logTsys, = (9.18) where G is the net antenna gain up to an arbitrary reference point or reference plane in the downlink receive chain (for an earth station). Conventionally, in commercial practice the reference plane is taken at the input of the low-noise amplifier (LNA). Thus G is simply the gross gain of the antenna minus all losses up to the LNA. These losses include feed loss, waveguide loss, bandpass filter loss, and, where applicable, directional coupler loss, waveguide switch insertion loss, radome loss, and transition losses. Tsys is the effective noise temperature of the receiving system and Tsys = Tant + Treevr. (9.19) Tant or the antenna noise temperature includes all noise-generating components up to the reference plane. The reference plane is a dividing line between the antenna noise compo- nent and the actual receiver noise component (Trecvr). The antenna noise sources include sky noise (Tsky) plus the thermal noise generated by ohmic losses created by all devices inserted into the system such as waveguide, bandpass filter, and radome. Trecvr is the actual receiver noise temperature, which has equivalence to the receiver noise figure. A typical earth station receiving system is illustrated in Figure 9.20 for a 12-GHz down- link. Earth stations generally have minimum elevation angles. At 4 GHz the minimum 14 We called this path analysis in LOS microwave. CONCEPTS IN TRANSMISSION TRANSPORT Antenna elevation angle = 10°, clear sky Feed loss = 0.1 dB Directional coupler loss = 0.2 dB W/G loss = 2.0 dB BP Filter loss = 0.4 dB Reference plane Frequency = 12 GHz Figure 9.20 Model for an earth station receiving system. elevation angle is 5°; at 12 GHz, 10°. The elevation angle is that angle measured from the horizon (0°) to the antenna main beam when pointed at the satellite. Take note that we are working with noise temperatures here. Noise temperature is another way of expressing thermal noise levels of a radio system subsystem or component. In Section 9.2 we used noise figure for this function. Noise figu