Q2Satellite Communication
Question
Q.2. For an earth station transmitter with an output power of 40 dBW (10000 W), a back off loss of 3 dB, a total branching and feeder loss of 3 dB and a transmit antenna gain of 40 dB, determine the EIRP.
Answer
For an earth station transmitter with 40 dBW output power, 3 dB back-off loss, 3 dB branching/feeder loss and 40 dB antenna gain, the EIRP is calculated as 40-3-3+40 = 74 dBW.
Given: output power Pout = 40 dBW (=10,000 W), back-off loss = 3 dB, branching and feeder loss = 3 dB, transmit antenna gain = 40 dB.
Effective Isotropically Radiated Power (EIRP) is calculated by starting from the amplifier's rated output power, subtracting all losses between the amplifier output and the antenna aperture, and adding the antenna gain:
Result: EIRP = 74 dBW, equivalent to 10^(74/10) = 25,118,864 W ≈ 25.12 MW. This EIRP value represents the effective power that would need to be radiated by a hypothetical isotropic (omnidirectional) antenna to produce the same power flux density at the receiving satellite as this actual directional transmit system produces — it is the key transmit-side quantity used in the satellite link budget equation to compute the received carrier power at the satellite.
Physical significance of each loss term: the back-off loss represents the deliberate reduction in HPA drive level below its rated saturated output, applied specifically to keep the amplifier operating in a more linear region of its transfer characteristic and thereby limit intermodulation distortion when multiple carriers are amplified together (as in FDMA operation) — this is a designed operational trade-off between maximum available power and signal quality, not an unavoidable physical loss. The branching and feeder loss represents the real resistive/insertion loss in the waveguide runs, combiners/splitters, and other passive RF components between the HPA output and the antenna feed, and must be minimized through careful RF system design (short waveguide runs, low-loss components) since every dB lost here directly and permanently reduces the achievable EIRP for a given HPA rating. Because EIRP appears directly (linearly, in dB terms) in the satellite link budget's received carrier power calculation, this 74 dBW figure would be combined with the free-space path loss and the satellite's own receive antenna gain and system noise temperature to determine whether the resulting carrier-to-noise ratio meets the minimum threshold required by the chosen modulation and coding scheme for reliable communication.