Extract from:

Microwave System Design Tools and EW Applications, Second Edition, by Peter W East.
Publisher: Artech House Inc, January 2008.

Antenna Near-Field Power

Applet Help

Applet Notes:

The applet plots the power density and power received in the near-field on- and off-axis of a large rectangular aperture.
Two plot options are given in the drop-down menu, Power Density, or Power Received by an antenna of scrollbar-defined aperture.
The powers are determined by dividing the transmitting and receiving antennas into sub-elements and integrating the powers over all transmitter and receiver elements. To achieve a reasonable accuracy for received power, many elements are used, which makes the calculation somewhat slow for large apertures and high frequencies.
Default values may be modified and the display updated using the scrollbars and Enter button. Plot resolution of coarse, medium, or fine, is adjusted using the C,M,F radio buttons.
The display automatically scales, dependent on the parameter values selected.
The red curve indicates the power responses using the long-range inverse square-law formula.
The receive antenna position may be offset both vertically and laterally by means of dedicated scrollbars.
The integration assumes that the apertures are rectangular, but weighting by the edge taper should still give realistic values for elliptical apertures.
Mouse movement in Range-Power space displays the powers relative to the mouse x-axis range value.

User Notes:
To speed up response in the Power Received mode, whilst testing the effects of parameter ranges, choose coarse plot resolution (C radio button). Once the wanted geometry is found, fine mode provides for increased accuracy. Also, for better detail in the very near zone adjust the upper scale range and use F for fine resolution.
The Fresnel zone range is calculated from the standard formula, 2D2, where D is the transmitting antenna largest (width) dimension.
Antenna width and height dimensions are in multiples of the signal wavelength (denoted by WL).
At close ranges, below about one-fifth of the Fresnel zone range and especially for low edge tapers, the power density flattens to approximately that expected by dividing the prime source power by the antenna physical area. Because of limitations of the integration approximation, accuracy no better than 1 dB can be expected.