The design of a new zero-field electron magnetic resonance spectrometer including cryogenic capabilities and a new tuning mechanism for sweeping over approximately two octaves is described, The tuning mechanism is based on a one-loop, two-gap resonator mounted on quartz rods in such a manner that rotating the rods changes the width of the gap and the resonant frequency of the resonator, The spectrometer was used to measure the zero-field emr spectra of several copper carboxylate dimers at 90 K and 70 K. The parallel hyperfine structure was well resolved in all of the spectra and this allowed accurate determination of D, E, and A(parallel to). A(perpendicular to) was found to be poorly defined in general and least square refinements attached large errors to this term in the spin Hamiltonian. The zero-field terms in the spin Hamiltonian at 90 K were determined to be D=9.828 GHz, E=83 MH2, A(parallel to)=217 MHz for [Cu(CH3CO2)(2)(CO(NH2)(2))](2) . 2H(2)O; D=9,985 GHz, E = 43 MHz, A(parallel to) = 221 MHz for [Cu(C2H5CO2)(2) (H2O)](2); D= 10.107 GHz, E = 49 MHz, A(parallel to) = 218 MHz for [Cu(C2H5CO2)(2)(CO(NH2)(2))](2); D=9.979 GHz, E = 192 MHz, (parallel to) = 223 MHz for [Cu(C6H5CO2)(2)(C6M5CO2H)](2). A model which assumes A(perpendicular to)=0 is discussed in detail as it was found that all of the features observed in the spectra could be interpreted using simple explicit expressions derived from the model. (C) 1997 American Institute of Physics.