Hi Richard,
I think the noise variation is very similar to that on 136k and relates to
the position of the the dawn/disk edge. In the morning as the dawn sweeps
west across Europe the night-time skywave delivery of all this noise (both
atmospheric and man-made) is wiped out by D-layer absorption. We in the UK
get no noise from the west as its is mainly water. (the condition is exactly
the opposite on the East coast of the States). There seems to be less
lightning over water too.
The catch is that just around dawn there is a very quite period....this is
very encouraging and many have tried to make good DX in that period
.....unsuccessfully. I believe that this period is dogged by intense
absorbtion because at dawn the D-layer is first illuminated from underneath
(NM6M Bob Brown QST about 1999) by solar rays that have grazed the ground
before impinging on the D-layer about 1000km W of the ground dawn line. Thus
in this quiet period there is just no propagation by skywave so that DX is
not possible. Quite often the best time has been just before (say 60 mins)
dawn (at mid path)before the absorption starts.. N<>S paths are more
seriously affected than E<>W paths.
Once the sun rises and the ionosphere is illuminated from above then normal
daytime skywave returns slowly with the best singnals around mid-day at
mid-path. 500k is complicate dby the possibility of two "reflection heights"
which give rise to the deep daytime fading not seen on 136kHz, though this
my only occur later in the day.
There are difference between 136 and 500k but I think the noise problem is
similar.
Best of luck
Alan G3NYK
----- Original Message -----
From: "Richard Newstead" <[email protected]>
To: <[email protected]>
Sent: Sunday, June 07, 2009 9:39 AM
Subject: LF: 500kHz Diurnal Noise Variation
> I wonder if anyone has attempted to plot the diurnal noise levels on
> 500kHz? I wonder if, for instance, very early on a Sunday morning is
> particularly quiet?
>
> 73
>
> Richard
> G3CWI
>
>
|
