[CTSARA] Some Lessons From Last Night's Digital Bootcamp

[Jon Perelstein]

So we learned a couple of interesting lessons last night.  

1.  When operating SSB (as opposed to FM). the convention is to use upper 
sideband on all bands, even 40 and 80.

2.  Some of the modes, such as MT63, can take up to 10 seconds before the 
computer is translating the data, which can confuse you when you're trying to 
tune in the signal.  This tells us two things:

	* When you see the signal on the waterfall, bracket it and then have a bit of 
patience to see if it's translating.  As you become familiar with the various 
modes, you'll know which have delays and which don't.  In general, the more 
error detection/correction in the mode, the more of a delay.  Thus, there is 
virtually no delay with PSK-31 (which has no error detection/correction) whereas 
there is about a 10 second delay with MT63 (which has some of the best error 
detection/correction)
	* Our doctrine for digital transmissions in emergency conditions should include 
a 30 second preamble so that the receiver can get a good tune on the signal
3.  While it would appear that a mode like PSK31 is better for EMCOMM than a 
mode like MT63 because of the ease of tuning in the PSK signal, PSK is a mostly 
horrible mode for EMCOMM because it has no error detection/correction and is 
subject to interference in the IF stage of the radio (more below).

4.  We all KNOW that antenna polarization (horizontal vs. vertical) doesn't 
matter on HF, right?  Well, not always.  Polarization doesn't much matter if 
you're bouncing a signal off the ionosphere (which is what we're mostly doing on 
HF).  However, if you're relying on ground wave or just direct line of sight, it 
can matter as we saw with John (AB1HI) last night.  Presumably we will be using 
VHF/UHF for our local communications, so it should not be an issue for us, but 
it is something to remember when operating HF.  

And we all know that the convention is verticals for VHF/UHF FM, and horizontals 
for VHF/UHF SSB, right??

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Interference in the IF stage

The typical rig IF stage is at least 2.5K wide.  That means that it is receiving 
and working with anything within that 2.5K bandwidth.  Usually not a problem for 
phone or CW, but potentially a big problem with narrow digital modes like PSK.

Why?  Because a strong signal from one station can drown out the signal from 
another station and make it unreadable, even if the two signals are 1K or 2K 
apart.  They're still within the IF bandwidth.  Turning off AGC helps since AGC 
will attempt to lower everything in the 2.5K bandwidth.  Mechanical filters are 
an even better solution -- the 300Hz Collins filter I have on my FT-897 helps 
dramatically be eliminating anything outside that 300Hz range before it gets 
into the IF.  Best is DSP in the IF stage because you can get the bandwidth down 
to about 50Hz.  However, not all rigs have their DSP in the IF stage.  The 
FT-897, for example, has its DSP in the audio stage, and it's too late by then.

Jon
KB1QBZ


P.S.  Power settings on digital modes are a bit different than on phone or cw. 
 You can't crank up the power too much without distorting the signal.  As you 
continue cranking, you will wind up causing massive QRM to everyone on the band. 
 A discussion of power settings is going to be part of the next "lesson" that 
I'm producing right now.