TopBand: Re: Why the duct?

[Frank Donovan]

Tom,

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.

Anyone who has been a DXer for long has surely experienced long distance
propagation (over 10,000 km) with unusually strong signals.  In fact a 340
page book was written by on this topic by two members of the Academy of
Sciences of the USSR:

"Long Distance Propagation of HF Radio Waves"
Dr. Alexsandr V. Gurevich and Dr. Elena E. Tsedilina 
published by Springer-Verlag 1985
ISBN 3-540-15139-7
ISBN 0-387-15139-7
QC801.P46  

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."

73
Frank
W3LPL
donovanf@sgate.com


On Sun, 6 Jul 1997 w8jitom@postoffice.worldnet.att.net wrote:

> Here in Georgia I can hear and work the JA's almost anytime between 
> their sunset and my sunrise most days, but the signals PEAK at my 
> sunrise. At sunrise I can work the third and fourth layer stations, 
> if they can hear me.
> 
> I suspect the propagation is always there, but the sunrise 
> enhancement really helps.
> 
> The thing I don't understand or haven't heard is why the duct is a 
> necessary part of the explanation about anything. Maybe someone can 
> explain what propagation characteristic fits the ducting theory, 
> since there is no way to measure it or prove its existence other than 
> with effects we observe.
> 73, Tom W8JI 


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