Frequency Drift or Signal Drift

Frequency Drift or Signal Drift

Post by Ed » Sat, 20 Nov 1999 04:00:00

Newbe alert:

I've been playing with my DX-398 and I've been noticing that signals
come and go in strength quite a bit.  It's consistent whether I tune
up or down a notch on the dial.

Is the radio drifting or is it the signal itself.  I just want to be
sure I have a good example of the radio.

Thanks,

-Ed G

 
 
 

Frequency Drift or Signal Drift

Post by Jim Bowli » Sun, 21 Nov 1999 04:00:00



> Newbe alert:

> I've been playing with my DX-398 and I've been noticing that signals
> come and go in strength quite a bit.  It's consistent whether I tune
> up or down a notch on the dial.

> Is the radio drifting or is it the signal itself.  I just want to be
> sure I have a good example of the radio.

I am not sure what you mean by "it's consistent whether I tune up or
down".  My guess is that you mean that when you get a strong signal
it stay strong even if you mistune your receiver a few 100 Hz, and
likewise if it is weak, it stays weak with the same mistuning.

If the above is the case then there is nothing wrong with either
the transmitter or the receiver -- you are just experiencing what
is called "fading" in shortwave lingo and called "multipath distortion"
for commercial FM.

What happens is that there are several slightly different paths that
the signal can take and it takes the signal a slightly different amount
of time to traverse each path.  These signals get added together at the
receiver.  If, for example, you only have two paths and the signal is
roughly the same strength on each, then when transit time along each
path is just right so the two signals add up "in phase" then you will
get a strong signal, when the transit times are such that the two
signals add up "out of phase" then you will get almost no signal.

Since the travel time along these paths depends literally on the weather
(and probably on the phase of the moon), the transit times are slowing
changing and this causes the strength of short wave signals to vary
drastically over time scales of minutes or less.

This is all perfectly normal.  If you meant something different when
you said that it was "consistent" then there is probably a different
explanation.

HTH -- Jim Bowlin