I have difficulties to understand how the below circuit can detect radar waves by only using C1
and the lengths of its wires to IC1
. Can this work? And if yes, how? What is the physics theory behind this?
This circuit uses a 1458 dual op-amp to form a radar detector. C1 is the detector of the radar signal. The first op-amp forms a current-to-voltage converter and the second op-amp buffers the output to drive the piezo transducer. R5 sets the switching threshold of the second op-amp; normally it is adjusted so that the circuit barely triggers on background noise, then it's backed off a bit. The response of the circuit may be tuned by adjusting the length of the leads on C1. For typical road-radar systems, the input capacitor's leads should be about 0.5 to 0.6 inches long.
("Radio-Electronics" Magazine, Jul,86 issue (C) Copyright Gernsback Publications, Inc., 1986)
Best Answer
This circuit supposedly detects radar speed traps passively. Just in case there is any confusion, it isn't part of a radar system - it's a detector that can sense the presense of (say) hand-held speed guns used by some police forces.
It "works" because at high frequencies the LM1458 op-amp is quite susceptible to EMI and this gets rectified at the input stages and a dc offset is created. It's a bit flakey in performance and was aimed at the DIYer so it isn't really anything reliable but I suppose some were made back then and may have delivered functionality.
The length of the tracks/wires that is formed in the C1-loop forms a resonant tuned circuit supposedly. Links to this circuit can be uncovered by googling "1458 radar detector" and there does appear to be a youtube video.
Here's an article from Analog Devices (MT-096 RFI Rectification Concepts) that explains the phenomena.