[aprssig] HF APRS igates on JS8

[Stephen H. Smith]

On 4/23/2020 10:36 PM, Joe Bennett wrote:
> OK. Just saw I was digi'ed by WA8LMF... Wonder who that is? :)
> 
> I misunderstood your earlier email as I thought that I was to be centered on 
> 1400Hz, which would be an offset of 1375Hz. Moved the Offset to 1400Hz and then 
> saw you gate me to the APRSIS... Now to figure out why we can't hear the other 
> Joe, N3HGB?
> 
> 
> 
> 
> -- Joe
> KA3NAM
> 
> 

Wow!  It actually worked.  Guess I've got it configured right!

I did cause some confusion.  I DID originally try to center the waterfall 
cursor on 1400 Hz.  This yielded an "offset of 1375; i.e. the lower edge of the 
cursor/signal.   I'm used to thinking of the center frequency of these 
MFSK-like modes (not one side) since I'm trying to center them in a narrow IF 
passband.

Then I realized the apparently the convention on this mode (and what the cursor 
points at) is to select the lower edge of the signal. I.e. what the "offset" 
selects.  Thus, I moved the lower edge, not the center, of the signal to 1400 
Hz.

This actually helps squeeze both the JS8 signal and the normal AX.25 packet 
APRS signal through the passband of the Collins mechanical filter in the Yaesu 
FT-857.  The AX.25 FSK signal is on the standard audio "KAM tones" of 1600 and 
1800 Hz.  The nominally 500 Hz bandwidth of the filter is more like 550 Hz.  I 
move the radio's passband tuning upward so the passband spans about 1375-1900 
Hz, so it passes both the JS8 tones and the AX.25 tones at the same time.

The narrow passband makes a massive difference compared to the normal SSB 
filtet width.  The S5 noise level on SSB drops to NO s-meter reading at all 
through the 500 Hz filter. By avoiding having the AGC activated on noise alone, 
the effective sensitivity of the receiver is significantly better.

Note that this is effectively a "dual-mode" igate.  Both the UZ7HO Soundmodem 
for AX.25 and the JS8Call program for JS8 are using the same sound system on 
the PC simultaneously.

_____________________________________________________

Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
EchoLink:  Node #  14400  [Think bottom of the 2-meter band]
Home Page:          http://wa8lmf.net

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