The UVX electron storage ring at the Brazilian Synchrotron Light Laboratory suffers from longitudinal instabilities driven by a HOM of one of the RF cavities. The operational difficulties related to these unstable modes were successfully overcome by determining the proper cavity temperature set point in combination with phase modulation of the RF fields at the second harmonic of the synchrotron frequency. However, a serious drawback of the method is to increase the energy spread of the electron beam, which is detrimental to the undulator emission spectrum. The use of higher harmonic cavities is a more appropriate technique since it provides damping of the longitudinal modes without increasing the energy spread. A full scale prototype of a 3rd harmonic cavity was manufactured at the LNLS workshops and had its main rf properties measured. Special care was taken to measure the shunt impedance of the fundamental resonant mode since it determines how many cavities will be necessary for the adequate operation of the system, which is designed to operate in passive mode. In this work we present the results of the bench characterization of the cavity. 748750Farias, R.H.A., Upgrade and commissioning of the LNLS RF system (2004) EPAC'04, 950. , http://www.JACoW.org, Switzerland, TUPKF001, July 2004Citadini, J., Commissioning of the LNLS elliptically polarizing UNdulator (2007) PAC'07, 149. , http://www.JACoW.org, Albuquerque, USA, MOPAN004, June 2007Abreu, N.P., Conceptual design of a 3rd harmonic cavity system for the LNLS electron storage ring (2006) EPAC'06, p. 1316. , http://www.JACoW.org, Edinburgh, June 2006, TUPCH129Maier Jr., L.C., Slater, J.C., Field strength measurements in resonant cavities (1952) J. Appl. Phys., 23, p. 68