Very cheap Ebay FM spy microphone equipment listening device tested and reviewed for range. Really surprised by how well these small listening spy equipment devices work.

Ebay link.

Frequency 85-115MHz FM transmitter module wireless microphone surveillance Black

Operating Voltage: DC 2V- 9V
Current: 9mA (3V voltage )
Reference distance: open ground 100 meters, 10-30 meters indoor environmental obstacles (3v voltage test, increasing the voltage from the greatly increased)
Operating frequency: 85MHZ — 115MHZ (usually between 99-101 boot default frequency)
Receive way: as long as the 88-108MHz FM radio can receive, with FM feature phones.
Frequency adjustment: adjustable capacitor
Output Impedance: 50 ohms
Size:2 cm * 1cm (L*W)

Package Included:
1PC*FM transmitter module wireless microphone Surveillance frequency 88-108MHz