TopBand: more on props

[Dan Robbins]

Been out of town for a few days so just catching up on all the propagation
discussion.  I agree with Tom, W8JI, that there is no reason to invoke
ducting as an explanation for the ionospheric propagation we experience on
160.  Many of the anecdotal tales of unusual propagation are really just
normal propagation, but unusual geography.

Let me try to explain this in the light of my experience.  When using the
backscatter radar or sounder, one of the great displays was a graph of freq
vs distance.  You could instantly see how any given frequency was being
propagated.  In the day, for example, I might see that 14 MHz had a dead
zone out to 500 miles, then fairly good coverage out to, say, 1800 miles.
At 1000 miles the second hop would begin to provide coverage, so in this
instance we would have continuous coverage past 500 miles for quite a ways. 
At 28 MHz the story might be totally different.  The dead zone might extend
out to 2200 miles, but there is a sliver of coverage between 2200 and 2400
miles.  (I have seen first hop slivers as narrow as 50 miles or less.)  This
sliver of illumination is repeated on the next hop, so there might be
coverage from 4400 to 4800 miles, and so on.  To a ham, if these slivers
come down in unpopulated areas or over the ocean, he hears no signals and
the band sounds dead.  You could have a major ham area at 1500 or 4000
miles, for example, and the band would still sound dead.  Now if FR5DX is
9200 miles away and he starts up, all of a sudden there is the band open to
an exotic DX spot and there are absolutely no signals in between - why, it
must be some unusual propagation, perhaps one of those ducts!  Wrong, it is
just normal, extremely normal propagation.  It happens all the time, it is
just rare that the ham has conditions where the alignment of the dead zones
and the illumination zones line up to provide this scenario.  That's the
nice thing about the backscatter display - it sees every hop and doesn't
rely on the population density for detecting band openings. 

Now go down to the low bands.  The same effect occurs.  Consider the freq vs
distance graph again.  In the time before sunrise, the graph might indicate
7 MHz might only be good for a sliver, just as 10 meters was during the day.
The distance will not be as far, though, maybe around 1500 miles.  On 80 we
see might see distances from zero all the way out to 1000 miles, but the
close in stuff is not that strong, the best props on 80 might be from
900-1000 miles.  The second hop has good props between 1800 and 2000 miles
but below that the signals are very weak.  The third hop is strong at
2700-3000 miles, but by now the lesser distances have pretty well been
filtered out.  So all of a sudden, there is Australia pounding in at
9000-10000 miles but hardly any Pacific in between.  WOW!  The exact same
effect happens on 160, too. Of course, on the low bands the absorption
controls whether signals will make it that far.  Another nice thing about a
big HF radar - they run a lot of power so I could see this effect night
after night after night, even when absorption was heavy.  This kind of
propagation is a regular occurrence.  It is only rare to hams because of
their power and non-continuous frequency limitations plus the means by which
they measure band openings.  If the signals come down where nobody is, we
hams have a dead band, at least philosophically.  Most of the earth is not
heavily populated with hams.

Let me reiterate - I saw this selective kind of skip night after night on a
regular basis.  For we lowband hams it requires not only certain levels of
absorption but also having DX in the right spot and being on a suitable
frequency.  During my work at the HF radar I was even able to match up stuff
I heard on the low bands with what I saw on the backscatter displays.  Long
haul DX out of the blue does not require abnormal ionospheric propagation.

If you want to find out how fast your car goes, you could compute the
rolling friction, aerodynamic coefficient, street weight plus driver, shaft
horsepower and a number of other factors and figure out how fast the car
will go.  On the other hand, you could find a nice long, flat stretch of
road and mash the gas pedal to the floor. The latter may be a tad more
dangerous, but fulfils KL7Y's corollary to Occam's Razor:  If an answer to a
problem may be determined by two or more methods, choose the method that
requires the least amount of math.

                                                Dan KL7Y   


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