Patent GB-1343498-A (GB)
Title: Multifrequency antenna system
IPC: H01Q21/30
Filed: 1971-05-19
Abstract: 1343498 Aerials RCA CORPORATION 19 April 1971 [27 May 1970] 15686/71 Heading H4A An aerial effective over two spaced frequency bands comprises first and second spaced elongated dipole elements which are of differing lengths and are parallel to each other. Each dipole includes two parallel rod-like conductors having central portions with electrical terminals connected to a common transmission line, and oppositely extending outer portions. The first dipole element is of such length as to act as a full-wave resonator at a frequency within the higher of said frequency bands, and the second dipole element is of such length as to act as a half-wave resonator at a frequency within the lower of said frequency bands. The central portions of the rod-like conductors of the second dipole element overlap. thereby providing an interaction with the first dipole to condition the performance of the aerial on the higher of the said frequency bands. As described, an aerial system comprises one portion which is effective in the UHF TV band (470 to 890 MHz) and another portion which is effective in both the high and low VHF TV bands (174 to 216 MHz and 54 to 88 MHz, respectively). Both portions are mounted on a boom 10, Fig. 1, which is carried by a mast 2, and have a common feeder 9. The UHF portion, at the front end of the the boom 10, comprises a sheet metal fan-like dipole element 21 with terminals 23, 25 to which the line 9 is connected, four directors 13, 15, 17, 19 whose extremities are bent backwards towards the element 21, and a corner reflector 27 having two members 28, 29 disposed at about 110 degrees to each other and supporting reflector rods 31, 31a, 32, 32a, 33, 33a. The VHF portion of the aerial system is behind the reflector 27  and comprises an array of four centre-fed dipole elements 55, 57, 59, 61 which increase in length from front to rear, directors 35, 53, and a reflector element 63. The element 55 is fullwave resonant at a frequency in the high VHF TV band and the elements 57, 59, 61 are halfwave resonant at frequencies in the low VHF TV band. The directors 35, 53 have lengths which correspond, respectively, to frequencies at the high end of the low VHF TV band and at the high end of the high VHF TV band. The reflector element 63 has a length which corresponds to a frequency at the low end of the low VHF TV band. The active elements 55, 57, 59, 61 and the parasitic elements 35, 63 each comprise two rod-like conductors which are carried by insulating supports of the kind described in Specification 1,319,798 and which are overlapped by various amounts at said supports, as is shown for the element 57 in Fig. 6, to provide additional shunt capacitances to improve performance and impedance matching. The director 53 is a single rod-like conductor, and the conductors 37, 39 of the director 35 are formed with loops 41, 43. The inner ends of the reflector element 63 and of the director 35 are connected to the boom 10. The conductors of the dipole elements 55, 57, 59, 61 are connected to the conductors 75, 77 of a balanced transmission line. The conductor 75 is above, and the conductor 77 is below the boom 10. A transposition between each successive dipole element is obtained by disposing the right hand conductor of one element in the upper groove of its insulating support (and the left hand conductor in the lower groove), then disposing the left hand conductor of the next element in the upper groove of its insulating support (and the right hand conductor in the lower groove), and so on. The conductors 75, 77 are connected to the terminals 23, 25 by transition sections, and two pairs of quarter-wave stubs, resonant respectively at the low and high ends of the UHF TV band are provided, to prevent coupling of UHF signals into the VHF portion of the aerial system (Figs. 3, 4, not shown).