Measurement of the Microwave Lensing shift in NIST-F1 and NIST-F2

With several Primary Frequency Standards (PFS) across the world demonstrating systematic fractional frequency uncertainties on order of 1 x 10(-16), it is crucial to accurately measure or model even small frequency shifts that could affect the ultimate PFS uncertainty, and thus ultimately impact the rate of Coordinated Universal Time (UTC) which relies on precision PFS measurements. Recently there has been controversy about the physical causes and size of PFS frequency shifts due to microwave lensing effects. We present here the first measurements of microwave lensing frequency shifts in the PFS NIST-F1 and NIST-F2. The measured frequency shifts agree well with the recent theory of Ashby et al [1].