10. A tranmitter/receiver as claimed in claim 5, further comprising means for rectifying the output signal of said oscillator, and a pair of subtraction circuits fed one from each mixer and from said rectifier means, said subtraction circuits being arranged to feed said demodulator with signals from which oscillator noise has been subtracted.
5. A transmitter/receiver as claimed in claim 2, wherein said aerial means comprises a transmitter/receiver aerial.