Transactions on Antennas and Propagation

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Transactions on Antennas and Propagation
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(c)
Fig. 20 Interval characteristics of
H
and
V
of inhomogeneous radome, CTR and VTR during antenna scanning. (a) Overlap ratio. (b) Range ratio. (c) jg computed by (7). From these results, it can be seen that VTR and inhomoge- neous radome affect the BSE indifferent ways. VTR mainly reduces BSE through flexible adjustments of the IPD distribution without imposing obvious effects on the IPD interval, whereas inhomogeneous radome can considerably reduce the
IPD interval but with very limited ability on changing the IPD distribution, that is, just varying from the incident angle distribution to the polarization angle distribution or something between the incident or polarization angle distribution. VTR, although tremendously improving the BSE, keeps a BSE of about 0.5 mrad (cf. Fig. a, close to the maximum BSE, at many scan angles in the whole scan range, mainly due to the existence of the relatively large IPD interval. Meanwhile, in- homogeneous radome only introduces large BSE at small scan angles, whereas at large scan angles, as the IPD distribution improves, causes much less BSE than VTR (in fact, almost no
BSE, cf. Fig. a) because of the tremendous IPD interval reduction. Moreover, Fig. 21 plots the overlap ratio and range ratio with the increase of curvature coefficient under several scan angles. It can be observed that, generally, overlap ratio and range ratio tend to be small with large curvature coefficient From Fig. 21 and Fig. a) which shows the transmission loss TL) of inhomogeneous radome with different
, it can be found that, for
>0.8, TL has a rapid decrease due to the impedance matching effect, whereas overlap ratio and range ratio have or begin to have a rapid decrease, which makes the
IPD distribution liable to resemble polarization angle distribution or be unpredictable. In this way, the maximum BSE tends to increase. This is the price of the excellent impedance matching performance of high- radome design. ab) Fig. 21 Interval overlap ratio and range ratio of
H
and
V
of inhomogeneous radome with different curvature coefficients. (a) Overlap ratio. (b) Range ratio. IV. C
ONCLUSION
In this paper, through the comparison with constant thickness radome (CTR) and variable thickness radome (VTR), the insertion phase delay (IPD) characteristics of inhomogeneous radomes are revealed and a method is proposed to estimate first radome IPD distribution and then boresight error (BSE) from parallel and perpendicular polarization transmission coefficients. Conclusions can be drawn as follows
(1) Inhomogeneous radomes which provide an excellent impedance match can improve the transmission loss (TL) whereas yield relatively larger BSE than VTRs, due to the effect of impedance match on the IPD interval of parallel and perpendicular polarization transmission coefficients. Given the
IPD of parallel and perpendicular polarization transmission coefficients, the radome IPD distribution can be predicted to make an efficient estimation of the BSE using the proposed method. This method can be helpful in radome design.
(2) Radome BSE depends mainly on IPD distribution and
IPD interval. Inhomogeneous radomes can considerably reduce the IPD interval but is limited in changing the IPD distribution, whereas VTR can flexibly adjust the IPD distribution to reduce
BSE and hardly affects IPD interval. Therefore, under small scan angles (corresponding to the radome nose area) where IPD distribution matters due to the sharp shape, VTR can yield much less BSE than inhomogeneous radome, whereas under large scan angles where IPD distribution is unimportant due to the gentle shape, inhomogeneous radomes can have much less
BSE and moreover, much less TL, too, than VTR. In addition, linear polarization is employed in this work. For antenna-radome systems with circular polarization, as the expression of T

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