A test head for electron spin resonance and paramagnetic electron resonance measurements comprises a resonator of circular cylindrical shape adapted to accommodate a coaxially arranged test tube, and a waveguide arrangement for exciting the H.sub.011 wave in the resonator, said waveguide arrangement comprising one section having its one end arranged adjacent one face of the resonator and communicating with the resonator via an opening provided in the face in eccentrical arrangement relative to the resonator. In order to enable the coupling coefficient to be varied and, thus, the resonator to be adapted to the waveguide section, the waveguide section carrying a H.sub.1n wave communicates with the resonator via a coupling slot the angular position (&phgr;) of which can be varied relative to the radial plane of the resonator passing through its center. Preferably, the waveguide section exhibits a circular cross- section and can be rotated about its axis. It guides the H.sub.11 wave, and the coupling slot is provided, symmetrically relative to the axis of the waveguide section, in a wall portion connected to the said waveguide section.
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