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 impedance R, series impedance Z, quality factor Q, R∕Q, etc.) are defined. Computational results for the aperture radius a, the guide radius b, Q, R∕Q, and R as functions of group velocity vg, the number of irises per wavelength N, and iris thickness d∕w are presented for structures with the phase velocity vp=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 frequency fπ∕2, deflector length l, vg, d∕w, and N is 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 Φ is considered. The chosen deflector structure has λ0=10.495 cm, N=4, Φ=☒π, 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 precise R∕Q perturbation method is detailed. Pertinent results are fπ∕2=2856.35 Mc, Q=8700, phase error per cell ΔΦ=2.7° rms, R∕Q=1.41 kΩ∕m, vg∕c=−0.0204, αl≈0.5. It is concluded that a klystron pulse of ≥14 MW is sufficient to give a 1 mrad peak deflection of 18 GeV∕c particles.