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1200/9600 Baud REAL test



For evaluating the optimum configuration for Handheld Satellite Messaging,
today, we did another practical test we have long wanted to do.  W4HFZ and
I compared 1200 and 9600 baud performance using the SAME radios under the
same channel under controlled conditions.  The results 'in this test' do
show about a 7 dB difference between 1200 baud AFSK and 9600 baud FSK.

TEST CONDITIONS:  Two THD7 HTs were separated by 100 feet of RG58 with
70 dB or more of SMA RF attenuators on both ends and with RF clamp on
ferrite chokes at 4 places along the coax.  Radios were operated at
Extremely Low power setting.

PROCEDURE:  Attenuators were adjusted to find the 50% and 100% performance
rate at 1200 baud, then we changed to 9600 baud and re-adjusted
attenuators to get the same performance.  S signals are the number of LCD
segments lit.

BAND  FREQ    DATA RATE   50%          100%      Comments
----  ------- ---------  -------     -------     ----------------
 A    145 MHz 1200 baud  S1 -120dBm  S1 -117dBm
 A    145 MHz 9600 baud  S5 -113dBm  S5 -110dBm  About 7 dB worse

 B    435 MHz 1200 baud  S0 ?        S0 -116dBm
 B    435 MHz 9600 baud  S3 -111dBm  S3 -109dBm  About 7 dB worse

S-METER:  Although the above test shows that there is a difference in
the S meter on the two bands, the RF signals are about the same.  Within
each S meter range, there appears to be about 3 dB and that is the
difference between solid and marginal decoding.

PACSATS:  The above tests are between two identical radios.  My PACSAT
experience has shown that the 9600 baud PACSAT downlinks often decode with
even less signal, often at S0!  So they do seem to have optimized signals.

CAVEATS:  Even though the radios are identical, it is still an apple and
oranges comparison, because, of course, the audio path and filtereing is
different  between the two modems.  But if we "asume" that kenwood did the
best they could with each, within their other constraints, then this does
give us a data point that tends to confirm the performance difference
between 1200 AFSK and 9600 FSK baud.

Given that there is a further 9 dB or so PATH loss difference between VHF
and UHF, this gives us a baseline for trying to design the optimum simple
2-way digital messaging satellite links to handhelds.  On the downlink, we
want to deliver a message to an HT in your pocket.  On the uplink, we want
to be able to transnmit to the bird with the whip held in the clear.

Just thought you might be interested in the results of this test.

de WB4APR, Bob


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