Radio transmitters were shortwave communications devices.
The Third Doctor altered a normal radio so that it could be a transmitter instead of a receiver. (TV: The Sea Devils)
Ivo used a radio transmitter to contact the rebels, despite the Three Who Rule's ban on advanced technology. He hid it in a podium. (TV: State of Decay)
Keillor used a radio transmitter to contact Gavrok and inform him of Delta's location. Gavrok used a high impulse beam locked onto the signal to detonate the transmitter so he would not have to pay the informer. (TV: Delta and the Bannermen)
The Brigadier contacted his base for orders with an army-type radio transmitter. (TV: Battlefield)
Research stations used radio waves for tests and often transmitted them into space to search for signs of intelligent life. In the 1970s, the Nestenes tried to materialise on Earth through radio waves at one station, but they were stopped by the Third Doctor and the Master. (TV: Terror of the Autons)
While in Daniel Eldred's rocket, the Second Doctor, Zoe Heriot and Jamie McCrimmon used radios to contact Earth, the Moon and one another. (TV: The Seeds of Death)