The prior art split-ring resonator for use in an electron spin resonance (ESR) spectrometer comprises a circular cylinder made from an electrically conductive material and housed in a metallic tube. The cylinder is provided with a split extending axially of the cylinder. A sample to be investigated is inserted into the cylinder in the axial direction. In accordance with the invention, a shielding member which is wider than said slit and made from an electrically conductive material is disposed so as to shield the slit from the inside of the cylinder, whereby the resonance frequency of the resonator is not significantly affected by the presence or absence of the sample or by the amount of the inserted sample.

A dielectric resonator is used in a magnetic resonance examination apparatus comprising a magnet system (31, 32) for generating a steady magnetic field in an examination space (2), a transmitter device (12, 13, 18, 28) for generating an RF field to be superposed on the steady magnetic field in an object (30) to be examined, a device for producing a resonance step-up of the RF field active in the object (30) to be examined, and a device for detecting magnetic resonance signals generated in the object (30) to be examined. Stronger B.sub.1 fields are created by at least one dielectric resonator (1, 5, 10, 16, 17, 20, 21) which neighbors the object (24) to be examined and which comprises a dielectric having a relative dielectric constant .epsilon..sub.r >20, the dimensions of the resonator being chosen so that therein, using a neighboring transmitter device, (12, 13, 18) there can be generated at least one resonant mode with a frequency which is in the vicinity of the Larmor frequencies to be detected of the nuclei to be excited in the object (24) to be examined, for example, protons.

An RF probe for acquiring MR data comprising four loop means. There are first and second loop means that are substantially round, centered about a longitudinal axis of said probe and spaced apart substantially the length of said probe to define a cylindrical shape therebetween. There are third and fourth loop means being substantially saddle shaped spaced apart, oppositely disposed and extending between said first and second loop means. Conductors attached to said first and second loop means at points on said first and second loop means spaced apart by 180 degrees couple said third and fourth loop means to said first and second loop means to form a point of psuedo-saddle coils providing extremely homogenous signals.

A multiplexer comprises a number of directional filters 17.sub.n connected to a transmission line feeding an antenna 2, 3. Signals to be multiplexed fed to the filters 17.sub.n may be sent via respective switches 16.sub.n. Unlike prior multiplexers where each directional filter defines a respective channel of the multiplex, the channels of the multiplexer of the invention, apart from one at the end, are defined by the band pass response of one directional filter and the band stop response of another directional filter, since the band pass responses of the directional filters from the input connected to the switch to the output connected to the transmission line, and the corresponding band stop responses between the two output ports connected to the transmission line, overlap each other. The same arrangement may be used for demultiplexing.

An extremely small and inexpensively manufactured physics package for an atomic frequency standard can be provided with a microwave cavity having non-critical dimensions that is driven in a substantially TEM mode by a lumped LC means, the cavity resonant frequency being primarily determined by the lumped LC means. The lumped LC means can be any structure or combination of elements providing, at a selected microwave reference frequency, a resonant inductance and capacitance. Examples of such lumped LC means include, preferably, a rod or wire conductively attached to a wall of the microwave cavity as a lumped inductance and extending into the cavity to form, at its other end, a gap with an opposing cavity wall as a lumped capacitance; or a pair of rods or wires conductively attached to opposing walls and extending therefrom as a lumped inductance to form a gap therebetween as a lumped capacitance.