Theory
The solar power supply consists of a small 18V PV panel which charges a 1000uF electrolytic capacitor. The capacitor keeps the circuit running during brief interruptions of light, such as a bird flying over the PV panel. The 18V is regulated down to 9V with the 78L09 regulator IC to provide a steady 9V supply for the rest of the circuitry. With the PV panel shown above, the circuit will only work when direct sunlight is shining on the panel. A larger panel that can provide 22mA at 12V during cloudy conditions would extend the circuit's operating conditions.

The Electret microphone is biased with a 33K resistor, the resistor value can be changed to vary the amount of modulation and optimize the performance of specific microphones. The microphone signal is amplified by a 2N3904 audio amplifier. This signal is sent to the 2N2222A oscillator stage where it changes the oscillator's frequency (FM). The oscillator's operating frequency is set by L1, the 6pF capacitor and the 5-20pF variable capacitor. With L1 wound as specified on the schematic, the circuit will operate near the low end (88Mhz) of the FM broadcast band.

The output of the oscillator circuit is taken from a tap on the oscillator coil L1 and fed to the RF amplifier 2N2222A transistor. The output of the RF amp is run through a low pass PI filter to remove unwanted RF harmonics before the signal is sent to the antenna.
Specifications

Output Frequency: 88Mhz nominal, can cover 88-108Mhz with coil adjustments
Input voltage: 11-18VDC
Operating current: 22mA @18VDC
DC input to RF amp: 81mW
RF output power: 40mW (approx.)

Construction
The prototype circuit shown in the top photo was built using the "dead bug" construction method, it was laid out as the circuit was designed. A second-generation version of the circuit was built using a home-made printed circuit board, this is shown in the second photo. The frequency stability of the transmitter was greatly improved when it was built with the circuit board. Artwork for the PCB is available at the end of this page.

It important to mount the oscillator components solidly so that they don't move around and cause unwanted frequency shift. The component leads for all of the RF wiring should be kept short. The coils were wound on a #2 Philips screwdriver shaft and stretched out a bit. To improve the circuit's frequency stability, wind the oscillator coil on a 1/4" form, then heat the coil in an oven at to anneal the metal. A layer of polystyrene "Q dope" can be painted onto the coil to further improve the stability.

Another trick that will improve the transmitter's frequency stability is to build it into a metal box that is surrounded by an insulating material such as styrofoam or bubble-wrap. If the transmitter box is mounted in the shade, it will be less likely to change frequency due to solar heating and cloud shading.
Antennas
This circuit will work with a variety of antennas. An adequate short-range antenna can be as simple as a 1' to 2' wire connected directly to the circuit. A resonant antenna such as a tuned dipole or a vertical antenna will greatly extend the range of the transmitter.

A resonant half-wave diple antenna for 90Mhz can be made with two 2.6 foot pieces of wire fed in the middle, using the classic dipole formula: quarter wave length (feet) = 234 / frequency (Mhz). the PV panel and wiring should be kept away from the antenna, or in the case of a short whip antenna, the PV wiring can be run in the opposite direction as the antenna to act as the other half (counterpoise) of a dipole.
Alignment
The circuit can be aligned in the laboratory by putting 12V to 18V DC across the PV panel to power the regulator. Tune your receiver to a blank spot on the lower end of the FM band and adjust the frequency calibration trimmer until you hear the microphone signal. Turn the trimmer very slowly, alignment takes a light touch. Don't turn the receiver volume up too much or you will get audio feedback. A frequency counter may be useful for setting the output frequency. It may be necessary to retune the frequency a bit after the circuit has warmed up in the sun.

The output capacitor should be tuned for the maximum transmitted signal, this setting varies with different antennas. The best way to do this is to connect the antenna to the transmitter and monitor the signal with an oscilloscope (100 Mhz bandwidth) connected to a nearby antenna. Adjust the control for the highest signal. If you have a receiver with a signal strength indicator, that can also be used for monitoring the transmitter's output level. Adjustment of the output capacitor will pull the oscillator frequency a bit, it will be necessary to alternate between oscillator and output adjustments to fully align the circuit.
Use
Place the PV panel in the sun and tune your receiver to the bug's signal, listen to the world outdoors. An analog receiver is best for picking up the signal since, unlike a digital receiver, it can be fine tuned to track the signal. I use a 1970s vintage Pioneer receiver to good effect. Once the bug's temperature has stabilized, its frequency should not drift very much.

The microphone enclosure and placement can be tuned to optimize sound reception in a particular direction. A good directional microphone can be made by putting the mic element into one end of a short piece of PVC pipe. Inserting a thin tube of porous foam into the pipe can lower the resonant nature of the cylinder.
Parts

1X GM 684 60 mA 18V PV panel (available from Electronix Express) or equivalent
1X 78L09 voltage regulator IC
1X 1N4001 diode
1X 2N3904A transistor
2X 2N2222A transistors
1X 1000uF 25V electrolytic capacitor
1X Electret microphone
4X 100nF capacitors
2X 22nF capacitor
1X 1nF capacitor
1X 3pF silver mica capacitor
1X 6pF silver mica capacitor
1X 10pF silver mica capacitor
1X 20pF ceramic disk capacitor
1X 27pF ceramic disk capacitor
2X 5-20pF (or similar) miniature variable capacitor
1X six hole ferrite choke or equivalent
1X 100 ohm 1/4W resistor
1X 470 ohm 1/4W resistor
1X 10K 1/4W resistor
1X 20K 1/4W resistor
1X 33K 1/4W resistor
1X 47K 1/4W resistor
1X 1M 1/4W resistor
1X 1-3/4"x3" copper plated blank printed circuit board
1' length of #20 tinned copper hookup wire for making two coils
1X weatherproof plastic box (recommended)