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Radar receiver antenna leakage

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Salvador12

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I ask this mostly about the small radar "detectors" receiver units used in cars for police radar but I suppose this also applied to larger radars.
Is it possible to have a radar receiver that doesn't leak it';s own Local Oscillator frequency at all?

I understand that in heterodyning you always have to use a local oscillator and then to mix the output of it with the incoming radio wave in order to get some IF frequency which can then be amplified and the output checked whether it is in the known radar bands to give a result (beeping etc)

Is it true that in the cheapest radar receivers they simply use the antenna itself as the mixer? They just dump the LO output to the antenna and then let the incoming radio wave and the LO driven signal overlay within the antenna?

I see from block schemes that in the more expensive ones they use a separate mixer and before the mixer /right after the antenna is a separate RF amplifier, does this RF amplifier work like a buffer so that any leakage from the mixer can't simply end up int he antenna ?
 

Is it true that in the cheapest radar receivers they simply use the antenna itself as the mixer? They just dump the LO output to the antenna and then let the incoming radio wave and the LO driven signal overlay within the antenna?
The antenna is not the mixer, the LO and input are combined at the antenna port The local oscillator is allowed to leak out of the antenna.

I see from block schemes that in the more expensive ones they use a separate mixer and before the mixer /right after the antenna is a separate RF amplifier, does this RF amplifier work like a buffer so that any leakage from the mixer can't simply end up int he antenna ?
Yes the amplifier increases the sensitivity and reduces the LO leakage, it does not eliminate it the reverse isolation is only a few tens of dB.

Is it possible to have a radar receiver that doesn't leak it';s own Local Oscillator frequency at all?
Yes, some military radar waring receives have non-existent LO leakage, they use wide band logarithmic detectors. Reducing the LO leakage to very low levels is too complex and expensive for the type of receiver you are thinking of. it needs significant filtering and isolation.
 

The antenna is not the mixer, the LO and input are combined at the antenna port The local oscillator is allowed to leak out of the antenna.
But isn't that the same as I said? If they are connected together at the antenna port then it is essentially connecting the LO to the antenna, and then the antenna radiates just like any other RF source would be connected to an antenna is it not the same principle?

Yes the amplifier increases the sensitivity and reduces the LO leakage, it does not eliminate it the reverse isolation is only a few tens of dB.
Do you mean that as few tens of a single db, or tens of db as in "10" or "20" db?
 

But isn't that the same as I said? If they are connected together at the antenna port then it is essentially connecting the LO to the antenna, and then the antenna radiates just like any other RF source would be connected to an antenna is it not the same principle?
A mixer requires a nonlinear function, the antenna should be linear, the circuit is acting as a combiner the mixing function will take place in the mixer circuit what ever that may be, FET diode or even in some low performance circuits the VCO.


Do you mean that as few tens of a single db, or tens of db as in "10" or "20" db?
10 to 20dB, usually the gain of the amplifier plus a few dB. This depends on the reverse isolation of the amplifier and any screening around it. Getting good screening can be difficult in the 10+GHz region.
 

sure, it is all possible. If you made a radar receiver with TWO Local oscillators, say with a 1st IF at 30 ghz, and a 2nd IF at say 2 GHz, the LO frequencies would be way off from where someone would go looking for it.

You could get creative, and have a microwave/optical mixing device,, where the LO frequency is only from pulsing light, which will not leak out a microwave antenna.

You could add high quality (probably) microwave waveguide bandpass filters that would drop the LO leakage by 80 dB. that with some backward isolation in a front end LNA might get you to the -110 dBm sort of leakage level, which would be hard to detect.

You could use a subharmonic mixer, where the LO frequency is 1/2 of where it needs to be. the mixer has diodes set up to effectively double that frequency when being used as a downconverting mixer. So it there is any LO leakage, it is at 1/2 the expected frequency. also would be easy to filter at the front end.
 

sure, it is all possible. If you made a radar receiver with TWO Local oscillators, say with a 1st IF at 30 ghz, and a 2nd IF at say 2 GHz, the LO frequencies would be way off from where someone would go looking for it.
So it's essentially a heterodyne but the first mixing stage is very close or at the received incoming frequency therefore if leaks out the antenna cannot be detected?
But isn't such a two oscillator approach a hybrid of heterodyne and homodyne aka direct receiver?
The first IF at 30ghz would essentially be a LO operating at input frequency as is done in homodyne/direct receiver schemes , how could you then take that output and use it for a heterodyne type mixer where two vastly different frequencies are combined in a mixer?

You could get creative, and have a microwave/optical mixing device,, where the LO frequency is only from pulsing light, which will not leak out a microwave antenna.
This would be a clever approach indeed, I wonder whether it is done somewhere

You could use a subharmonic mixer, where the LO frequency is 1/2 of where it needs to be. the mixer has diodes set up to effectively double that frequency when being used as a downconverting mixer. So it there is any LO leakage, it is at 1/2 the expected frequency. also would be easy to filter at the front end.
How does this work? What is the point of having a LO at 1/2 the needed frequency when you need to upconvert that frequency before introducing it to the mixer anyway?
 

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