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Carter-k8vt · January 18th 07, 01:47 PM posted to rec.radio.shortwave

wrote:

Why do you insist that the atmosphere treats the carrier differently
from the rest of the signal?

Because it does. See below...

Geez. You have a spectrum produced by
modulation. If the modulation is AM, then a carrier is present. Now you
are saying the atmosphere is sucking out the narrow band carrier and
leaving the wideband spectrum untouched. Fiction at best.

Bzzzzt. Wrong!

Yes, the "atmosphere" [ionosphere] DOES "suck out a narrow band" or even
a single frequency. Ask any amateur radio operator that has used RTTY
(radio teletype).

The RTTY "modulation mode" used is FSK or frequency shift keying. At any
given instant, the transmitter is sending either a "mark" or "space",
essentially two carriers if you will, 170 Hertz apart that represent the
5-level Baudot code as used in ham RTTY.

As an aid to tuning, an oscilloscope is used as a tuning indicator; the
mark signal from your RTTY decoder is connected to the horizontal plates
of the 'scope, the space signal to the vertical plates. On the screen of
the CRT (due to the persistence of the CRT phosphors and your eyes),
this shows what appears to be a "+" sign, also known as the classic
"cross display". When you see the cross on your screen, you know you are
tuned in properly.

So, "What does this have to do with the discussion above?" you ask.

Remember, you are looking at essentially two "carriers", 170 Hz apart,
one on the horizontal axis and one displayed on the vertical axis.
During disturbed ionospheric conditions, many times you will see one
signal or the other disappear; i.e., the cross turns into a single line,
either a "-" or a "|", depending if the mark or space faded--and yes,
sometimes both fade, but it is more common to see one or the other
disappear.

This phenomenon is known to hams as "selective fading", is quite common
and is interesting to observe.

So, yes, the ionosphere CAN suck out one signal separated from another
by as little as 170 Hz.

Carter
K8VT

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