TopBand: Re: Why the duct?

Mon, 7 Jul 1997 08:04:56 +0000

> From:          Frank Donovan <donovanf@sgate.com>

Hi Frank,

Thanks for the data.

> In MD, the  O N L Y time we can work JAs on 160 is from a few minutes
> before sunrise until ten minutes after or so.  The typical scenario is
> that we hear the W1/2 stations working them easily, and we hear just a
> trace of the loudest JA stations.  When sunrise approaches, the JAs build
> up from 10-30 dB, then gradually fade away starting 10 minutes after
> sunrise.  If Ole Sol doesn't show more activity soon, we may have another
> good 160 season to JA this December/January.

In both Toledo and Cleveland, Ohio, I could work JA's from sunset in 
JA to sunrise in Ohio. I'll guarantee there was almost never  a 
time the east coast guys would be working JA's (and NEVER VS6DO or 
others further south) when I couldn't hear em fine from Ohio. The same 
is true down here. Asian  signals are more consistent day after 
day here in Georgia (further from the polar path) but they are not 
any more or any less "selective".

> Anyone who has been a DXer for long has surely experienced long distance
> propagation (over 10,000 km) with unusually strong signals.

Sure, I've seen it happen on rare days when the MUF is low.. and 
also when going into or coming out of a geomagnetic storm.. 

> To quote the preface: "Due to the Earth's sphericity and stratified
> structure of the ionosphere, waveguide channels (ducts) appear in which
> radio waves propagate over long distances.  These ducts have a complicated
> structure, they are inhomogenious, interrelated, and vary with time
> variations in the ionosphere.  To understand the main features of
> long-distance propagation, it is necessary to study the properties of the
> waveguide channels.  The interlayer duct is of particular interest."

How did they measure or confirm the existence of the duct, other 
then by postulation?

I like W1BB's theory, expressed to me back in the 60's. Stu 
postulated at sunrise and sunset the ionosphere acted like a 
big wall behind the receiving or transmitting location, and it focused 
the small weak 160 meter signals like a giant poorly reflective dish 
on one area at a time just ahead of the densely ionized region in 
sunlight.

I think Stu was correct, especially since I'm very sure losses at 
low incident (grazing) angles are VERY high near the LUF of a 
path. I simply fail to see how a long duct through the 
lossy ionosphere would have low loss. 

The further I push my receiving noise floor down, the longer these 
ducts last. Over saltwater paths and away from the magnetic poles, 
they last a long time. A two or three hundred mile drive east south 
east either puts me in the duct for Europe, or the salt water makes 
the difference. And it will do so night after night.

Stations in SC and Georgia that enjoy salt water paths (not NEAR 
their antennas, but many MILES away) to Eu will beat me several dB, 
but when "looking west" across the USA for any distance I beat the 
same guy on a regular basis.

If the signals duct, why does the saltwater path help?

73, Tom W8JI 

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