EIRPs are achieved using antennas with gains ranging from around 20 dBi to 66 dBi.

Corresponding powers necessary to deliver to the antenna ranges from around 1 watt to several hundred watts. BUT maximum power capability of the transmitters in some earth stations is several hundred kilowatts. It is necessary to use the transmitters very inefficiently when they are amplifying more than one carrier

Losses in the networks required to combine the outputs of a number of transmitters.

Main types of PA in earth-stations are:

• Travelling wave tube (TWT) amplifiers
• Klystron amplifiers
• Solid-state (SS) power amplifiers

Travelling wave tube amplifiers (TWTA) have bandwidths of 500 MHz and more and powers from a few watts to many kilowatts.

Klystrons have bandwidths of 40 to 80 MHz and are tuneable over 500 MHz or more. Powers from several hundred watts to many kilowatts. Cheaper, easier to set up, operate and maintain than TWTAs.

Solid-State Power Amplifiers are comparatively cheap and reliable. Power is relatively limited compared with TWTAs and Klystrons.

The most important characteristics of HPAs, apart from frequency, power, bandwidth and linearity are:

• Gain: 
  The gain of TWTAs and klystrons ranges from about 35 to 50 dB and Intermediate Power Amplifiers (IPAs) are often needed between the modulators and power amplifiers.
• Variation of group delay with frequency: 
  This is another cause of intermodulation.
• AM/PM conversion: 
  This causes intelligible crosstalk and intermodulation noise.
• Noise and spurious outputs: 
  All amplifiers generate noise; the noise figure (NF) of microwave power amplifiers is usually about 30 dB but special low-noise tubes are available as pre-amplifiers; Pas may also generate spurious tones and may modulate output signals as a result of ripple on power supplies.