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Near Field and the Phase Difference

 

Up until now, we have mainly dealt with the far field. In a residential location, however, we can, for local interfering sources, easily be dealing with the near field. For the boundary between far and near field, a distance of 1/6λ may be maintained, but this is not a strict limit and must sometimes be set at 1λ or more. The dimensions from the source with respect to the wavelength are very relevant in this matter. The near field radiation is completely different from that of the far field and the relationship between phase difference and direction is no longer obvious. Unexpected phase differences can occur, which, however, remain constant in time!

With the loop antennas at right angles to one another, this translates into a phase difference that is no longer in the 0 or 180 range. The phase difference can have any value for a large-sized local source. Overlap in phase difference of a local interfering source and a desired source can then occur by chance.

If we set the loop antennas parallel in the same direction, we can see something similar. The phase difference can be larger than we expect based on the distance between the antennas. This is favourable, as the local interfering source and a desired source cannot overlap in this case.

In this context, it should also be mentioned that coupling with objects (such as antennas) in the near vicinity can influence the phase difference. This coupling can normally be minimized by turning the loop antenna(s).

 

 

Last update: September 24, 2006

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