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788IEEE TBANSACTIONS ON ANTENNAS Ah?) PROPAGATION, N O V E D E R 1972 mode leaking t.hrough the slot. It is also remarkable that the at,tenuation due t o radiation for 7t = 1 mode remains very small for slet width to diameter ratios up to 0.25 but rises rapidly afterwards to values comparable to those for zeroth-order mode. The improper nature of the modes just described can be easily seen from the numerical results as Im h is always negative. In the open region I the field has the asymptotic behavior DSe,(h,+)He,,(') (h,p) It is known in general that in leaky guides which cannot support surface-wave modes (i.e. containing neither dielectrics nor impedance structures) the mode spectrum is purely continuous and in excit.ation problems fields must be represented by integrals over continuous spect.rum modes. However highly convergent field representations for the near-zone fields can be obtained in terms of leaky-guide modes which correspond to poles on the wrong (nonphysical) branches of Green's function. The leaky-mode represenhtions may be obtained by deforming the contours of integration over the cont.inuous spectrum, i.e., on the branch cuts, into the wrong Riemann leafs of the function [5]. Radiation field of slot antennas can also be obt.ained usihg Kirchhof's type integrals over the aperture distribut,ions which are determined mainly by leaky modes.Therefore the preceding analysis may be useful as it gives the location of leaky mode poles as well as their spat,ial struct,ure. The problem of the slotted semielliptical guide attacked represents one of the rare instances where an exact analytical solution can be obtained for a leaky-wave structure.Abstract-The radar backscatter of natural snow surfaces was measured at 10 GHz and 35 GHz and at grazing angles from 1' to 0.3'. For horizontal polarized radiation the terrain clutter per unit area (mz) at 10 GHz of a flat snow terrain decreases from -50 dB at 1' to -70 dB at 0.4". The return is approximately 10 dB lower for vertical polarized radiation. The terrain clutter was found to depend on the free water content of the snow. The radar cross sections of ice blocks placed on the snow surface is roughly proportional to the square of the area of the ice block facing the radar at 10 and 35 GHz and is approximately 20 dBsm below the return expected for a perfectly reflecting plane surface. At 95 GHz the ice blocks become diffuse reflectors. work was supported by the Advanded Rekearch Projects Agency under Manuscript received February 3 1972' revised M a y 15. 1972. This Order 1616. P. Hoekstra is with the E. S. Army Cold Regions Research and Engineering Laboratory Hanover N H 03755.