RF power amplifiers and preventing damage when the antenna is disconnected
The best way to protect against a disconnected or malfunctioning antenna system is an SWR feedback circuit where the power is reduced when a high SWR is detected. Some amplifiers will have a maximum SWR listed in the data sheet which will give you some idea of how likely they are to be destroyed with a high SWR. 

SWR or Standing Wave Ratio (indirectly) measures the amount of power reflected back towards the transmitter relative to the amount of power produced in the transmitter (it really measures the ratio between the max ac voltage along the transmission line and the minimum ac voltage). Power being reflected towards the transmitter can be damaging because it can cause the output transistors to exceed their voltage or current ratings. Power is reflected any time there is an impedance mismatch between the transmitter (usually 50 ohms), the feedline (50 ohms) and the antenna (ideally 50 ohms, can be anywhere between 1-2 ohms to several kilohms depending on the antenna, or 0 and infinite ohms if the feedline is shorted or no antenna is connected).