During the past few years, many efforts have been made RF coils can be easily incorporated (8). These advantages are, however, overshadowed by the inconvenient sample to develop new resonance structures for EPR applications. The main aim of these is to increase the S/N ratio of the handling of powders and solutions. detected signals, especially when working with samples of Searching for possible solutions, our attention has been small volume. Various technical problems, related in particuattracted to the Alderman-Grant resonator {AGR} (9, 10). lar to the introduction of additional features such as double-This structure is well studied for radiofrequency applications resonance RF coils and optical access, also contribute to the in high-resolution NMR and MRI. One of the versions of complexity of the design problem.
this resonator appeared to be transformable for use in the The common trend in the design of resonators with a high microwave region (11). The original AGR is manufactured filling factor is to use undersized structures with dimensions from copper or aluminum foil with chip capacitors, and ideoless than the wavelength. Resonators developed for pulsed logically represents a high-frequency analog of saddle coils EPR such as the loop-gap resonator {LGR} (1), the slotted (Fig. 1). Two H-shaped foil pieces {1} are bent around a tube resonator {STR} (2), and the ''Mims''-type strip line cylindrical former {8}; four chip capacitors {5} are solresonator {MSLR} (3) and some of its latest modifications dered between the wings of the structure to close the current (4, 5) represent such a quasi-lumped circuit approach. The loops. Two metal strips {3} placed under the wings and LGR is used most frequently at frequencies from L to X separated from them by a thin insulating layer {7} act as band. It shows good sensitivity due to its high filling factor the ''guards'' preventing the electric fringing fields from combined with a fairly high microwave magnetic field homopenetrating into the sample. The Alderman-Grant resonator geneity. However, for samples with high dielectric losses, is known to have extremely good separation of the electric the relatively poor separation between the electric and magand magnetic radiofrequency fields (which is usual for denetic components of the microwave field causes the actual vices with all the electric field concentrated in the lumped sensitivity to be strongly reduced. This situation is greatly capacitance). Extensive studies of the AGR geometry have improved in the so-called bridged loop-gap resonator led to an optimal strip-to-window-width ratio of 78ะ/102ะ {BLGR} developed by Pfenniger et al. (6). The principal (10) which provides the highest homogeneity of the magproblems in loop-gap resonator design occur when ENDOR netic field inside the structure. operation is required. It is difficult to avoid coupling of the To work in the microwave region, the structure has been RF coils to the microwave field. Therefore, one is forced to scaled down, resulting in a substantial decrease in the strip place the RF coils outside the resonator, leading to a reduced and wing inductance. The chip capacitors were substituted RF amplitude at the sample location. In contrast, because of by bridges like those used in the BLGR (Fig. 2A). The first its partly open structure, the STR is very suitable for ENDOR structures were made of silver foil, but later models were applications. For the same reason, optical access is very painted on quartz tubes using silver paint. The mw-AGR easy. These advantages must be paid for by a rather poor has three principal resonance frequencies, one corresponding filling factor. The MSLR combines a very high filling factor to the principal AGR mode. Two other frequencies correwith a very low Q value. Originally, it was designed for spond to LGR modes with slightly different resonance fresingle-crystal pulsed EPR studies and observation of the quencies; these modes are due to the currents in upper and linear electric field effect {LEFE} (7). Also in this design, lower BLGR-like structures (involving circular currents in the wings and bridge capacitors). It is relatively easy to design the structure with these frequencies far apart from