 as prime focus feeds in small symmetric and offset reflectors. The basic design will have an aperture diameter of about one wavelength and is essentially an open-ended circular waveguide propagating a TE 11 mode. The radiation pattern can be improved by adding one or more rings or chokes around the aperture, Figure 17.11(c). These have the effect of changing the distribution of current on the flange and creating a more symmetric radiation pattern. The theoretical design of the open-ended waveguide is straightforward, but the analysis of the choked version is much more complicated. As a consequence most small feeds are designed empirically with measured data. 17.4.4.3 Multi-mode conical horns These improve the performance of conical horns by generating a second mode inside the horn in such a manner that the aperture fields are linearised. The second, TM11 mode, is generated by a step change in the conical horn diameter and the length of the horn is determined by the need to have the modes in the correct phase relationship at the aperture. The dual mode horn gives low crosspo- larisation over a narrow band of frequencies. Although narrow band it is simple to make and of low weight. The concept of adding higher order modes in a horn can be extended for other purposes. In tracking feeds a higher order mode is used to provide tracking information. The inherent crosspolarisa- tion which occurs in offset reflectors can be cancelled by the appropriate addition of higher order modes (Love, 1986). Finally the main beam can be shaped to provide higher efficiency in prime Figure 17.12 Microwave line of sight reflector antenna focus reflectors although only over a narrow frequency band. 17/12 Antennas Sidelobe Envelope as Defined by FCC Rules & Regulations Part 25.209 Peak Sidelobe Envelope Smoothed Sidelobe Envelope Degrees from Peak of Main Beam Figure 17.13 FCC earth station antenna pattern specification radiation patterns of the array are mainly determined by the element for microwave radio antennas. The front-to-back ra