The Frequency Meter BC-221-AK is a precision instrument designed to measure or radiate radio frequency impulses between 125 and 20,000 kHz. It is portable and completely self-contained instrument used mainly to calibrate field radio receivers and transmitters by direct comparison with the calibrated variable frequency oscillator of the frequency meter. The control panel itself is covered when not in use by a hinged lid that contains the units Calibration Book.
Since it is completely portable and self-contained, iy was a workhorse for adjusting aircraft radio transmitters and radio receivers and transmitter on the field during wwII.
Each meter contains a 1,000 kc crystal oscillator circuit with is used both as a source of accurate radio frequency signals at multiples of 1,000 kHz and for calibrating the variable frequency oscillator of the instrument at a number of points. These points of frequency calibration are called "Crystal Check Points". Each BC-221 was individually calibrated and the calibration book marked with the serial number of the unit.
Adjusting aircraft radio transmitters and radio receivers and transmitter on the field.