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NERO Research Tiny Transmitter

Tiny 144 Mhz homing transmitter

At this page we will explain how to build a very small tracking transmitter used by Radio Amateurs for fox hunting. The objective of a fox-hunt is to locate a hidden transmitter in as short as possible a time. This basically covers the need of many rocketry amateurs who are very eager to find their precious equipment back.

There was an article in the "73 Amateur radio today" magazine of may, 1993 called "Homing in" which described a good design that completely matched our requirements. It was small, powerful and easy to build. Complete with schematic and PCB the article was clear to setup an experiment and check the possibilities of the transmitter.

Figure 1 - Tiny 144 Mhz transmitter
Figure 2 - Tiny 144 Mhz transmitter

The PCB layout shown in the above figure was the one actually used for the three first transmitters. The right side is the bottom side, the left side is the component/top side. You can print the PCB on a 300 DPI printer without changing the size. If printed on overhead sheet, the picture will occur is positive when you look from the non-print side of the sheet. This will facilitate the photo process since the ink is directly on the PCB and not on the other side of the sheet.

The get the clean ".gif" file, click on the layout. The previous attempt to provide a ".pcx" file didn't want to work.

Here is the relevant text from the article (design is by Ken Bauer KB6TTS).

Figure 2 is the schematic of the RF and pulsing circuits. Components values are for 2 meters (144 Mhz). For the 1-1/4 meter band, change C2 to 47 pF, C4 to 22 pF andC6 to 10 pF. Y1 will be 74 to 75 Mhz. For operation with a 9 volt supply, change R1 to 10K, R2 to 5.1K and R5 to 22K.

The design is straightforward and reproducible. Very hot RF transistors are needed for the unit to work well at low voltage and provide plenty of energy at the third harmonic of the oscillator. The Motorola MM-BR951L is a good choice but not available in Europe. For that reason it is replaced by the BFP 193 which requires the PCB shown above. It is tested and works fine with this minor modification. Q1 forms a Colpitts oscillator with the crystal, C1 and C2. The crystal sees the combined capacitance of C1 and C2 as its 20 pF load and itself looks inductive as these elements form the main frequency control tank. L2 prevent the circuit from oscillating on the fundamental frequency and keeps it in the overtone mode. The trick is that the oscillator will only work on frequencies where its emitter sees capacitance to ground. The combination of L2 and C2 must be such that they look capacitive at the overtone frequency but inductive at the lower fundamental frequency, thus preventing oscillation there.

Tank circuit L1/C4 is tuned to the transmitter output frequency, making Q1 an oscillator/tripler stage. Q2 is a buffer that amplifies the oscillator output and isolates it from load variations at the antenna. U1 pulses the Vcc supply to Q1 and Q2 with on and off time periods set by R5, R6 and C8. Change these components as you whish to give the right pulsing rate and duration for your application. It is important to use the CMOS version of the 555 timer to maximize battery life.

The crystal is a third overtone type at two meters and fifth overtone at 1-1/4 meters. It should be specified for 20 pF load capacitance, series resistance not more then 60 ohms, and frequency tolerance of 25 parts per million or better.

If your idea of project building is dropping component leads through holes in a nice big circuit board and fastening them down with a soldering gun, you will need to change your ways for this project. Making an "Agent 007 size" transmitter calls for the same technology used to make today's pocket-size dual-band walky-talkies. All capacitors and resistors are chip components except R5, which is 1/8 watt carbon. The transistors and IC are in SMD package.

This is definitely not a beginners project. If you have successfully built a few circuit boards and know your way around VHF RF circuits, you should be able to build and adjust this little rig. Otherwise, seek help from a local technically-inclined ham.

Surface mount construction may seem a bit scary, but it just means acquiring a few new skills. Set your soldering gun aside and use a fine tipped iron, 18 to 25 watts. You will also need steady hands, a strong light, a low power magnifying lens and a pair of fine point tweezers.

Once you get the hang of it, soldering a component onto the board is not difficult. Pick up the part with the tweezers and "wet" one terminal using the soldering iron and a tiny amount of solder. Use the tweezers to hold the part down on the board in its place, then momentarily touch the iron to the pre-tinned pad to flow the solder and secure the component. Then tack down the other leads, being careful not to overheat the part so that is shifts position on the board.

L1 and L3 are made from AWG 32 (0.203 mm) enameled wire, wound over a 0.06" (1.5 mm) diameter drill bit, then slipped off and soldered onto the board. Start with the turns close spaced. You will spread the turns as necessary to tune each stage.

The primary goal of tune-up is to peak up output on the transmit frequency and minimize output on the fundamental and all other harmonics of the crystal. A spectrum analyzer is the ideal tune-up instrument, but most of us don't have one at home. With a bit of patience, you can substitute the S-meter of a wide-range receiver.

As an example, the output of a 146.565 MHz unit should be checked at 48.855, 97.710, 195.420, 244.275 and so forth, tuning to minimize all spurs and maximize the 2 meter signal. Tune-up is easiest with continuous output, so jumper the collector and emitter of Q3 during this process. Whether you use an analyzer or receiver, connect the mini-T's output to the indicating instrument through a resistive step attenuator, so you are measuring the true output instead of direct radiation from the stages.

Ken's board has no provisions for ID or modulation. You will want to add these as appropriate for your application. Many CW keying circuits have been published and they can readily be adapted to key the Vcc line of this rig.

Component list:

Component: 9 volt version 3 volt version
C1 22 pF 22 pF
C2 100 pF 100 pF
C3 1 nF 1 nF
C4 47 pF 47 pF
C5 10 pF 10 pF
C6 33 pF 33 pF
C7 4.7 pF 4.7 pF
C8 100 nF 100 nF
C9/C10 1 nF 1 nF
R1 10 K 5.1 K
R2 5.1 K 10 K
R3 330 330
R4 22 K 39 K
R5 10 M 1/8 W 10M 1/8 W
R6 1 M 1 M
R7 5.1 K 5.1 K
Q1/Q2 BFP 193 BFP 193
Q3 2N3906 2N3906
L1 4 turns AWG 32 *) 4 turns AWG 32 *)
L2 270 nH 270 nH
L3 5 turns AWG 32 *) 5 turns AWG 32 *)
Y1 48.855 MHz 48.855 MHz

*) see text.