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[aprssig] APRS propagation monitor

Stephen H. Smith wa8lmf2 at aol.com
Wed Jun 6 19:53:34 UTC 2012

On 6/6/2012 3:27 PM, Charles Blackburn wrote:
> don’t forget, soundcard packet isnt as good as using a tnc obviously, but any 
> slight differences in the sound card can cause issues.
> charlie
> ___

Simply NOT TRUE!

I conducted an extensive test under controlled conditions earlier this year 
that included:

    TNC-2 (MFJ-1270 with TAPR 1.1.9 Firmware)
    KAM (Original in packet mode)
    D700 Internal TNC
    [I would have loved to try some of the newer DSP-based hardware TNCS such 
as the PK-900 or SCS modem, but didn't have access to any.]

    UZ7HO Soundmodem
    AX.25-SCS modem for the now-discontinued KB2SCS  APRS-SCS app

I used a repeating packet test signal generated by a Tigertronics TigerTrack 
APRS Tracker feeding the external modulation input of my IFR-1500 service 
monitor's signal generator.   The IFR's digital attenuator allows me to 
accurately vary the signal level down to -127 dBm in 1 dB steps.

In turn, I fed this test signal into several different 2M transceivers including

   Kenwood TM-G707
   Kenwood TS-711 (All mode 2M FM/SSB base station similar to an HF rig)
   Kenwood TM-211
   Kenwood TM-221
   Yaesu FT-1500
   Bearcat 760 Scanner

In turn, I tried each of the TNCs listed above with each of the radios.  The 
test consisted of reducing the generated RF level until packets started being 
missed.  I arbitrarily choose "1 missed packet out of 10" as my "FAIL" threshold.

The UZ7HO Soundmodem outperformed everything else tested, hardware or software, 
by around 3 dB!

This was all done in FM mode.

My next step is to re-purpose a Yaesu FT-100 "DC-to-light" HF-VHF-UHF SSB 
transceiver, with blown finals, into a calibrated  SSB signal generator. I plan 
to run the 1-watt-out driver stage into a calibrated step attenuator network to 
create a variable signal source similar to the IFR, but on SSB.   This will let 
me compare the performance of these various TNCs operating in 300 baud 200-Hz 
shift HF mode as one might encounter on 30 meters HF APRS.



Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

High-Performance Software-Only Packet TNC

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