Multipath means that the field intensity at a particular point is the sum of a number of waves that arrive from different directions or from different sources. It arises from signal transmission paths such as edge reflections from the mounting structure around an antenna and general reflections from objects near the antenna. Nearby reflections only seem to modify the antenna pattern, while reflections from additional objects cause rapid ripple with changing pattern angle. Multipath causes degraded system performance or measurement errors. Of course, multipath can improve performance as well. In fact, we add nearby objects, such as ground planes, to improve antenna performance.

We specify pattern response in terms of the power response, but we add fields. An extra signal −20 dB relative to the main signal is 0.01 in power but 0.1 in field strength (voltage). Since the extra signal can have any phase relative to the main signal, it can add or subtract. Given an extra signal MP(dB), the pattern ripple is

  (1)

where MP(dB) has a negative sign. Scale 1 gives the relationship between peakto- peak amplitude ripple and the level of the multipath signal. Equation (1) is numerically the same as the relationship between return loss and 20 log(VSWR). The multipath signal can change the phase when summed with the main signal over a range

given by

  (2)

Scale 2 calculates the peak phase error due to a multipath signal.

SCALE 1 Signal peak-to-peak amplitude ripple due to multipath signal.

SCALE 2 Peak phase error due to multipath signal.