GB1523115A - Aerial - Google Patents

Abstract

1523115 Multi-frequency aerial system ANTENNA SPECIALISTS UK Ltd 19 Dec 1975 [24 Sept 1974] 41418/74 Heading H4A An aerial system for operation at two or more frequencies comprises a first conducting part tuned to a first frequency, and a second conducting part linked to the first by a length of transmission line. The length of the line is selected such as to provide a high impedance at the first frequency so that the second conducting part is then non-operative, and its characteristic impedance is chosen so that the line, together with the two parts, resonates at a desired second frequency. In the Fig. 2 embodiment, the first conducting part is a centre-fed tubular dipole 12, 13 within a glass fibre tube 10 and having a downlead 15. Bayonet fittings 4a, 3b and 5a, 4b join this assembly mechanically to a lower elongate support (not shown) and to an upper aerial element so that the whole system is of vertical whip antenna form. The bayonet fittings also constitute the second conducting part and are linked to dipole elements 12,13 by short-circuited (or open-circuited) lengths of transmission line 16, 17 so that dipole 12, 13 is resonant at one frequency, whereas the dipole plus bayonet fittings plus transmission lines together resonate at a second frequency. The upper aerial element includes a loading coil 20 and is tuned to a third frequency. The transmission lines 16, 17 may be of coaxial form as shown, or of flexible coplanar microstrip form. In the latter form, the widths and spacings of the microstrip conductors are chosen to provide a suitable characteristic impedance. Aerial dimensions and operating frequencies are specified.