We illustrate communication system design and path loss by considering a sample link budget example. The 5-m-diameter reflector is pointing at a satellite in an orbit 370 km above the Earth with a telemetry antenna radiating 10W at 2.2 GHz. Since the antenna pattern has to cover the visible Earth, its performance is compromised. Considering the orbit geometry and antenna pointing is beyond the scope of this discussion. The range from a satellite at 370 km to a ground station pointing at  is 1720 km. The satellite antenna pointing angle from nadir is , and a typical antenna for this application would have gain = −2 dBiC (RHC gain relative to an isotropic antenna) and an axial ratio of 6 dB. Assume that the ground station antenna has a 2-dB axial ratio. We apply the nomograph of Figure 1 to read the maximum polarization loss of 0.85 dB since we cannot control the orientation of the polarization ellipses. The link budget needs to show margin in the system, so we take worst-case numbers. When we apply Eq. (1):

for path loss, we leave out the antenna gains and add them as separate terms in the link budget (Table 1):

FIGURE 1  Maximum and minimum polarization loss. (After A. C. Ludwig, A simple graph for determining polarization loss, Microwave Journal, vol. 19, no. 9, September 1976, p. 63.)

The link budget shows a 4.4-dB margin, which says that the communication link will be closed. This link budget is only one possible accounting scheme of the system parameters. Everyone who writes out a link budget will separate the parameters differently. This budget shows typical elements.

Radar systems have similar link budgets or detection budgets that consider S/N:

The radar has a required S/N value to enable it to process the information required, which leads to the maximum range equation:

  (2)

Equation (2) clearly shows the role of the transmitter, EIRP; the receiver and antenna noise; G/T ; and the requirement for signal quality, , on the radar range for a given target size σ.

Equation (2) applies to CW radar, whereas most radars use pulses. We increase radar performance by adding many pulses.We ignore the aspects of pulse train encoding that allow coherent addition. Radar range is determined by the total energy contained in the pulses summed. We replace EIRP with  (energy) since . It is the total energy that illuminates the target that determines the maximum detection range. Using antennas in radar leads to speaking of the radiated energy correct for pulsed systems, but when we do not integrate pulse shape times time, the antenna radiates power. To be correct we should call radiation that we integrate over angular space to find power, “power density.” To say “energy radiated in the sidelobes” is poor physics unless it is a radar system, because it is power.

TABLE 1  Link Budget