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Far field calculation

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AmirMohammad

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Hi
Far field for linear antennas can be calculated using formula:
R>2*D^2/lambda
but for a broadband reflector antenna that radiates at different frequencies and not have a linear dimension such as a linear antenna what should we do?
 

normally this is thumb rule but for big antennas u can use compact ranges or near field ranges where u measure near field data and transform it to far field data
 

I remember, for reflector antenna, the largest dimension can be taken as its diameter. You may need to check this in some book or document.
 

There are several far-field approximations (see Stutzman and Thiele pg 24):

Rff>2*D^2/lambda
Rff>>D
Rff>>lambda

D=maximum dimension of antenna

You pick the longest of the three.

Rff>>lambda is used for electrically small antennas, such as an HF ferrite rod, or can also be used for dipoles, patches, monopoles, etc.. Although I don't think you would measure a 1 MHz ferrite antenna at a distance of 3 km.

Rff>>D is used when the antenna D is on the order of 1*lambda. Maybe for low directivity arrays such as a couple of dipoles.

Rff>2*D^2/lambda is used for arrays where D>>lambda. If you had a 30 GHz planar array that was 1m^2 you would not encounter the far field at 10*lambda=10cm. Make sense? 10m may be too close. 200m as given by the last equation is good, but do you have a 200m chamber? This is why near-field measurements are made on arrays.

To summarize, the old rule of thumb (Rff=>10*lambda) is no good for large arrays, hence the last equation.
Click to expand...
 

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