substantially coextensive with the longitudinal surface 3 of the resonator 2 and extending slightly beyond the width of the slot 6 the portion 10 of the feed line 9 extending beyond the slot 6 being defined as the u201c overhang u201d all but the end region 7 of the longitudinal surface 3 of the resonator 2 is painted with a metallised paint 11 as shown in fig4 the metallised paint 11 may be loaded with silver or other metals and is preferably applied as two coats the end region 7 of the longitudinal surface 3 may be masked prior to painting so as to keep the end region 7 free of paint 11 furthermore the longitudinal surface 3 is adhered to the grounded substrate 1 by way of a metallised adhesive 100 which may also be loaded with silver an embodiment of the present invention that has been constructed and tested by the present applicant will now be described a half split cylindrical ceramic dielectric resonator 2 having a relative permittivity of approximately 110 a radius of 7 5 mm and a longitudinal surface 3 of length 20 mm by width 7 mm was fitted onto a grounded substrate 1 having a.