Topband: Re: [160m] The N2XE Beacon Tribune

[Tom Rauch]

I'd like to point out a couple things about listening to any
very low power beacon. Some of this is spurred by an e-mail
I just received about a VK's signal having a short "peak".
Hopefully this will translate to an exchange of interesting
effects of propagation on signals near or in the noise
floor. I wonder if anyone else noticed the effect I will
describe at the end of this, but first I'd like to make a
comment.

Everyone should keep in mind any test like this really isn't
a test of miles-per-watt or receiving ability compared to
others in different directions or at other distances. It may
be the only way to express results, but it really does not
translate to "miles per watt" over any other distance or
path. (I think N0FP pointed this out earlier.)

For ionospheric propagation (and real world antennas) there
will always be some direction and distance where "miles per
watt" peaks, and this can be a rather dramatic effect. At
shorter distances path loss is generally lower because of
increased wave angle. This effect is caused by the wave not
having to refract a signal at a low angle to cover larger
distances, where the longer path causes the wave to travel
along and through the ionosphere at low angles where it is
subjected to higher "attenuation per mile". There will
always be some distance where everything works best and
"miles per watt"  is maximized.

Longer paths on low frequencies can involve multiple hops
between earth and the ionosphere, and this places an
additional reflection loss in the path at each hop. It also
increases losses associated with the wave passing though
lower ionospheric layers that have fairly high attenuation.
The wave might have to go through a lower layer many more
times at longer distances.

We also don't know the pattern of the transmitting antenna,
or how propagation losses vary with direction. For example
consider the difference if the source transmitted from an 80
meter dipole at 35 feet rather than a dipole at 160 ft, or
in various directions with a typical dipole pattern (or even
a vertical in a cluttered environment).

The result of all of this is unfortunate for comparing
systems in different locations. We will find a "sweet
direction and distance" that is significantly better for
setting miles-per-watt records with any given transmitting
system. This doesn't mean anyone is not doing work, it just
means we can't say one station receives any better than
another station unless they are very close together so the
path is essentially the same to both. It also clearly means
a signal over a 500 mile path cannot be compared to similar
"miles per watt" at 300 miles or 800 miles, and especially
it doesn't mean a thing when compared to a Europe path.

Now for the curious effect I observed.

While I didn't spend a large amount of time listening to 80
and I never listened to 40 at all, I did notice one thing
that I found interesting. Listening to the very low power
transmissions on 80 meters I observed a very pronounced
signal peak just before sunrise. The effect was very much
like the effect called "search light" or "spot light"
propagation. While the signal was largely in the noise and
unreadable from 0900 Z  until 1120 Z, I observed a very
clear "strong" peak between 1125 and 1135 Z where the signal
was the equivalent of  about a "559" or better DX report. By
1138 copy was back to nil with an increase that provided a
short 2 minute long readable signal starting at 1143 Z .
After 1145 Z that was it for the signal. I never really
noticed this effect over the same path when power levels
were higher and signakl levels stronger, although I'm sure
the path loss went through similar variations. This is why
even one dB sounds like a large change when signals are in
the noise, and is meaningless with "579" signals.

I wonder is a lot of the "search light" effect is really
just normal small or modest level increases. I wish I could
record absolute signal levels over a long period of time,
but I can't.

73 Tom