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signal processing S21

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quocvietltbu

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Hi everyone
I'm having a problem processing as follows:
I want to measure distance using radio waves .I use two antennas transmit and receive signals. signal transmitted to the obstructions
signal is measured parameters S21, it is the ratio obtained in wave antenna and the transmitting antenna emit waves. S21 parameter depends on the phase of the incident wave and the wave of the transmission time and frequency
signal S21 obtained a matrix of values ​​of the real and imaginary part of it corresponding to each frequency
so now you want to measure the distance d that is how the signal S21 obtained
some guidelines are modified IFFT and d = nλ + x
thanks a lot
 

albbg

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The procedure could be:

1) calibrate the instrument connecting port1 with port2, mode: transmission. You can use a single frequency
2) connect the two ports to the antennas; the same cables used for the calibration have to be used to connect the two antennas
3) then you can measure s21 =a+jb or directly the phase (in radiants) of the signal. Set the instrument to measure unwrapped phase
4) if you have measured a+jb, then phase = arctan(b/a)
5) convert the phase into time delay usign the formula td = phase/(2*pi()*frequency)
6) the distance is given by d = c*td/sqrt(ε) where c is the velocity of ligth and ε the dielectric constant of the medium (air = 1)

This method should work in a dispersion free and homogeneous medium.

I'm not sure the network analyzer can measure large phase delay. You can try measuring the lenght of a long cable; after calibration connect a cable of known length and ε (see data sheet of the cable) where you will connect the antenna, apply the method and see if the measurement is correct. Let me know if it works.
 

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