locked #JT9 - Fast Mobile vie satellite #JT9


Andy Talbot
 

I've just recently been conducting some (one way) tests using JT9G-Fast and
JT9H-Fast through the QO-100 satellite. Running 10 Watt to a Quadrifilar
Helilx (QFH) antenna mounted on the roof of the car. Tx bursts, consisting
of four frames of either modes, repeated every 10 seconds gave a regular
low duty cycle test signal to listen to.
WSJT-X was set up at home, monitoring the satellite downlink.

Driving around local suburban and rural roads, generally at speeds up to 40
mph / 64kph and usually somewhat lower, I achieved successful decodes of
51% of the transmission bursts using JT4G-Fast, and 39% of all those using
JT9H-Fast. Considering the latter has a lower duty cycle (~ 1.7 second
transmission every 10s against JT4G-Fast 3.4s every 10s), and its inherent
3dB lower sensitivity I'd say the faster mode is the winner, in spite of
the slightly reduced decode rate success.

A plot of S/N for all successful decodes showed values ranging from 0dB to
-9dB (ref 2.5kHz).
Seems to me that if you need a one-way telemetry/alerting/warning
/messaging protocol through that sat from a mobile, the JT9G/H-Fast has a
lot going for it. Low duty cycle transmitters capable of 10 Watts at
2.4GHz are typically used in standard Wi-Fi booster amps.

I tried MSK144, but the link budget was at least 3dB short of even being
remotely useable, and would have needed something like 10dB increase n EIRP
to give a similar reliability.

A full write up of these tests will appear in due course

Andy
www.g4jnt.com