I built this very simple SAW transmitter, with a simple "spring" antenna for the right frequency. To test it, I tuned a portable receiver to around 433.92Mhz and fine tuned it until I heard silence. But I noticed that when tapped the board, the antenna would vibrate and I could hear the "boing!" in the receiver.
Why does this happen?
The frequency of this circuit is primarily controlled by the SAW resonator which indeed operates on the piezo-electric effect. However, "acoustic" in "SAW" is somewhat of a misnomer, since the "acoustic" waves in question are at radio frequency. In the industry, these "acoustic" frequencies are considered "ultrasonic". That resonator is fairly insensitive to audio frequency vibration, but the coil antenna is extremely sensitive. The modulation you hear is most likely from the spring antenna as you originally assumed. Change in its inductance has a secondary influence on the circuit's frequency and/or amplitude. I suggest a simple experiment to confirm: Anchor the PCB to something solid, stable, and non-conductive such as wood. Find a nylon or wood rod or other object with similar properties. Tap the antenna and then tap the SAW resonator with that rod. Observe which location causes the greatest modulation. Another experiment can be conducted by securing the coil antenna to a similar rod with RTV. Now observe if that reduces the modulation you observed. This modulation is not usually an issue with most applications of this circuit since it is most often used for simple remote controls. IF this modulation is a problem in an audio application, design the antenna into the PCB so it is less sensitive to audible acoustic vibration.
