# A New Sum-Channel Radiating Element for a Patch-Monopole Monopulse Feed
## 2 Materials and Methods  
The main course of the research consisted of a study and review of the literature study, then the design synthesis, which was followed by a few prototype iterations to optimize the design. The initial patch dimensions can be obtained from resonance-frequency approximations [22]. Any general-purpose antenna-simulation software, employing, for example, the method of moments, suffices for simulating and optimizing such a feed. At the current stage of research, the feeding circuit consists of coaxial lines below the cavity floor, which complicates electromagnetic modeling of the whole structure. We only simulated the cavity part of the antenna for coarse-tuning of radiation properties, and the experimental work was mainly focused on prototype fabrication and measurements. The optimized physical and electrical dimensions (at 2.21 GHz) of the proposed monopulse feed design are shown in Table 1 and correspond to the marking in the technical drawing in Figure 2.  
Figure 2: Top and side views of the proposed feed design with input (\(\pm\Sigma\), \(\Delta\)) and parameter (dimension) marking. The parameters include cavity diameter (_d_), cavity wall height (_h_), patch height above cavity floor (_e_), patch diameter (_a_), patch feedpoint offset (_f_), slot dimensions (_u_, _v_), and monopole length (_m_).  
The idea for prototype fabrication is the same as that of the design itself: simplicity and straightforwardness. Prototypes were fabricated manually, by using only some standard tools such as a ruler and compass, metal cutters, a drill, a soldering iron with approximately 50 watts of thermal power, standard tin and flux, etc. The used materials and components were also standard. The cavity and the patch element are both made from 0.4 mm thick brass sheet. The patch element is held up by the center conductors (and/or dielectric) of both sum-channel feedpoints, made from flanged SMA (SubMiniature version A) connectors. The monopole is easily made from a standard SMA cable connector with a flanged mount and a suitable coaxial cable (such as RG-405) with an outer cladding stripped to make a monopole. A fabricated prototype is shown in Figure 3.  
The sum-channel patch requires a balanced feed for a symmetric, nonsquinted radiation pattern and least coupling to the difference channel. The prototype uses a simple balun made out of a parallel tee connection of coaxial cables that are \(\frac{1}{2}\)\(\lambda\) and 1 \(\lambda\) in length. The patch is best fed at a point where the impedance is around 100 \(\Omega\)[23]. A parallel connection of the two arms results in an impedance of around 50 \(\Omega\). The monopole feedline is made for the best impedance matching of the difference channel. The length differences in the sum and difference channels manifest as an additional constant phase shift of the resulting monopulse ratio, which needs to be spatially aligned with the antenna (calibrated). The final feed prototype during radiation-pattern measurements, including the feeding circuit, is shown in Figure 4.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Unit** & **d** & **h** & **e** & **a** & **f** & **u** & **v** & **m** \\ \hline mm & 96.0 & 11.0 & 3.5 & 69.2 & 11.0 & 7.0 & 18.5 & 27.5 \\ \(\lambda\) & 0.71 & 0.08 & 0.03 & 0.51 & 0.08 & 0.05 & 0.14 & 0.20 \\ \hline \end{tabular}
\end{table}
Table 1: Physical and electrical dimensions (at 2.21 GHz) of the proposed monopulse feed design.  
Figure 3: A fabricated prototype of a patch-monopole monopulse feed.  
The radiation efficiency is expected to be high since the prototype is made out of good-conductivity metals and low-loss dielectrics: air in the cavity, PTFE (polytetrafluoroethylene, also known as Teflon) or PTFE-like in coaxial lines, etc. Simulations show a sum-channel radiation efficiency above 99% for an ideally-fed antenna and above 95%, including a simple coaxial network balun. The experimental verification [24] in a less-than-ideal reverberation chamber revealed a sum-channel radiation efficiency above 80%.