Transmit-receive switching circuit for radio frequency circulators

Apparatus for switching a receiver and a transmitter to a common antenna comprising:

• combining means, electrically coupled to said receiver and said transmitter, for providing signals from said transmitter to a common signal path and for providing signals from said common path to said receiver in a manner maintaining isolation between said receiver and said transmitter;
• a circulator exhibiting a predetermined characteristic impedance and including first, second and third ports, said first and second ports being electrically coupled to said common signal path and said common antenna, respectively, and
• selectively actuable impedance means, electrically coupled to said third port, for exhibiting an impedance approximately equal to the characteristic impedance of said circulator when said transmitter is transmitting and exhibiting an impedance substantially less than said characteristic impedance when said transmitter ceases transmitting.