Is it possible to predict the signal form at position B by the measurement at position A?

[csurocket]

Hi, I need to know the signal form and phases of position B, but I only have one antenna at position A, which is 1m away form B. The signal come form a source far away.   I think it is possible to predicte it.

Like this: First transform the signal received at A in time domain into fourier domain and then caculation the phase changes of frequency components between A and B. After that transform it back to time domain, it should be the signal series at frequency B.

I have simulation this process by Matlab, but it seems it has some problem. What is the problems with this methods? Is there any methods to realise the predict? Any reply will be appreciaged.

[johnsold · Accepted solution]

If your antennas are in free space so that you can assume that you do not have any multipath issues, then it should not be too difficult to predict the relative phase between A and B.

You need to know the angle between the line from A to B and the line from B to the source.  You need to know the wavelength.  You need to do some trigonometry.  In the simple case where all three are colinear the phase shift is 2*pi*(A - B spacing)/wavelength. For other angles you will need to calculate the appropriate triangle.

If the signal has a complex modulation, especially if it a wideband signal, there may not be any simple or straightforward way to calculate the phase shift.

Lynn 

[csurocket]

Thank you for your reply. So you mean the forms of radio wave keeps the same in its propagation way? If I take  pictures of the wave at A and B seperately, will the pictures be the same?

[johnsold]

Well, yes.  And no.

For simple signals in free space the answer is yes. If the signals are wideband and they pass through a dispersive medium or network, then no. Dispersive means that the propagation behavior changes with frequency. So signals which bounce off the ionosphere can be distorted because the reflectivity of the ionized parts of the atmosphere vary with frequency and position.

Most likely in your application nothing will change much between two antennas spaced 1 m apart.  What is the wavelength and distance to the source?

Lynn