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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.

What Does This Really Mean?
 
The vast majority of radomes are multi panel assemblies that are made out of curved or flat, one or multi-layer dielectric panels, connected together by flanges that are bolted together around the perimeter of the panels.  For (MSF) Metal Space Frame and (DSF) Dielectric Space Frame radomes those joints are metal or dielectric frame beams.  For sandwich radomes these joints are solid laminate panel flanges and for air inflatable radomes, which are seemingly one-piece radomes the joints are created by the wall material overlapping.  Regardless of the joints physical character, its EM waver scattering properties are very different for the scattering properties of the panel surface and thus, the EM distortions introduced by the joints have to be accounted for separate from those introduced by the panel.  To account for these distortions the radome industry adopted what is referred to as the IFR (Induced Field Ratio) approach.

Vast majority of radomes are multi-panel or multi-piece assemblies that are made out of curved or flat, one or multi-layer dielectric panels connected together by some kind of joints placed around the perimeter of panels.  For MSF (Metal Space Frame) and DSF (Dielectric Space Frame) radomes those joints are metal or dielectric frame beams.  For sandwich radome joints are solid laminate panel flanges.  For airbag and other seemingly one-piece radomes role of joints are played by the wall material overlaps hidden within the radome wall.  Regardless on joints physical character its EM wave scattering properties are very different from the scattering properties of the panel surface and thus, the EM distortions introduced by the joints should be accounted apart from those introduced by the panel.  Since its complexity the scientifically rigorous consideration of the EM distortion inserted by the radome joints is impractical, the radome industry adopted approximate but workable and at the same time accurate enough approach that is based on the IFR (Induced Field Ratio) concept [1, 2].    

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.

What is the IFR and what does it stand for?

Vast majority of radomes are multi-panel or multi-piece assemblies that are made out of curved or flat, one or multi-layer dielectric panels connected together by some kind of joints placed around the perimeter of panels.  For MSF (Metal Space Frame) and DSF (Dielectric Space Frame) radomes those joints are metal or dielectric frame beams.  For sandwich radome joints are solid laminate panel flanges.  For airbag and other seemingly one-piece radomes role of joints are played by the wall material overlaps hidden within the radome wall.  Regardless on joints physical character its EM wave scattering properties are very different from the scattering properties of the panel surface and thus, the EM distortions introduced by the joints should be accounted apart from those introduced by the panel.  Since its complexity the scientifically rigorous consideration of the EM distortion inserted by the radome joints is impractical, the radome industry adopted approximate but workable and at the same time accurate enough approach that is based on the IFR (Induced Field Ratio) concept [1, 2].    

The IFR is a two complex dimensionless numbers that fully characterize scattering properties of the radome joints.  For each particular joint design and frequency these two complex numbers can be measured or calculated.  As soon as joints IFR values are known, any distortions of antenna radiation pattern under the radome can be calculated using the reasonable amount of computer and engineering resources.  In short, the main idea behind the IFR concept is to give the radome designer the ability although approximate but from all practical tense and purposes accurate and workable way to calculate any distortions of antenna radiation pattern inserted by the radome.

The formal definition of the IFR values is illustrated in Figure 1 where, the IFR is defined as a ratio between complex magnitude Es of the EM wave scattered by the joint and complex magnitude of the incident EM wave E0.  Due to the significant difference between joint length and width, the IFR value for incident wave whose polarization is oriented along the joint axis and IFR value for incident wave whose polarization is oriented perpendicular to the joint axis might be noticeably different.  That is why for the comprehensive description of joints scattering properties the joint IFR must be represented by two complex numbers: IFR‖ and IFR┴.

The IFR values represent the “contrast” between scattering properties of the panel and the joints.  In the hypothetical (and unrealistic) case when the scattering properties of panel and joints are the same the IFR values would be equal to zero.  Alternatively, bigger IFR values correspond with bigger distortions of antenna radiation pattern due to the panel joints.  When the joint scattering properties correspond with pure optical blockage (when metal beam width and depth become much bigger than the wavelength, for example), the IFR values approach to -1 + j0.  For the MSF radomes at frequencies below 1 GHz and for the DSF radomes at frequencies above 1 GHz the magnitude of IFR values might exceed 1.  That is the reason why the DSF radomes are uncompetitive with the MSF radomes above 1 GHz frequency range (below 1 GHz range the DSF radomes are uncompetitive with the sandwich radomes).  From the joints transmission loss standpoint only the real part of complex IFR values makes a difference.  The radome boresight error is mostly defined by the phase of the IFR values – if it is equal to zero the radome joints do not cause any boresight error.  Other distortions of antenna radiation patter due to the panel joints depend on both, real and imaginary parts of the IFR.

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