# How do I compute fade margin and reliability for a passive repeater link?

You must first determine if the passive repeater is in the “near-field” of one of the terminal antennas.

The calculated field factor needed for this determination is defined as 1/k where:

 1/k = Field factor (unitless) d = Distance from the near terminal antenna to the passive reflector in feet A = Passive reflector area in square feet l = Wavelength in feet a = One – half the horizontal angle included between the two terminals in degrees

If the field factor (1/k) is 2.5 or less, then the passive is in the near field of the terminal. If 1/k is greater than 2.5, the passive is in the far field.

If the antenna is in the near-field of the passive repeater, the entire circuit encompassing the antenna and passive may be considered as an antenna system. An example of this is the typical “fly-swatter passive” with a reflector (acting as a mirror in a periscope fashion) mounted at 45° to the ground at the top of a tower illuminated by an antenna near ground level looking directly up. In this case it is necessary to determine the gain of that system. The gain is proportional to the projected area of the passive repeater and the efficiency of illumination, and is usually greater than that of the illuminating antenna. A book entitled “Passive Repeater Engineering” showing the equations is available by clicking Passive Repeater Engineering.

To use the Microwave Link Module, you consider the system as a single path with the path length equal to the distance from the far terminal to the passive repeater. The antenna gain at the passive end is equal to that of the antenna/passive system gain. A detailed methodology for this calculation can be found in the above referenced. All other program entries are the same as those of a normal path.

If the passive is in the far-field of both terminals, it is necessary to assume that there are two independent paths. The gain of the passive is again dependent upon the projected area of the passive and the illumination efficiency.

The formula for the gain in dBi is: Using the Microwave Link Module, this case is treated as two paths. At the passive repeater end of each path, the gain used is that determined from the above equation. There are no additional losses at the passive end of the circuits since no waveguide or other transmission elements are involved. The transmitted power (EIRP) used for the second path (passive to far end) is equal to the “Received Signal Level” determined in the calculation for the first path (near end to passive). All other parameters entered are those you would normally enter for a two link system.