What is considered the "farfield" of an array of antenna elements?

[b4bb4ge]

What is considered the "farfield" of an array of antenna elements. I realize that the Far Field of an element is > 2D^2 / lambda; where D is the largest dimension of antenna. But in an array of antennas is this the size of a single element or the size of the array it'self?

I realize the "farfield" is when the EM wave looks planar, therefore you would think that a more directive antenna would have a planar EM wave closer to the aperture because it is more directive, but if you use the entire array as "D" then the farfield could be quite far away?

I have looked through Balanis' and Johnson's books with no luck.

Thanks for your help,
Babbage

 

[WimRFP]

Re: Farfield of antenna array

Hello

It is the size of the antenne where there is significant current. for your array it will be the size of the complete array.

The Far field distance relates to the distance where the point source approximation is valid. That is that each doubling of distance gives 6 dB reduction in power flux density (or E-field). If no reflections are present, in the far field region the Friis formula is valid. Note that you need to sum the far field distances for the transmit and receive antenna.

You are right several antennas can have reasonable plane wave field well within the far field distance, but the point source approximation doesn't hold. This is used in compact antenna measuring ranges. You are also right that the far field distance can be large.

With some degradition in accuracy, one can reduce the measuring distance to D^2/lambda. In reality some reduction of measuring distance may even enhance accuracy as the influence of unwanted reflections becomes less.

When you use field taper (to reduce side lobes), there is less current at the extremeties of the antenna hence far field conditions occur at even shorter distance.

If you have something in simulation, you may simulate this by measuring S21 for increasing distance. You can use a simple dipole as receiving antenna in your simulation.

 

[b4bb4ge]

Re: Farfield of antenna array

Thank you that is quite beneficial, I will try to simulate an array in FEKO to see this effect.