مرکز منطقه ای اطلاع رساني علوم و فناوري - The radiation characteristics of the conical horn-reflector antenna excited in higher modes

Abstract :

A conical horn-reflector antenna has some gain for higher mode excitation in the main direction which means the direction of the maximum radiation for the dominant mode excitation. This paper gives the general expression of the gain function of the conical horn-reflector antenna excited in higher modes in the Fresnel and the Fraunhofer regions, and represents graphically the computed results for the first five higher modes of the circular waveguide: $TM_{01}\\deg$ , $TE_{21}\\deg$ , $TE_{11}\\deg$ and $TE_{31}\\deg$ modes. Graphs show the gains in the main direction in the Fraunhofer region as functions of the flare angle of the conical horn, the gains in the main direction in the Fresnel region as functions of the distance between the antenna and the point of observation, and radiation patterns. Then simple approximate expressions of the gains for higher mode excitation in the main direction in the Fraunhofer region are derived and verified to give results which coincide very well with the computed results mentioned above. By using these approximate expressions, the gain for higher mode excitation in the main direction is generally discussed. In consequence, it is cleared that the gains for $TE_{on}\\deg$ and $TM_{on}\\deg$ mode excitations in the Fraunhofer region are zero, that the gains for $TE_{m}\\deg$ mode excitation is independent of the flare angle, and that in general the gain for higher mode excitation depends on the flare angle $\\Psi$ in the proportional manner to $\\tan^{2(m-1)} (\\Psi /2)$ for $TE_{mn}\\deg$ mode and $\\tan^{2(m+1)}(\\Psi /2)$ for $TM_{mn}\\deg$ mode, and the proportional coefficient decreases as $n$ increases. The gain for $TE_{12}\\deg$ mode excitation is maximum and it is -10.6 dB over the gain for the dominant mode excitation. From this fact it is recommended to choose the diameter of the feeding circular waveguide small enough to cut off the $TE- {12}\\deg$ mode propagation. Among the gains for the higher modes which have lower cutoff frequencies than that of $TE_{12}\\deg$ mode, the gain for $TE_{21}\\deg$ mode excitation is maximum, and is -14.9 dB over the gain for the dominant mode excitation when the flare angle is 30 degrees. To suppress the gain for $TE_{21}\\deg$ mode excitation below -20 dB, the flare angle must be smaller than 16 degrees.