[aprssig] VARA-FMSK16-OFDM Shootout A Complete Fiasco

Stephen H Smith WA8LMF2 at aol.com
Sun Feb 12 15:19:00 EST 2023

The test run announced last week was a complete fiasco with numerous hardware 
and software problems.

For example, I discovered the FLdigi program's audio detector  on receive is so 
sensitive that transmitting on one mode would cause the other instance of 
FLdigi (on receive) to hear the RSID code and auto-switch to to the same 
mode!    So I would beacon on MFSK16 -- then the next beacon (from the other 
copy of FLdigi) would also beacon on MFKS16 instead of OFDM.

I think some sort of sneak audio path in the hardware of the computer or 
interface, or perhaps possibly stray detection of transmitted RF.

I did days of testing in my carport before heading out on the trip.   For the 
at-home tests, I had a 10dB 50-watt attenuator in the line between the 
transceiver and the mobile antenna.  Partly to keep the nearby base station 
receiver from overloading so badly, and partly to hide the severe de-tuning of 
the mobile whip when parked under the carport canopy, from the transceiver.  
(In the interest of keeping the transceiver PA as linear as possible.) Under 
these conditions there were no problems.  On the road, without out the 
attenuator with the antenna matched and radiating full power, the problems began.

Also, my experiment to have VSPE (Virtual Serial Ports Emulator) allow three 
applications to share the same serial port for PTT keying worked fine on the 
desktop pre-test, but came unglued in the real world. It would work for a 
while, then randomly lockup and stop doing PTT.    I suspect this had something 
to do with the DigiRig interface I used having NO isolation between the 
computer side and the radio side; i.e. common ground connections, aggravated by 
automotive electrical noise circulation on the vehicle ground systems. (Again, 
the home desktop pre-test worked perfectly for days when the the transceiver 
(Yaesu FT-891) and computer were being powered by clean DC power from a 12 VDC 
100AH battery bank being float-charged by a good 20A DC power supply.

At the same times,. a second VSPE serial splitter setup sharing a single GPS 
receiver with 5 destinations (MS MapPoint for navigation, the main two-meters 
APRS instance of UIview, and three copies of UIview being used for the 
experiment on 60 meters) would also quit. The normal setup (MapPoint, two 
UIviews, a GPS monitoring tool "GPS View" and sometimes Delorme Topo NA)  has 
been absolutely bullet-proof with this setup  in the past for over 100,000 
miles of mobile operating.

I lost a lot of tine stopping at highway rest areas rebooting the Toughbook and 
restarting applications in a precise sequence.  On the return trip from the 
hamfest, I began to suspect the interface and noise/ground loops as the problem.

I stopped by the side of route Michigan 115 and switched the interface to my 
own homebrew totally passive tone-activated self-powered and TOTALLY ISOLATED  
interface (that I had been demoing at the hamfest). I lost an hour 
re-configuring and tweaking multiple applications on the Toughbook; then 
resumed the drive.   The GPS splitting and PTT was now bulletproof!

But I hadn't noticed that in the meantime the audio tone freqs in both copies 
of FLdigi had shifted downward 100 Hz, ruining the remainder of the test!

I plan to run a proof-of-concept test on a short drive from East Lansing to 
Grand Rapids (about 70 mi away) sometime next week. Before running a "real" 
test farther afield enroute to a ham fest in a month or two.

All this comes with the territory of mobile radio systems engineering which I 
have been doing for decades!

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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