[aprssig] Interesting Findings - 300 Baud AX.25 on VHF-FM APRS

Stephen H. Smith wa8lmf2 at aol.com
Sat Apr 2 03:20:44 EDT 2016

I am anticipating a possible APRS public service event in mountainous terrain 
on a non-144.390 channel. Thus no existing digipeater infrastucture.

On a whim, I decided to experiment with 300-baud HF-style packet on VHF-FM 
(instead of the usual 1200-baud mode) to determine what, if any, advantage 
could be gained in marginal signal situations.

For my table-top closed-circuit test, I used one of my Acer netbooks running 
the UZ7HO Soundmodem soundcard software TNC. The Soundmodem program can be 
instantly switched between the 300-baud mode normally used on HF, and the 
1200-baud mode normally used on 2M.  Further, it has a built-in display of 
successfully-decoded packets.

I routed the audio-out of the netbook through one of my homebrew interfaces 
into the external modulation input of my IFR-1500 service monitor, and set the 
deviation of this "transmitter" to exactly 3.0 KHz.  The IFR-1500 has a 
digitally-controlled  attenuator on it's RF generator that can vary the RF 
output in exact 1 dB steps.  The IFR's generator was hardwired to the antenna 
port of the receiving radio with a 3-foot BNC-BNC cable.

At the receiving end, I used a Yaesu FT-1500 FM monobander transceiver. I 
coupled the non-deemphasized raw discriminator output of the 6-pin mini-DIN 
"data" port of this radio through a second interface into the audio-in of a 
Winbook 7" Windows tablet running Windows 8.1 and another copy of the UZ7HO 
Soundmodem. The internal "Realtek HD" audio system on this tablet is actually 
an excellent performer; far better than most built-in PC sound systems.

My test consisted of manually punching off beacons on the sending-end 
Soundmodem, while watching the internal "decoded packets" monitor of the 
receiving-end Soundmodem display. I arbitrarily defined "perfect" reception as 
50 successive beacon packets without a miss. I gradually reduced the RF 
generator output in 1dB steps until packets started being missed at the 
receiving end.   I moved the RF level up and down around the "fail" point 
several times to verify the repeatability of the threshold level.  The results 
were striking:
In the 1200-baud mode, -109 dBm (.795 uVolts) was required for "perfect copy". 
At this point, the un-modulated generator carrier was yielding soft 
"fine-grained" hiss in the FT-1500's receiver, but no "pop corn" noise. I.e. 
something resembling 20 dB quieting in a classic SINAD test.

In the 300-baud mode, only -113 dBM (.501 uVolts) was required for "perfect 
copy".   At this RF level, the unmodulated generator "dead carrier" was 
yielding rather severe "pop corn" noise similar to about 10 dB quieting on a 
classic SINAD test.
In other words, just switching from 1200 baud to 300 baud bought a 4 dB 
improvement in receive performance. I would expect something like this on 
HF/SSB, especially with a DSP variable-bandwidth IF system that can be screwed 
down around the width of the transmitted signal.

I was surprised that it worked in such a similar manner on FM.  The 
Soundmodem's internal audio DSP incorporates does incorporate 
variable-bandwidth AUDIO filters keyed to the selected baud rate.

With a short compressed-mode APRS posit format  (not mic-E) generated by 
UIview, I discovered that the transmission-time penalty for using the slower 
more noise-immune 300 baud mode was actually very small.   Using a minimal 
packet with just position, a WIDE1-1 path, and TXD set to 250 mS, the 1200-baud 
packets took around 2 seconds while the 300 baud ones took 2.5-3 seconds.

In real life, I will be using a TinyTrak III in Mic-E mode, making the 
transmitted strings even shorter.  I intend to set the Primary mode to 1200 
baud while the Secondary mode will have identical settings but at 300 baud.

The UZ7HO Soundmodem is ideal for this kind of operation.   It is capable of 
operating like a dual-port TNC with TWO completely separate software modems 
internally.  Each one can be set to any speed from 300 baud through 2400. 
Normally these are used separately with the left and right channels of a 
STEREO-input soundcard to create something similar to a KAM dual-port hardware 

However the program also offers a mode where both modems can take their input 
from a SINGLE audio input, and the two decoded streams can be output through a 
single AGWpe-compatible TCP/IP port, and at the same time to a KISS-over-IP port.

By setting one of the modems to 1200 baud and the other to 300 baud, the 
program can "automagically" receive either baud rate from a single radio with 
no manual switching required.  All I have to do is switch the mobile TinyTrack 
from Primary to Secondary mode!

Further, the latest version of the Soundmodem now offers a built-in rudimentary 
digipeater. This is a basic literal callsign-matching non-N-n-decrementing digi 
suitable for the basic WIDE1-1 "fill-in" digi function. (You can enter a list 
of multiple callsigns to literally match; i.e. something like 
"MyDigicall=WA8LMF,WIDE1-1,TMP1-1" )  This list is separate for each of the two 

Running the Soundmodem alone on the cheap ($65) Windows tablet can instantly 
create a digi that will repeat either 300-baud or 1200-baud automatically.

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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