The ImmersionRC RF Power Meter is a versatile compact device and a novelty in 2018 for transmitter-power measurement.
For comparative measurements and approximate values, it measures only by moving the Power Meter and the signal source together. For accurate measurement, the Power Meter is connected directly to the SMA connector of the transmitter.
The ImmersionRC RF Power Meter provides fast pre-flight control of powered devices, helps diagnose problems, compare the strengths of different antenna signals, and captivates women according to the official site . And no wonder, the device is calibrated to the frequency: 35; 72; 433; 1200; 2400; 5600-6000MHz (switchable at 50MHz), so you can also measure RC devices with it.
Founder of ImmersionRC Tony Cake and Joshua Bardwell introduce the ImmersionRC RF Power Meter - https://www.youtube.com/watch?v=MQu7SRDOab8
Official manufacturer's manual here .