[aprssig] Kenwood D700 Internal TNC Amazingly Mediocre!

Stephen H. Smith wa8lmf2 at aol.com
Fri Aug 5 23:53:34 EDT 2016

I had always assumed that the built-in TNC of the D700 was fairly optimized, 
due to being mated directly to the radio's discriminator, avoiding audio 
channel distortion, audio level issues, squelch rise times, de-emphasis issues, 

I just got back from a road trip to the Colorado Rockies for the annual 
Evergreen Jazz Festival.  While there, I did some coverage testing in the 
mountains around Evergreen of a potential portable digipeater and tracker for 
the festival shuttle bus, that may be used next year.

This is very rugged mountain terrain with few line-of-sight paths and lots of 
multipath.  I used the D700's internal TNC with one copy of UIview on my 
Panasonic CF-51 mobile Toughbook.

I also connected the 6-pin mini-DIN "data port" of the D700 through one of my 
homebrew sound card interfaces to a Behringer UCA-202 external USB "soundcard". 
In turn, the UCA-202 was used by the UZ7HO Soundmodem software TNC.  The 
Soundmodem was linked to a second copy of UIview running on the same computer. 
(The Soundmodem faithfully emulates the AGW Packet Engine interface supported 
by most current APRS clients.)

The tests consisted of placing my FT-1500/TinyTrack 3 ammo-box tracker at the 
hillside location I picked for the digipeater, based on radials I drew on 
Delorme TopoUSA. The tracker was set to beacon every 20 seconds on 144.37 MHz. 
I then drove the 4-mile loop route taken by the shuttle bus during the event.

[The original intent of using the soft TNC on the mini-DIN port at all was to 
try using 300-baud HF-style packet on VHF/FM to gain 3-4 dB of noise margin 
compared to 1200 baud.]

I was completely shocked at the difference between the D700's hardware TNC and 
the software TNC, both at 1200 baud.  When the signals were clean and 
fully-quieted, there was essentially no difference between the two TNCs.  When 
the signals fell out of full quieting, or were "fuzzed up" with multipath 
distortion, the Soundmodem massively outperformed the D700 internal TNC.

Driving the 4-mile winding loop around Evergreen, only about a third of the 
loop showed on the D700 hardware TNC, while 100% of the beacon transmissions 
showed on the instance of UIview linked to the Soundmodem soft TNC.

When I saw how well the Soundmodem worked at 1200 baud, I abandoned the idea of 
going to 300 baud for this project.  (Staying at 1200 means I can intermix 
off-the-shelf 1200-baud TNCs, radios and soft TNCs, while 300 baud would 
require that every node involve a PC of some sort running the 300-baud software 


  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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