Hi All,
Trying to find out more about near field effects, I have read that the transition from near to far
field happens at the "Rayleigh distance", sometimes called the "far field
distance". An estimate for this distance is given by the formula (2 d^2)/(lambda) where d is
the maximum dimension of the radiating structure. (See for example:
http://www.ee.surrey.ac.uk/Personal/D.Jefferies/antennas.html)
For a quarter wave vertical this gives a value of one eighth of a wavelength,
not too different from the 1/2Pi formula quoted elsewhere. However for a
typical amateur 10 metre vertical, the formula gives a ridiculously small value
of less than 0.1 of a metre at 137 kHz.
It seems that the Rayleigh distance is commonly used when considering microwave
antennas, dishes, horns and the like.
Can anyone explain to me why it is not appropriate for our proportionally small antennas? In particular are the Near Field effects of our small antennas over-estimated when we suppose them to be still significant out to lambda/(2Pi)? Is the latter formula only appropriate to professional antennas?
73, John, G4CNN
email: [email protected]
web page: http://www.g4cnn.f2s.com
___________________________________________________
GO.com Mail
Get Your Free, Private E-mail at http://mail.go.com
|
