[CQ-Contest] Grey zone absorption?

Clive Whelan clive at gw3njw.fsworld.co.uk
Sat Dec 22 20:54:00 EST 2001

This post to uk-contest produced a very small response. Anyone 
here have a slant?


This is a tad off topic, but I know of no better source than 
contesters to discuss.

Over many years, substantially in contests, it has been 
noticeable on 21 and 28 ( particularly) that it is difficult to 
raise even quite loud stations when firing out of the twilight 
zone. This happens often, even when the other station doesn't 
reply to anyone else.

This phenomenon was particularly noticeable in last weekend's 
ARRL 10m contest.

In particular I hit the shack with toast and coffee in hand 
about 06:30. Quite a few strong East Eu sigs already racking up 
the QSOs, but could not raise any of them ( 400 watts, vertical 
antenna). Then I heard A61AJ and raised him (06:53) without too 
much trouble ( why the difference?), followed by P3F(06:55) when 
the Russians were still not workable. I managed the first 
Russian at about 07:00, but it was not until after 08:00 that 
the rate picked up at all.

Then at 10:27 I was called by K1KI a big gun, but quite weak, 
and clearly he was still in his own twilight zone. Shortly after 
a weakish K3ZO ( what's going on!) got into the log through the 
now rock crushing QRM from Eu.

Later still as dusk came to GW , it became more difficult to 
raise US and Caribbean stations. 

In a recent sked with Ed who operated as FM/G3SQX, he told me 
that he experienced the same thing. In his case the contest 
started at 20:00 local which is dusk in FM, but less so in the 
in the USA. He experienced the same difficulty raising US 
stations in this situation, and made very few QSOs before going 
to bed. Later on in daylight he had no difficulty at all in 
generating pile ups even with 100 watts and a vertical.

So the question is , why is the rx/tx path apparently not 
reciprocal in such situations, and further why is the 
transmitting station disadvantaged . Is there perhaps absorption 
in (one of?) the lower layers of the ionosphere, which affects 
both signals but perhaps not enough to sink the *received 
signal* below the receiver noise floor.. Conversely is there 
perhaps greater absorption of signals travelling upwards rather 
than downwards through  the F ( or others) layers in twilight.

Perhaps this is a well known scientific phenomenon, but 
certainly not to me! Your perspective is solicited.



gw3njw at gw7x.org
Contest Cambria-http://www.gw7x.org

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