Use an old Motorola analog AMPS cellular phone and an external frequency transverter to transmit narrowband FM in the 902-928 MHz (33 cm) amateur radio band.

By using a standard Motorola cellular phone (in TESTMODE) as a low-power signal source (825-850 MHz), with the microphone enabled, and an easy to construct frequency transverter, you can get on the air in the 900 MHz ham band.

For receiving, you'll need to use a separate receiver, i.e. Radio Shack scanner.  It's not possible to convert the cellular phone into a 900 MHz narrowband FM receiver without alot of tweaking.  It is possible though!  It envolves just using the phone's 45 MHz (or 109.6 MHz) IF strip for FM demodulation... but that's getting ahead of ourselves.

Now, this isn't a transmitter setup that will break any world records, it's just for fun.  The 900 MHz band is horribly underused and needs to be reclaimed.  This is a little project which can help to reverse that.

A Motorola phone in TESTMODE has an option to enable carrier (transmit) on any of the cellular channels between 825 and 850 MHz, with the standard 30 kHz spacing.  By using this as a signal source and mixing it with a simple 80 MHz crystal clock oscillator, you can then transmit betwen 905 and 928 MHz.  The transmit audio option (microphone enable) will then allow you to modulate the new transmit frequency.

Motorola TeleTAC 200 Operation

Press [PWR] to turn the phone on.

Immediately enter: [FCN] [0] [0] [*] [*] [8] [3] [7] [8] [6] [6] [3] [3] [STO]

The screen will start flashing numbers.  Example: 314 059

In case you have trouble remembering the number sequence, it spells out "TESTMODE".  Your phone is now in test mode, which is where we'll begin to turn it into an excellent, stable, 900 MHz transmitter.

Press [#] to get to the U5   ' prompt.  This is the test mode prompt where all the commands will be entered from.

Enter the frequency you want to transmit at.  This can be anything from 824.04 MHz to 848.97 MHz in 30 kHz steps.  You'll need to enter the frequency in "Motorola Test Mode" channel format.  Example: to transmit at 837.00 MHz, cellular channel 0400, you'd enter:

[1] [1] [0] [4] [0] [0] [#]

Next is the most important step, you need to turn the microphone (transmit audio) on!!

Do this by entering: [1] [0] [#]

Connect the Motorola cellular phone to the transverter.  The transverter RF power input should be limited to around 0 dBm (1 mW).

To turn the transmitter carrier ON enter: [0] [5] [#]

To turn the transmitter carrier OFF enter: [0] [6] [#]

Verify it is indeed transmitting by listening for a feedback squeal on a communications receiver.

See the Motorola Bible v3.0 for more information on programming Motorola phones in TESTMODE.

Schematics

• Schematic

Pictures / Notes

• Prototype Transverter Overview  Connected to a Motorola TeleTAC
• Tapping a Motorola Cellular Phone's RF Amplifer Output  Attempting to use the module at 900 MHz.
• Enable Testmode on Motorola Bag-style Cellular Phones  Ground Pin-21 on power up.
• Inside a Old Nokia Cellular Phone  The power amplifier is a PF0030 and the 45 MHz IF is based around the Motorola MC3371.

• Motorola Bag Phone Handset Control  Control a Motorola cellular phone's handset with a simple PIC circuit.
• PF0031 Amplifier
• Hacking The Original 915 MHz WaveLAN
• An Experimental AMPS Micro Base Station

Datasheets

• Mini-Circuits VNA25 MMIC Amplifier  (55k PDF)
• Hitachi PF0031 6 Watt 900 MHz RF Amplifier  (65k PDF)