The design of the iris‐loaded deflector for the Panofsky type rf beam separator now under construction at the Brookhaven AGS is presented. General expressions for the transverse momentum acquired by an ultrarelativistic particle traversing the deflector are derived from the field equations. Useful figures of merit (shunt impedanceR, series impedanceZ, quality factorQ, R/Q, etc.) are defined. Computational results for the aperture radiusa, the guide radiusb, Q, R/Q, andRas functions of group velocityvg, the number of irises per wavelengthN, and iris thicknessd/ware presented for structures with the phase velocityvp=c. They are compared with experimental results reported from deflection tests. The design procedure of a deflector is developed for the realistic case that the shunt impedance varies with group velocity. Optimization of operating frequencyf&pgr;/2, deflector lengthl, vg, d/w, andNis carried out. Higher order modes in iris‐loaded waveguides are investigated and methods for the suppression of mode degeneracies are outlined. The effect of the mechanical tolerances on the phase shift per cell &PHgr; is considered. The chosen deflector structure has &lgr;0=10.495 cm,N=4, &PHgr;=½&pgr;,d/w=0.8, 2a=48.13 mm, 2b=116.71 mm,l=3.07 m, rounded iris edges, and lateral rods as mode stabilizers. ``Cold'' measurements on cavities and waveguides are described, and a preciseR/Qperturbation method is detailed. Pertinent results aref&pgr;/2=2856.35 Mc,Q=8700, phase error per cell &Dgr;&PHgr;=2.7° rms,R/Q=1.41 k&OHgr;/m,vg/c=−0.0204, &agr;l≈0.5. It is concluded that a klystron pulse of ≥14 MW is sufficient to give a 1 mrad peak deflection of 18 GeV/cparticles.