jim weir diagnosed a similar problem in my radio.
the connector between the coax termination and the external aerial had become slightly corroded increasing the impedance.
increased impedance prevented the signal getting out into the antenna wire.
evidently it was radiating inside the aircraft and getting picked up by the mike boom cable, fed back into the first transistor in the radio and causing a horrendous squeal.
took the antenna off, cleaned off all the corrosion with brasso and screwed back together.
needs to be redone about every 5 years.
I seemed to receive ok but couldnt transmit.