RESOURCES: IFR Definition

The formal definition of the IFR (Induced Field Ratio) of a blocking obstacle, such as a radome beam or joint, is as follows. The IFR is the ratio of the forward scattered field from the obstacle when illuminated by a uniform plane wave, to the hypothetical field radiated in the same direction by the fields of the same illuminating plane wave when confined to a aperture of the same width as the projected width of the obstacle. It should be noted that the latter field is purely hypothetical, as any practical method used to confine the plane wave fields to a finite aperture will in some way disturb the field uniformity.

The IFR is a complex dimensionless parameter, and is essentially a measure of how strongly the obstacle will block the incident fields by scattering in the forward direction (the direction of propagation of the incident wave). Because it is complex valued, however, it is important to note that its phase as well as magnitude will determine the influence of the obstacle on the net fields in the forward direction. It should also be noted that the IFR is a function of the polarization of the incident wave, and is generally fully charcterized by two values: one for the incident polarization parallel to the obstacle axis, and the other for the incident polarization perpendicular to the axis. For a perfectly conducting obstacle such as a metal beam, in the limit of the projected width in wavelengths approaching infinity, the IFR will approach a value of -1+j0, which corresponds to purely geometrical blockage with the obstacle casting a geometrical shadow. For obstacles such as dielectric beams and composite panel joints, which are constructed from dielectric materials, the IFR will remain complex valued even in the limit of the projected width approaching infinity.

REFERENCES

A.F.Kay, "Electrical Design of Metal Space Frame Radomes", IEEE Trans. Antennas & Propagation, Vol.AP-13, pp.188-202, March 1965.

W.V.T.Rusch, J.Appel-Hansen, C.A.Klein & R.Mittra, "Forward Scattering from Square Cylinders in the Resonance Region with Application to Aperture Blockage", IEEE Trans. Antennas & Propagation, Vol.AP-24, pp.182-189, March 1976.