Motorola RNet 150 Transmitter

 
A 2 channel Motorola RNet 150 Telemetry Radio was used to make a transmitter for the BEAR 1 & 2 APRS tracker.

The receiver and unnecessary components were removed to reduce the size, weight and turn the transceiver into a single channel transmitter. Extensive circuit modifications were also made to improve the transmitter efficiency.

The heat sink is present in this photo, but the transmitter will be used without it to reduce weight. Transmitter duty cycle is very low with APRS and the output transistor temperature did not increase without a heat sink, at least not enough to be easily detected.
Bottom view of modified transmitter

 

BEAR-2 APRS Transmitter

 
With the addition of a cross-band repeater on Bear-2, it was evident that more shielding and isolation of system components was required. This will become even more important on future flights when more equipment and transmitters will be added for photography.

Top View

The transmitter was enclosed in a box made from
.01" thick copper.

Feed-thru capacitors were used for power, PTT and
audio lines.

A Tx LED (relocated from the BEAR-1 tracker PC board) and a PTT test push button were added.
A higher battery voltage then used on BEAR-1 will be used on BEAR-2 which will increase the output power so a threaded coupling was added to the RF output transistor to allow using the copper case as a heat sink.
Bottom View

The Finished Transmitter

Size = 1-1/2" x 2-3/8" x 5/8"
Weight = 38.4 grams (1.35 ounce)
Standby current = 0.16 ma
2.4 watts @ 6.0 VDC - 1.2 A

 

APRS Tracker
Specifications
Voltage - minimum for APRS data 4.05 VDC
Voltage - minimum for analog telemetry 5.15 VDC
Voltage - maximum 12.0 VDC
Current (not transmitting) 63 mA
Current (2.4 watts RF output) 1.2 A @ 6.0 V
Current (3.4 watts RF output) 1.4 A @ 7.0 V
Current (4.5 watts RF output) 1.6 A @ 8.0 V
 
Temperature Telemetry −90° to +100°C
Atmospheric Pressure Telemetry 0 − 15 PSI
Battery Voltage Telemetry 0 − 8.5 VDC
 
APRS Tracker
 Transmitter Specifications
  Battery
Voltage		 Current
Drain		 Output
Power
Idle Current 4.04−9.0 0.16 mA ----
Output Power
and
Current Drain
vs.
Battery Voltage.		 4.04    .005 W
4.3 0.28 A .05 W
4.4 0.33 A 0.1 W
4.5 0.62 A 0.5 W
5.0 0.93 A 1.4 W
5.5 1.08 A 1.9 W
6.0 1.20 A 2.4 W
6.5 1.30 A 2.8 W
7.0 1.40 A 3.4 W
7.5 1.50 A 3.8 W
8.0 1.60 A 4.5 W
8.5 1.70 A 5.0 W
9.0 1.80 A 6.0 W

 

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