the assembly being held in place by an appropriate dielectric mechanical structure which is omitted from the figure for the sake of clarity an element 62 having high self inductance is also placed within the cavity 17 and is connected electrically by each of its ends 63 and 64 to the two edges 65 and 66 of the slit 61 thus the inductance of the circuit lc is increased resulting in a decrease in its resonance frequency f 0 and an increase in its bandwidth u03b4f 0 in terms of construction the element 62 may consist of a single assembly extending over the whole length of the cavity 17 or as shown in fig6 may consist of a plurality of elements arranged in sequence with each other each element being connected by its ends to the edges of the slit 61 according to a sixth variant aspect shown in fig7 each resonant element has a slit 61 formed on one of the lateral faces 16 of the resonator together with separate inductances 71 which are preferably distributed over the whole length of the slit and each of which has its terminals connected to the two edges 65 and 66 of the slit 61 as in the previous case this arrangement increases the inductance of the circuit.