Radar Basics

Transmit/receive leakage with a continuous wave radar

For continuous wave radar modules in free ISM frequency bands, an EIRP is specified by law, which cannot be exceeded. However, the power provided by the transmitter for this purpose is reduced by losses. To achieve maximum range, these losses should be considered when calculating the maximum permitted transmit power. These losses are shown in Figure 1.

The power at the receiver input can be calculated ignoring possible multiple reflections with the following equation:

(1)

• P_t = power out of transmitter
• P_l = power leaked directly from transmitter to receiver
• P_r = power scattered by target and collected by the antenna
• |Γ| = magnitude of the antenna voltage reflection coefficient (related to its VSWR)
• L_x = transmission line loss (≥1)
• L_c = circulator loss (≥1)
• I_circ = circulator isolation (fraction of incident power leaked in the reverse direction)

target return

Isolation

Antenna
mismatch

The echo signal is possibly modified by Doppler frequency and delay time. The leakage of the isolation and the reflections from the mismatch of the antennas, on the other hand, have the same signal shape and can be made to be in phase opposition by suitable selection of the length of the feed line so that they cancel each other if not outright, but at least reduce each other.