The size of the transmitter designed by tomtechtod is almost as small as the 337 button battery that supplies it. The board of the mini PCB covers a large number of patch components, most of which are packaged in 0201. Even so, the transmitter is a simple design with two transistor audio levels that amplify signals from MEMS microphones and feed in oscillators using surface acoustic (SAW) resonators to maintain stability. The transmitter has adjusted the frequency band of 433 MHz low-power equipment. From the video below, it seems to have a good frequency coverage with a random long line antenna, which may be transmitted by 50m. Tomtechtod has released all the build files, including gerbers and BOM with DigiKey part number, so it should be easy to make one for your Fieldcraft suite.

Working at 433MHz, it is very stable with SAW resonator and uses a single 1.5V button battery!