PURPOSE: To switch a frequency optimum for ionospheric communication without using human labor in the case of transmitting radio waves by using an ionosphere.

CONSTITUTION: A high frequency signal from an oscillator 1 to generate the high frequency is turned to a higher harmonic wave by a higher harmonic wave generator 2. This higher harmonic wave is turned to one frequency signal by a filter 4 equipped with a voltage variable capacity diode 3. The power of this frequency signal is amplified by a power amplifier 5, and the signal is switched to a transmitting state by an antenna switcher 6 and supplied to an antenna 7. The radio waves reflected by the ionosphere are received by the antenna 7, switched to a receiving state by the antenna switcher 6 and supplied to a receiver 8. The signals demodulated by the receiver 8 are impressed to a peak detector 9. The peak detector 9 detects the maximum value of the demodulated signal. This maximum value is converted to a control voltage by a controller 10 and supplied to the voltage variable capacity diode 3. Thus, since the frequency optimum for ionospheric reflection is selected by the filter 4 and supplied to the power amplifier 5, the optimum frequency can be automatically switched.