Personal Radio Station

Fig. 1 shows a high quality radio transmitter for the A.M. broadcast band. Although the
schematic looks somewhat complex, the circuitry is easy to build and adjust. And unlike
simpler circuits, the frequency stays put and the fidelity is excellent. A simple output meter
confirms proper signal level and checks antenna tuning while "on the air". Add an audio
mixer, tape recorder, and perhaps a CD player and have a near-professional micro-power station.

Most values are not critical but a few choices must be made carefully for best results. The
output tank is tuned to the crystal frequency by selecting the values from the chart. For
example, for a 1 MHz transmitter, the chart indicates 500 pf and 35 uh. This chart assumes
that a 220 pf capacitor is already connected between the collector and base of the output
transistor as indicated in the schematic so the indicated capacitance is in addition to the 220
pf. A variable inductor or capacitor will allow the tank to be fine-tuned for the maximum
meter reading with no antenna connected (a few volts with a 10 megohm voltmeter or about
50 microamps with a current meter).
After the antenna is connected, the loading inductor in series with the antenna is selected for
the minimum meter reading (best antenna loading). (A 3 foot antenna will need about 820 uH
for a 1.6 MHz output frequency.) Longer antennas or higher frequencies need less inductance
and shorter antennas or lower frequencies will need more. The meter reading should drop by
more than half with a reasonably good antenna but the reading can be ignored if sufficient
transmit range is achieved. The antenna, which is short relative to the wavelength, is hard to
match well because it has a very low radiation resistance in series with a very small capacitor.
(The power dissipated in the radiation resistance is the power that is transmitted.) The loading
coil helps to resonate out some of the series capacity resulting in more antenna current and
thus more radiated power. Some retuning of the tank may be desirable when the loading coil value is changed.

A remote radio playing back through a baby monitor or walkie-talkie makes a good signal
quality monitor for antenna tuning and positioning.
The crystal can be practically any surplus crystal with a fundamental frequency between 530
kHz and 1.7 MHz in 10 kHz increments but the higher frequencies work best. Choose a
crystal frequency away from strong local stations at or above 800 kHz for best transmit range.
Proper operation of the oscillator may be verified by probing the junction of the two 1000 pf
capacitors with a high impedance oscilloscope probe connected to a scope or frequency counter.
Full modulation is achieved by applying about 2 volts peak-to-peak to the base of the current
source transistor in the differential amplifier. The modulation voltage varies the current in the
diff. amp. away from the nominal 20 ma. setpoint and this modulated current is converted to a
clean, high voltage sinewave by the output tuning circuit. The modulated signal may be
observed with an oscilloscope connected to the antenna terminal if desired.
Circuit construction is mostly non-critical but a few points should be observed. Ground-plane
is not mandatory but it helps control parasitic feedback elements when less than perfect layout
techniques are used. The two capacitors across the base-collector leads of the diff-amp
transistors should have short leads. Bypass the 15 volt supply well, perhaps with additional 1 uf capacitors not shown in the schematic.
The 100 ohm emitter resistor in the modulator may be bypassed with a 22 ohm resistor in series with a 470 uf capacitor to increase the modulation sensitivity to about 1 volt peak-to-peak which is typical of many sources.
Eliminating the 22 ohm resistor will increase sensitivity to under 100 mv but the linearity will
suffer somewhat.
An amplifying audio mixer may be added as shown in fig. 2 if more than one audio source is
to be used. The gain resistor might be near 2.8k for typical 300 mv sources or considerably
higher for lower level sources. If the signal level is different for each source then vary the 600
ohm resistors to compensate. A larger resistor will reduce the gain. Set the main gain resistor
for the weakest source then increase the 600 ohm resistors in the other channels for the proper
balance. A fancy mixer panel could be constructed with potentiometers in place of the
resistors. Remember that some op-amps are not sufficiently fast to amplify high fidelity audio.
For simplicity, choose an internally-compensated audio op-amp such as the LM833. Since the
LM833 is a dual op-amp the second amp could be used as a separate pre-amp for a
microphone or other low-level sources using the same schematic as the mixer. The output of
this amp simply feeds one of the mixer source inputs.

Applications:

A continuous-loop tape could give sales information to passing cars. Place a sign that says,
"tune to xxxAM for information," next to the house or car that is for sale.
Transmit special seasonal music at Christmas or Halloween to enhance your decorations. (Use
a similar sign.) Transmit a cassette player or other audio source to the car radio for better sound.
Make a pair of toy AM band two-way radios by adding inexpensive AM radios. Or talk
between cars on a trip using the car radio for reception.
Make a baby monitor that works with any AM receiver.
Transmit control tones to a number of cheap AM receivers for unusual remote control applications.
Build a fully functional radio station for the kids - complete with vu meters, slide faders, and
an "on the air" light.
Besides making a nice general purpose radio transmitter the Personal Radio Station is suitable
for some nice practical jokes:
Hide the transmitter with a cassette tape player in your personal effects as you ride in the back
seat of a friend's car. (Leave out the meter circuit to keep the size down.) Ask your friend to
tune in that new radio station - since your transmitter is crystal controlled it will be at the right
place on the dial. What your victim hears is up to you. The circuit will work reasonably well
with a single 9 volt battery instead of 15 volts.
How about a less than desirable school lunch menu for the kids. Or, if you are younger, an
unexpected school closing for the day. (I didn't really suggest that one, did I?)
A news announcement of your marriage proposal will get results. Local news personalities
will probably be delighted to help make a tape.

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