I've wondered for a long time how phase information is captured for wireless signals. It's used in applications like QAM for decoding symbols and radar beam forming to to locate targets, but I've never been clear on how we can get phase information from voltages measured as a time series of real numbers.
Some signals from medical applications like MEG and EEG do not have phase information. What is the difference between such signals and radio signals? Does it have to do with being able to make certain assumptions about the nature of the signals being sinusoidal?
Best Answer
Radar beam forming is performed by feeding an array of small aperture antennas at different phases of the RF carrier. The phase shifting can be accomplished by digital processing, lumped circuits, spacial positioning, or delay lines formed from various electrical lengths of transmission lines. Phase shifting is the basis of nearly every multi element antenna. Phase shifting is also generally reciprocal for receiving and transmitting on the same antenna.
So generally we are not interested in the exact phase of an RF signal but rather its relative phase to another path of the same signal. This extends beyond antennas to include selective fading, multipath distortions, standing waves and other topics.
QAM is a different beast. The QAM signal modulates a carrier frequency. So the phase of the carrier is not of interest for decoding.
Radio signals propagate due to the accelerating nature of the sinusoidal waveform and because they are composed of an electric and magnetic field at right angles to one another.