796 ASPECT ANGLB (&g) ASPECT ANGLE (deg) -MEASURED ______ bIODIFIBD GEOWETRICAL DIFFRACTION THEORY Fig. 3. Principally polarized radar cross section of pyramid at 4.03 GHz. i . 2 0 p j -13 VERTICAL 0" -----------_--%-HORIZOhTAL 0" I -33 0 3 b ASPECT ANGLE (&g) -MEASURED -----MODIFIED GEOhlETRlCXL DlFFRACTlON THEORY Fig. 4. Principally polarized radar cross section of pyramid at 1.00 GHz.base diameter 2a = 7.1 in and height L = 24.0 in (cone half-angle = 9"). Fig. 3 compares predict.ions a t the two linear polarizations with experimental data at 4.03 GHz in the aspect interval, 0 5 * 5 9", a t roll angles of 0 and 45". It is seen that the agreement at vertical polarization is quite good, with the largest deviation being about 5 dB at. a roll angle of 45". The horizontal polarization calculations display a null near 9" for the zero roll angle orientation, which occus in t.he neighborhood of an idection point in the data. As the roll angle increases, the position of the null shifts to larger aspect angles as the in€lect.ion point in the data becomes less pronounced. Thus although the horizontal predictions are not in agreement with the data for p = O", beyond an aspect angle of 6", the agreement. improves markedly as the roll angle is increased. The 1-GHz measurements and predictions are shown in Fig. 4. Since the measurements and the predictions shorn very little variation with roll angle, only the 0" roll angle results are included here.It is seen that both the calculated and measured returns show very small variation wit.h aspect angle over the interval of interest.The theoretical predictions fall about 2 dB below the measured results, however, which is considered to be within the accuracy of the measurements. This agreement of the high-frequency prediction with the measurements at 1 GHz (ka = 1.9) is not completely unexpected. The results of Keller [SI and Bechtel [4] indicate that the nose-on returns from right circular cones can be successfully approximated by unmodified geometrical difFract.ion theory in cases when the base diameter is smaller than a wavelength, and Keller's result5 show good agreement for values of ka similar to t,he preceding. It is not unlikely, therefore, that the improvement. on Keller's "caustic correction" provided by the full integration around the ring discontinuity allows the predictions to be extended throughout the aspect interval of interest for values of l e a as small as 1.9. ACKNOWLEDGMENT The author gratefully acknowledges the efforts of Dr. J. W. Rakestraw and Dr. R. E. Slattery, who supervised the model fabricat.ion and the measurements. IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, NOVEMBER 1972 . -REFERENCES [5] D M Raybin "Radar cross section of s herical shell segments," Aug. 1965. IEEE' Trans. h t e n n a s Propagat.. vol. A8-13, pp. 754-759. Sept. 1965. [6] J B. Keller "Backscattering from a finite cone-Comparison of theory and e;periment," IRE Trans. Antennas Propagat. (Commun.), V O~.Abstract-A study is made to evaluate the radiation resistance of dipole and loop a...
