The birdcage coil is still by far the most common design for MRI quadrature coils, mostly thanks to its intrinsic efficiency and B₁⁺ field uniformity. However, the “degenerate” version of this design¹, is rarely used for multichannel transmit arrays, especially at ultra high field (≥7T). We introduce here a combination of analytical theory and simulations to design a knee-sized transmit-and-receive degenerate birdcage at 7T. These methods resulted in excellent performance, despite the asymmetry introduced by the splittable design. A prototype was constructed, and demonstrated satisfactory by workbench and scanner measurements on phantom and human volunteers.The birdcage theory^2,3 was rewritten in term of variables more suited for the “degenerate” application, obtaining a mode equation dependent only on the mutual inductances, the ratio R and series combination T of leg and end-ring capacitance values. As shown in Figure 2, while a true degenerate solution does not exist³, several almost-degenerate spectra can be obtained, depending on the value of R. The mode equation can be inverted to obtain a relationship between the resonant frequency and the mutual inductances, to be used in an iterative way to find optimal values of R and T. The theory was used in combination with circuit and numerical simulations using Agilent ADS and Altair FEKO to obtain an estimate of optimal capacitance values, B1 efficiency, and S matrix. All FEKO simulations were performed using a cylindrical phantom model (OD=12 cm, σ=0.6, er=78). The coil model was imported into CST to evaluate the SAR and B₁⁺ inside a realistic human head model (Ella from the Virtual Family). The coil was then fabricated on an ABS cylindrical former (ID 180 mm, OD 240 mm, length 236) using Kapton and ARLON FR25 as PCB substrate for the coil legs and end rings respectively. The 0.05 mm Kapton shield consist of 2-layers of overlapping strips. Workbench measurements were performed with a saline 0.1M NaCl cylindrical phantom (OD 12 cm). A Butler matrix⁴ was simulated (using ADS and FEKO) and built to interface the degenerate birdcage to the 2-channel MRI scanner. It consisted of three layers containing four microstrip quadrature hybrids each and microstrip connection lines of variable length. The substrate was 3 mm thick RO3010. Different layers were connected by carefully calibrated RG405 semi-rigid coaxial cables. The power loss, amplitude and phase balance were measured and compared to simulations. A set of 8 TR switches was built on 3mm thick RG5880 substrate. The circuit uses two λ/4 lumped element lines with shunt Microsemi UM4906PIN diodes. Insertion loss, transmit power loss, preamp isolation and preamp gain were measured.The simulated and measured capacitor values are shown in Table 1. The simulation correctly predicted the R value, while some adjustment (around 15%) was needed to correctly tune the coil to 300 MHz. The mesh that spanned the split required slightly different values of both R and T. As shown in Figure 3, the simulated coil is correctly matched and decoupled, although some residual coupling (-11 dB) exists between next-neighbor meshes. The measurements show an average reflection coefficient of -16.8 dB and coupling coefficients better than -10.4 dB. The simulated and measured B₁⁺ maps are shown in Figure 4. The coil in CP mode has good efficiency (44.4$\frac{μT}{\sqrt{kW}$ in the simulation and 56.5 $\frac{μT}{\sqrt{kW}$ in the measurement) and the qualitative agreement with simulation (and the birdcage theory) is good for all modes. The simulation on the human phantom shows a CP mode efficiency of 44 $\frac{μT}{\sqrt{kW}$ and a maximum local SAR of 1.05 W/kg normalized to 1 μT average B₁⁺ across a central slice. Regarding the Butler matrix the mean phase error was ±3.8^o for the measurement and ±1.4^o for the simulation, while the measured power loss was 14.6%. The TR switches insertion loss during reception was 0.219 dB, while the attenuation during Tx was 0.161 dB. The preamp isolation was -50 dB. The coil, in combination with the Butler matrix and T/R switches, was used in CP mode for knee imaging in vivo (see Figure 1). A knee-sized splittable degenerate birdcage was constructed at 7T, with good performance in terms of B₁⁺ efficiency and element decoupling. The modified birdcage theory, together with circuit and numerical simulations, provided an effective strategy for design and construction. The coil was successfully used in vivo for knee imaging and MSK applications. This coil can be used for RF shimming and pTx solutions on both 2-channel and 8-channel MRI systems.