Speech transmission by microwave:description
The VSWR meter is a voltage amplifier tuned at a center frequency of 1 KHz.
The gain can be varied over a wide range (using range and coarse/fine gain control).
The microphone output is connected to the meter input (BNC(F)).
An amplified input is available in the meter back panel (BNC(F)).
Although the spectrum of speech extends from 20 Hz to 20 KHz,
sufficient power exists around 1 KHz to make the tuned amplifier output understandable.
The reflex klystron power supply has a "MODULATION" switch which is set to external (EXT);
the amplified speech which is the modulating signal is connected to the "MOD.EXT" input connector (BNC(F))
and the beam and repeller voltages are appropriately set for high microwave power output
(the ac modulating voltage signal is added to the negative dc repeller voltage resulting in
amplitude modulation of the microwave carrier).
The modulated carrier is transmitted by the transmitting antenna.
The parabolic antenna has a narrow beam width which results in a longer link length.
A horn antenna can also be used as the transmitting antenna.
At some distance, an aligned receiving horn antenna is connected to a detector which is tuned for maximum output.
The crystal detector diode has a square law characteristic which demodulates the microwave signal from the horn antenna.
The demodulated speech signal from the detector is connected to the input of a VSWR meter using a coaxial cable.
Using appropriate gain, the amplified speech from the meter back panel is connected to a speaker.
Since the impedance of a speaker is low, a direct connection results in loss of gain and low volume audio output.
By connecting the meter to the primary of a transformer and the speaker to the secondary,
the reflected speaker impedance can be enhanced resulting in a high volume audio output.
We have satisfactorily implemented this system with a measured carrier frequency of 9.3125 GHz.