Antenna Gain
The antenna gain of an aperture antenna is given by the standard equations
G=10log [η4πA/λ2]
Where:
η is the antenna efficiency, typically 0.5 for millimetric frequencies (Papageorgiou, 2012)
λ is the wavelength in air, 3.9 mm at 76.5 GHz
A is the aperture area
The antenna diameter of a typical infrastructure radar is 150 mm, thus the maximum theoretical gain achievable is ~ 38.5 dBi. However, the beam is spread in the vertical dimension with a secondary antenna, thereby reducing the gain. To analyse the actual antenna peak gain an empirical measurement is required.
The EIRP of a typical infrastruture has been measured to be, in the worst case, 37.5 dBm at a particular instance, or 14.9 dBm EIRP if account is made for the fact that the antenna is rotating0. The EIRP is linked with the antenna gain G and the transmitted power Pt with the well-known relation
EIRP = Pt + G – L
Where L are the sensor losses, which can be neglected or of the order of few dB. Knowing the transmitted power of 10 mW (10 dBm) it is possible to derive the overall gain of 27.5 dBi at the radar boresight.
The antenna gain off the boresight is then reduced according to the beam profile in horizontal and vertical dimension.
Beam profile
The typical infrastructure radar has a spread vertical beam and a horizontal 2 degrees beamwidth. The beam profile is shown in Figure 29.
Figure 29 Infrastructure radar combined antenna beam. Note that the antenna is mechanically scanned through 360 degrees
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