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Re: PSK modems and noise



Hi Steve.
 
> I'm setting up a system here to use the 1200 baud pacsats, and am
> particularly interested in people's "real world" experience. 
> 
> The ARRL manual suggests that "PSK coded signals can be detected and decoded
> even when the signal is buried in noise" and also that simple antennae
> (J-pole) are the way to go.
> 
> Is this realistic? My G3RUH modem seems to need a signal well above the
> noise (by ear anyway) in order to "lock" in - and the pacsat signals are a
> lot below that level.

I think most people use antennas with gain for Pacsat communications.  It's
kind of frustrating to have to wait for an additional orbit to complete the
download of a file that could have been fully downloaded initially if the
downlink antenna only had some gain.  :-)

As far as noise is concerned, there are PSK packet signals I can copy through
noise I can clearly hear, and there are AFSK-FM (DOVE-OSCAR-17 and 2-meter FM
variety) packet signals I can't copy through noise I can't hear.

The performance difference between the two modulation formats is truly
amazing.

These observations are based on using a KD2BD Pacsat Modem with a Yaesu
FT-726R for PSK communications, and an MFJ-1270B TNC with internal XR2211
based AFSK PLL demodulator for AFSK-FM communications.

Incidentially, my modem AFC successfully tracks signals that are *BARELY*
audiable through the noise.  :-)


73, de John, KD2BD

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