Measurements of microwave emission from an electron beam have shown signals below the shielding cutoff frequency of the beam pipe for coherent synchrotron radiation. The frequency spectrum of the signal below this cutoff frequency agrees with calculations and bench measurements of the impedance of a vacuum component in the ring. The wake fields produced in this impedance are above the waveguide cutoff frequency of the beam chamber, allowing the fields to propagate around the ring and out a photon beam port. Data are presented that rule out the possibility that this signal is coherent synchrotron radiation. This unique method of measuring propagating fields is discussed along with its application to measuring high frequency impedances and their impact on beam properties.
