# A New Sum-Channel Radiating Element for a Patch-Monopole Monopulse Feed
## 1 Introduction  
The patch-monopole monopulse feed [1] is a relatively simple and compact multimode feed design with a circular cavity that is approximately a wavelength in diameter. This type of monopulse feed is suitable as a prime-focus illumination of a rotationally-symmetric (deep) reflector, a single-reflector paraboloidal (parabolic) antenna [2; 3]. It enables inexpensive monopulse antenna designs for stations that benefit from rapid and very accurate antenna pointing, such as LEO (low Earth orbit) satellite ground stations [4; 5; 6], as illustrated in Figure 1, ground stations of moving aerial or high-altitude platforms, long-distance links (for example, Lunar communications [7]), and similar.  
The compactness of the design results in minimal antenna-aperture shading and makes the feed suitable also for small(est) prime-focus paraboloidal antennas, only a few wavelengths in size. The simplicity of the design results in a very manageable fabrication complexity and low production cost. The compact design makes the feed also suitable for mobile radars, aerial-platform communications, and similar applications where compact monopulse antennas are sought.  
The main radiating element is a circularly-polarized patch exciting the TE\({}_{11}\) mode (circular-waveguide equivalent) as the sum channel. The second radiating element is a linearly-polarized monopole exciting the TM\({}_{01}\) mode (circular-waveguide equivalent) for the difference channel. The patch element in a relatively shallow cavity radiates like an open-ended (short) circular waveguide [8; 9; 10]. Two-dimensional monopulse operation requires a circularly-polarized incident wave, in contrast to the conventional monopulse design with an antenna array and a comparator network [11; 12; 13; 14; 15]. The tracking information is obtained by quadrature detection with the sum signal as the reference.  
This article briefly describes the research, development, and prototype measurements of a patch-monopole monopulse feed design with a novel, sum-channel, radiating patch element. The evolved design employs, as a circularly-polarized, sum-channel radiating element, a slotted circular patch [16; 17; 18] instead of an elliptical patch [18; 19] or similar [20; 21] from the previous design [1]. The slot allows for an additional separation of the sum-channel radiating element from the rest of the feed. The design variables are optimized mainly for good radiation properties, including the sum-channel aperture illumination efficiency in a typical satellite-receiving antenna (spillover illumination is minimized). The slot is designed so that the two fundamental modes of the patch are phase-shifted in quadrature for circular polarization. A technical drawing of the proposed design is shown in Figure 2.  
As in the previous design, the sum-channel patch is left-handed-circularly polarized (LHCP) and it is optimized for a symmetric paraboloidal antenna with a focal-distance-to-diameter ratio \(\frac{\mathrm{F}}{\mathrm{D}}\) of 0.4. The center frequency of operation is 2.21 GHz. Compared to the previous design, the new patch geometry provides an additional degree of freedom while the circular patch outline is somewhat simplified, allowing for a potentially easier fabrication. The new design shows a slightly improved monopulse channel isolation, and a high radiation-pattern symmetry.  
Figure 1: Illustration of the proposed feed in a satellite ground station with sum (\(\Sigma\)) and difference (\(\Delta\)) beam marking.