(jb)

Pulsed Tone FM Tracking Transmitter

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Introduction:

The following plans describe how to build a very small tracking transmitter that can be tracked using an FM broadcast band radio receiver. The transmitter is powered by a 3 volt to 9 volt battery and has a range of from 1/4 mile to over 1 mile depending on battery voltage, height above ground, receiver sensitivity, and antenna length. The circuit's size including the batteries is 3/4" by 1 3/4" when powered by two 3volt lithium batteries and twice that size when powered by a 9 volt transistor radio battery. A skilled technician can reduce the size considerably by packing the components tighter on the circuit board. This circuit is ideal for use as a beacon on cars and boats and can even be flown in small model rockets.

The transmitter requires patience and a steady hand to tune and it's frequency is prone to drift. Consequently, it is recommended that it be used with FM radios that can tune continuously across the frequency spectrum. That said, the transmitter is cheap to build, can be built from readily available parts, does not require any expensive test equipment, works well with cheap pocket FM receivers, and can be built by hobbyists with moderate soldering skills provided they don't try to make it too small.

The plans include a parts list, parts sources, schematic diagram, and recommended parts layout for perfboard construction


Theory of operations:

Please refer to the circuit schematic when reading the following description.

The transmitter is composed of three smaller circuits; A transmitter, an audio oscillator and a circuit that pulses the tone oscillator and transmitter on and off.

The transmitter is a variation of a very common FM oscillator circuit that is extensively used in circuits of this type. It consists of one transistor (T2) whose oscillation is determined by a parallel resonance circuit composed of C5 and L1. C6 provides the feedback needed for the oscillation and can be between 3 picofarad an 20 picofarad. R4 determines the DC biases point for the transistor and should provide enough current to prevent clipping of the RF signal. T1 is a Unijuction Transistor (UJT) which forms a simple oscillator of about 1000Hz. R1 and C2 determine the frequency.

The entire circuit is pulsed on and off by the action of a "blinking" LED. The LED will visibly blink when powered with voltages of over 8 volts and will effectively switch the circuit on and off down to about 2.5volts, although the LED will not blink noticeably.

Parts:

Parts are available from a wide variety of suppliers. For convenience only two sources will be listed. They are Mouser Electronics (800)-346-6873, http://www.mouser.com and Radio Shack, just about everywhere in the US.

Resistors
( use 1/8 watt or 1/10 watt resistors)
............................. ..................................... Mouser .......................... Radio Shack
R1........................ 100K .......................... 278-100k .......................... The smallest size
R2........................ 680 ............................ 278-680 ............................ RS has is 1/4 w
R3........................ 330 ............................. 278-330 ............................ buy the value
R4........................ 15k ............................ 278-15k ............................. pack 271-312
R5........................ 2.2k ........................... 278-2.2k
R6........................ 220 ............................ 278-220
Capacitors:

C1....................... .01 microfarad ............. 581-UEC103J2 ................... 272-1065
C2....................... .01 microfarad ............
C3....................... .01 microfarad ............
C4....................... .01 microfarad ............
C5...................... 12 to 100 picofarad ......242-3610-100 ....................... not available
.......................... adjustable
C6...................... 5 pico farad ..................140-CD502S-005J ................ 272-806
Coil .................. Five (5) turns of 26 gauge magnet wire wound on a 1/8" or 5/32" form and removed. (salvage from small electric motor or transformer) Total coil length is 3/16".

................................................................... 501-MW26H-1LB ................ 278-1345
Semiconductors:

T1...................... ECG 6410 UJT.............. 526-NTE6410 ....................... not available
T2...................... ECG 123........................ 526-NTE123AP .................... 276-2009
D1..................... Blinking LED................. 351-8001.............................. 276-036
Antenna:

18" to 27.5 " of stiff wire (steel guitar string works well)

Battery:

3 volt lithium battery type........................ CR1632 ................................... DL1620
.................................. ..................................or
.................................. ............................. (614-CR1632)
9 volt transistor radio battery

Other:

perfboard.................... ............................ 153-1108 .................................. RS 276-1395
(cut to desired size)
Battery holder .................................. ...... 534-071 .................................. 270-326
9volt snap connectors ............................. 12BC311 .................................. 270-325

Assembly:

Please refer to "parts layout" illustration.

The transmitter can be assembled in a variety of configurations although good VHF circuit technique should be followed. The illustration included with these plans shows one of many possible layouts. It is one which has been tested and found to work well.

The components are assembled on a small piece of perfboard of approximately 3/4" by 1 3/4" . The leads are mounted through the holes as shown and soldered together on the back side of the perfboard. Some jumpers are made on the backside of the perfboard to connect the various components. Note that a Printed Circuit Board (PCB) pattern is also include should you wish to make a PCB. The asembly is the same for PCB construction except the jumpers used on the backside of the perfboard are not required since the foil pattern replaces them.

1. Position the components as shown in the pictorial and push the leads through the holes as indicated.
2. Solder all the leads that are in common holes together on the backside of the perfboard.
3. Trim the excess leads from the components soldered in the previous step.
4. Using some of the longer leads that are penetrating through the backside of the perfboard make the connections as shown in the pictorial on the bottom side of the circuit board by bending the leads and soldering.
5. If the transmitter is to be powered by two(2) lithium batteries use a stiff steel wire (possibly obtained from a metal paperclip) to fashion the battery hold down clip and the negative terminal contact as shown in the pictorial.
6. If the transmitter is to be powered by a 9 volt transistor radio battery , solder a 9 volt battery snap connector to the perfboard instead of the battery hold down clamp described in step "6". Connect the red wire from the battery connector to the hole marked "+" and the black wire to the hole marked "-". Glue a 9 volt battery holder to the end of the circuit board to hold the battery.
7. Connect the antenna as shown. The antenna can be any piece of wire of the appropriate length. However, a stiff wire made from a steel guitar string works exceptionally well. The antenna can be any length from 18" to 27", however an 18" antenna is probably the most dependable.

Tuning:

8. Connect the batteries.
9. Turn on any FM radio and tune to a clear portion close to 105MHz.
10. Place the radio about 4 feet away from the transmitter.
11. Secure the transmitter in a nonmetallic vise or brace in a wooden block. The idea is to secure the transmitter so that it will not move when it is tuned.
12. Using a nonmetallic alignment tool very very slowly turn the screw on C5. Do not touch the circuit and position the antenna so that it extends away to the side. (Note: a nonmetallic alignment tool is nothing more than a wooden or plastic screw driver with a very thin and flat tip. One can be easily made by whittling a flat tip on a 6" to 10" stick or wooden dowel rod.)
13. As C5 is turned there will be many spots where a signal will be heard. However, there will only be one spot that will be the loudest and represents the exact frequency. It will be as loud as most of the commercial radio stations and will be several orders of magnitude louder than any other spot. Tune C5 several times working slowly into the loudest signal.
14. Retune the receiver slightly to peak the reception to the exact signal. When the transmitter is properly tuned a loud and clear "beeping" should be heard and when the radio is tuned to either side of the signal the "beeping" should decrease in loudness.

Suggestions:

This transmitter can be easily "detuned" by just about anything that comes near the circuit board or antenna. Tuning is also effected by the movement of battery wires and the battery. Therefore, it is recommended that the transmitter be placed in a small box and the circuit board, battery holder, battery wires, and antenna secured to the box so that nothing can move or slide about. Silicon rubber glue works well for this purpose. A small hole can be made in the box for tuning C5. The box need not be a standard project box but can be just about anything from a model rocket nose cone to a dog collar as long as it holds the circuit components securely.

Because of the tuning stability it is important to use a stiff wire for an antenna for example a steel guitar string. A length of from 18" to 27" can be used and some experimentation should be anticipated if optimum range and stability is desired. A good approach is to cut the antenna about 27.5" long , tune the transmitter, check the range then remove 1/4" of antenna, retune, and recheck the range. Repeat until the antenna is less than 18". Be careful to record the range and antenna length on each retuning. When completed, cut an antenna the length which worked best and install on the transmitter. Note that this technique works best when the radio receiver has the antenna disconnected or in the down position, so that the distance between the transmitter and receiver can be shortened. It is also helpful if your receiver has a "signal strength" meter, but it is not necessary.

The coil is 5 turns of 26 gauge magnet wire wound on the shaft of a 5/32" drill bit and then removed. The coils are spaced so that the total length is 3/16". The best way to make the coil is to cut a piece of magnet wire 1 inch long and clean the varnish 1/8" from each end before winding the wire around the shaft of the drill bit. The wire is stout enough to hold it's form when it is removed from the shaft of the drill bit. The coil requires less than an inch of magnet wire and can be easily salvaged from an old toy electric motor or transformer. Successful coils have been made from a variety of wire types including standard telephone wire.

Receiver sensitivity and tuning are important consideration when utilizing the transmitter. A receiver should be selected that can tune continuously across the radio frequency range. The new digital radios that jump from channel to channel will be hard to tune and prevent the use of the transmitter in the dead spaces between channels. Also, it is best to select a radio which can tune 1 MHz or so above the FM band because this is an area which is frequently free of commercial broadcasts. Of course, radios with antennas are a plus. The "walkman" type of radio which does not have an external antenna does not provide the range that a radio with an external antenna does. Radios with external directional high gain antennas are best and can improve the range many-fold.


More Suggestions (for model rocket and general use):


The Antenna:

Antenna length has an important relationship to transmitter range and also, to how easy it will be to store in the model rocket. The user will need to decide if the longer transmitting range of a long (properly tuned) antenna is worth the inconvienece of having to fold it into the rocket. It is the author's experience that a 6 inch antenna will give a range of 200 meters to a "standard" pocket FM receiver and will work effectively in locating rockets that drift a half mile down range. Almost always the recovery team will know the direction down range of the rocket and will not have any trouble coming within a hundred meters of the rocket. The problem of locating the rocket comes in the last 100 meters. A transmitter with a lot of power can confuse the search because the signal is so strong that it is hard to discern direction or strength. With a lower powered transmission, signal strength and direction are easier to determine even down to a distance of a few meters.

Because of the tuning stability, it is important to use a stiff wire for an antenna, for example a steel guitar string. A length of from 6" to 20" can be used and some experimentation should be anticipated if optimum range and stability is desired. A good but laborious approach is to cut the antenna about 20" long , tune the transmitter, check the range then remove 1/4" of antenna, retune, and recheck the range. Repeat until the antenna is less than 6". Be careful to record the range and antenna length on each retuning. When completed, cut an antenna the length which worked best and install on the transmitter. Note that this technique works best when the radio receiver has the antenna disconnected or in the down position, so that the distance between the transmitter and receiver can be shortened. It is also helpful if your receiver has a "signal strength" meter, but it is not necessary. Practical experience with this circuit indicates that an 18 inch or 6 inch antenna works best. Cut the antenna 1/4 inch longer to allow for insertion and soldering to the circuit board.

The transmission range can also be increased if the antenna has a small ball of solder attached to it's tip. This can be done by forming a small ball of solder on a small board or working surface and then piercing the molten ball of solder with the tip of the antenna. Hold the antenna steady until the solder ball hardens.

Maximum range can be achieved if a wire is attached to the negative battery terminal the same length of the antenna and extended in the opposite direction from the antenna.

Receivers (FM radios):

Receiver sensitivity and tuning are important considerations when utilizing the transmitter. A receiver should be selected that can tune continuously across the radio frequency range. The new digital radios that jump from channel to channel will be hard to tune and prevent the use of the transmitter in the dead spaces between channels. Also, it is best to select a radio which can tune 1 MHz or so above the FM band because this is an area which is frequently free of commercial broadcasts. Of course, radios with antennas are a plus. The "walkman" type of radio which does not have an external antenna does not provide the range that a radio with an external antenna does. Radios with external directional high gain antennas are best and can improve the range many-fold. Of course, one of the many models of scanners with less than one microvolt sensitivity fitted with an external FM directional antenna can improve the range over the cheap entertainment FM radio by a factor of 3 or 4.


Batteries:

The more voltage the greater the range and weight of the transmitter. The transmitter can be operated on from 6 volts to 12 volts by stacking 3 volt lithium batteries , 2 batteries equals 6 volts, 3 batteries equals 9 volts and 4 batteries equals 12 volts. The battery hold down clip will need to be properly sized to accommodate the number of batteries desired. If this is your first transmitter 2 batteries are recommended.

Make sure the batteries are securely attached to the circuit board. The most common failure of the transmitter is the inadvertent dislodging of the batteries during use. It is recommended that if the transmitter is to be used under conditions subject to jarring to tape over the batteries with electricians tape to insure that the batteries to not jar lose.


Tracking:

Most people are surprised to find that the telescoping dipole antenna found on most pocket FM receivers are directional. That is to say that the orientation of the antenna with respect to the transmitter will exhibit different gain. A little experimentation helps to determine the direction which has the most gain. Hang the transmitter on a tree limb and move about 50 to 100 meters. Hold the receiver and rotate it in several directions making note of the direction which produces the loudest beeping. Also, you might experiment by shielding the antenna with your body to see the degree of directional sensitivity.

The best directional antenna is a "beam" or yagi antenna. Most exterior TV antennas are of this type. A 3 element or more beam antenna connected to the properly matched input on a receiver will increase the gain by several orders of magnitude and provide optimal directionality.

If the transmitter is being used with the standard inexpensive pocket FM receiver, it will get much better range and direction if the jbGizmo FM antenna modification is used on the FM radio receiver. Instructions are included with these plans.







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jerry_baumeister@msn.com

Copyright ©2001, Jerry Baumeister
Revised - 6-23-01