they tend to stay there until the tilt of the wall of
the duct changes significantly.  That can happen from disturbances
but certainly at sunrise or sunset where the walls of the duct are
pinched down and closed.
 
In summary, signal strength considerations suggest that hops from
Pedersen-like refractions at the top of the night-time E-region
are the ones which propagate more efficiently and by surviving to
greater distances, they are responsible for long-haul DXing.
E-and F-hops may also survive but would be at considerably lower
signal strength.
 
I think that sums it up; the only other thing to worry about is
noise and at this point, I don't have much to say about that.
 
73,
 
Bob, NM7M
 




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