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18th Apr 2017, 12:38
sargs

Join Date: Feb 2008
Location: Not quite where I'd like to be
Age: 61
Posts: 101
Gouli:
The radar transmitter is pumping out pulses of energy of up to 1,000 watts. The receiver is designed to respond to returning echo signals measured in micro watts, a millionth of a watt. Even if the transmit/receive protection circuits work in around a microsecond, you will see nothing within a 300 metre range. If they take three microseconds to switch over, you are blind to everything within a kilometre. We are talking millionths of a second here to switch from 1000 watts out to a few millionths of a watt coming back! The technology is well proven, but transmit/receive delays increase as equipment ages, so close in blind spots get bigger. A crew may actually perceive the radar equipment as working better as it ages because of an apparent reduction in close in sea clutter.
As an ex-SAR radar operator I've tried to stay out of this debate - I used to operate a search radar, not a weather radar, but the statement above was a step too far. Minimum Range (Rmin) is a function of pulse width, or pulse duration (interchangeable terms), caused by a pulse requiring time to be transmitted, hit something and return. For example, with a radar with a pulse duration of 1 microsecond the pulse leading edge can travel out for 0.5 microsecond (at the speed of light, and when the receiver is effectively 'blind'), hit something, and travel back to the radar in the following 0.5 microsecond. Using the equation, Rmin = C x PD/2, if C = 300,000,000 m/s (approx) then Rmin in this case is 150 m. As Crab says, the Sea King radar had an Rmin of 75m due to its PD of 0.5 microseconds.

There usually aren't any complex "transmit/receive protection circuits" in the manner suggested as they are usually simple isolators (with no moving parts) that allow microwave energy to pass in one direction but not the other. You are mixing up circuits within the receiver which are designed to prevent very large targets swamping smaller ones by raising the receiver Automatic Gain Control too high.

"Transmit/receive delays increase as equipment ages?" That's just wrong.

Finally, I never perceived the apparent lack of information on my radar screen as an indication it was "working better."

Last edited by sargs; 18th Apr 2017 at 13:21. Reason: Mixing my MURs with my MINs...