TopBand: Why the duct?

[w8jitom@postoffice.worldnet.att.net]

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

> Another example of very selective propagation is JA-to-East Coast on 160.
> The window starts in New England at their sunrise and moves southwest from
> there.  When the W1s are working the peak of the opening we cannot hear
> the JAs at all, then 10 minutes later they can be S9 (more likely S6
> tho...)
> 73!
> Frank
> W3LPL

Hi Frank,

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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