GBPPR Mk-1 Family Radio Service ECM Device

Overview

A transmitter which transmits on all 22 FRS/GMRS channels and 38 subchannels.  The 836 channel/subchannel possibilities are used by most consumer-style FRS/GMRS radios.  Intended to prematurely trigger any RF carrier operated device using those channel/subchannels.

Operation

• Apply +12 to +24 Volts DC to the push-terminals on the top-left of the transmitter.  A short RED/BLACK jumper cable is included to connect to any external battery or vehicle power supply.  The end of the jumper marked "TO TX" should be connected to the transmitter's push-terminals.
• The RED push-terminal is POSITIVE.  The BLACK push-terminal is NEGATIVE/GROUND.
• Current draw is approximately 3 Amps max. during the transmit stage.
• The DC power input is reverse-voltage and over-current protected via a 5 Amp replaceable fuse.  A spare 5 Amp fuse is located in a sponge holder in the bottom compartment of the transmitter.
• Connect a suitable 462 - 467 MHz antenna (50 ohm impedance) to the female N-connector located on the top of the transmitter.
• Avoid touching the center conductor of the N-connector as static electricity can damage the RF power amplifier.
• The coaxial cable feeding the antenna should be low-loss RG-8, RG-213, LMR-400, or equivalent.  A short flexible N-jumper cable is included to connect the transmitter to any non-flexible coaxial cable.  Also included is a female N-to-female N adapter, in case the coaxial cable's connectors don't match.
• With a suitable antenna connected and DC power supplied, flip the switch labeled "PWR" (POWER) to the right.  The RED LED should illuminate, indicating that the unit is powered.
• After about 5 seconds, the unit will begin transmitting.  The GREEN LED labeled "TX" should flash each time the unit transmits.
• The switch labeled "HOP" and its positions "F" (FAST) and "S" (SLOW) indicate the channel hopping speed of the transmitter.  The FAST HOP option should be the default.  This rotates through all 836 channels/subchannels in approximately 30 minutes, transmitting on each channel for around 0.6 seconds.  In the SLOW HOP position, both times are doubled.
• The SLOW HOP position MAY be more effective in triggering RF carrier operated devices, but will take longer to go through all 836 channel/subchannel combinations.

Cautions / Notes

• When transmitting on channels 7 to 14, the RF output will be reduced and the GREEN LED will not be as bright.  This is because the FRS transmitter is forced into low-power mode on those channels.  I was unable to overcome this problem in the radio.
• Do not operate without an antenna attached or else the RF power amplifier will be destroyed.
• UHF frequencies are line-of-sight.  Get the transmitting antenna as high as possible and use the lowest loss coaxial cable as possible.
• The transmitter's antenna polarization must match the receiver's antenna polarization.  (i.e. vertical to vertical)  Otherwise there is an additional 20 dB mismatch loss.  This will reduce the effective transmitting range by almost 2/3rds.
• The antenna used should be at a "DC ground" or have some type of EMP or static protection to avoid damaging the transmitter.
• The transmitter hops through the channels sequentially.  A clever adversary could adjust their device to operate on a channel which may not be triggered on for almost 30 minutes.

Pictures

Outside overview.  It is built into the case of an old GRA-39 radio control set.  The two push-terminals on the left are for the +12 VDC power supply, the RF output (antenna) is a N connector, with a dust cap.  The two toggle switches control the system's power and the hopping speed.  The gray things are dust boots.  The red LED is a power indicator and the green LED lights everytime the system transmits.  The big, black light is not used.

The case is painted camel-humper beige.  The black sticker lettering is protected with clear finger nail protector, really.

Internal overview.  The BASIC Stamp 2 microcontroller on mounted on the left using little rubber washers and stainless steel hardware.  The RF power amplifier is mounted in the middle on a block of aluminium, to act as a heatsink.  The wires and coax cables are secured with zip-ties and plastic holders.

Behind the panel overview.  All the wire connections are sealed with heatshrink and all the stainless steel mounting hardware is sealed with Loctite.  Silicon sealant is also used around the N connector, fuse, LEDs, and the switches to help seal them against dust/moisture.

Close up picture of the low-dropout voltage regulator board.  This takes the incoming DC voltage and regulates and filters it to clean +12 VDC and +6 VDC for powering the RF power amplifier and the controller/FRS radio, respectively.  The Micrel MIC29302BU is configured for +12 VDC and the MIC29152BU is configured for +6 VDC.  See their data sheets for more info on them.

Another internal close up picture showing the BASIC Stamp II microcontroller and it's PC board.

Note the rubber washers and the secured wires.

Close up picture of the RF power amplifier.  It is based around the Mitsubishi RA07H4452M 7 Watt, MOSFET RF module.  The RF input is from the FRS radio - through a 15 dB attenuator.  The RF power control line (V_gg) is always set to around 3 volts via a Zener diode.  The big silver-plated copper wire loop on the RF output is a grounded 1/4-wave stub.  This helps to prevent any static electricity on the antenna from damaging the RF amplifier module.  A simple diode/transistor/LED is used as a RF sense for a "transmit" indictor light.

A Cobra PR950DX FRS radio is used as the exciter.  The "PTT", "MODE", and "UP" buttons are controlled via the BASIC Stamp.  The FRS radio is mounted in the bottom compartment of the GRA-39 case and is resting on some sponges.  A second little sponge holds a spare fuse.

Schematics & Source Code

• GBPPR Mk-1 FRS ECM Schematic  BASIC Stamp 2 Controller Board
• GBPPR Mk-1 FRS ECM Schematic  RF Power Amplifier
• GBPPR Mk-1 FRS ECM Source Code  BASIC Stamp 2 Controller

Software Truth Table

Step       PTT   UP   MODE (Radio Button)
0          1     0    0
1          0     0    1
2          0     1    0
3          1     0    0
4          0     1    0
5          1     0    0
6          0     0    1
7          0     1    0

Datasheets & Notes

• Higher resolution pictures and the original project article are available in GBPPR 'Zine Issue #15
• Mitsubishi RA07H4452M 7 Watt, MOSFET RF Module  (87k PDF)
• Schematic Diagram of a Cobra PR950DX FRS Radio
• Fair Radio Sales  Sells surplus GRA-39 control boxes.
• The Secret History of Iraq's Invisible War

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• Experimental Ideas for Counter-Terrorism
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• Simple CW GPS Jammer
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