Instant-on radars are not exactly instantaneous, some turn-on or warm-up time is required. In theory it is possible to transmit a steady stable signal in a few milliseconds (0.001 seconds) by leaving the transmitter on (no transmitter warm-up time) and dumping the signal into a dummy load until ready to transmit through the antenna. This technique requires a "make before break" switch (typical switching time 1 millisecond or so) to prevent microwave power from reflecting back to the transmitter causing damage. In practice (to save space, components, power, and cost) traffic radars turn the transmitter off (cut power from transmitter) until ready to transmit. This technique requires longer turn-on time to achieve a steady stable signal, typically less than a half second but can be as high as 2 seconds.
I found this information. this makes sense why my brief Ka signals come up on Tx State troopers 1-2 bleeps of Ka www.copradar.com/revised