The Transmitter Output Power is defined as the power produced in to a 50 Ohm load connected at the radio antenna port.
In transmit mode, the Synthesizer produces the desired Tx frequency with the FM modulation (speech or data) super-imposed. This part is called the Exciter. The transmitter then amplifies this signal up to the final power level to be transmitted, after which the signal passes through the Tx/Rx Switch before being filtered to remove any unwanted harmonics. Ideally then, all that is transmitted is the desired signal at the required power level.
A simplified block diagram of a typical Tait Mobile Transmitter is shown below:
The transmitter normally consists of three stages of amplification, known as the Pre-Driver, Driver and Final, although each of these stages may contain more than one active device.
The Pre-Driver is the first stage of the transmitter, and this amplifies the Tx Drive signal from the Synthesizer up from +5dBm to around +20dBm (100mW). This is followed by the Driver stage which takes the output from the Pre-Driver and further amplifies it up to a level of around +33dBm (2W). This signal is then passed to the Final, which amplifies the wanted signal up to a level of around +45dBm (32W). Between the Final and the antenna, we have the Tx / Rx Switch and the Harmonic Filter, which between them reduce the RF signal level by around 1dB, so the final transmitted signal level is +44dBm or 25W.
A related parameter you may hear mentioned is Effective Radiated Power (ERP). As opposed to the power produced into a 50 Ohm load connected at the antenna port, this relates to the power actually radiated when the antenna is connected. Effectively, ERP, is the Transmitter Output Power multiplied by the gain or loss of the antenna system.