Patent GB-1231022-A (GB)
IPC: H01Q5/328
Filed: 1968-10-19
Abstract: 1,231,022. Aerials. PLESSEY CO. Ltd. 21 Nov., 1969 [19 Oct., 1968], No. 49707/68. Heading H4A. A dipole aerial arrangement comprises a network incorporating an active impedance-converting device, said network being connected in series with one element of the dipole, and functioning to provide a predetermined feed point impedance/frequency characteristic of said dipole. As described, a dipole aerial placed vertically above a ground plane (9), Fig. 1 (not shown), has an upper element (6) of length l 1  and a lower element (5) of length l 2 , there being a feed point (8) between the two elements. The end of the element (5) nearest the ground plane (9) is connected thereto by a reactive network (10), the reactance of which so varies with frequency that the impedance at the feed point (8) maintains a desired value over a wide band of frequencies. One example of a network which can be used is shown in Fig. 14 and it comprises a wide band amplifier 70 having a voltage gain of + 2, a high input impedance and a low output impedance, and a series resonant type circuit 72 connected as a feedback path. If the impedance of the circuit 72 is + Z, the input impedance of the network is - Z. In another embodiment, the lower element 5, Fig. 3, of a dipole is in the form of a hollow tube 12 which is connected, at its upper end 14, to the outer conductor 16 of a coaxial feeder 18, whose central conductor 20 connects to the upper element 6 of the aerial. The diameter of the conductor 20 may be varied over its length, that is, over the sections l 3 , l 4 , l 5 , in order to improve matching, or the diameter of the said conductor may be uniform (Fig. 2, not shown). In a fourth embodiment, the outer conductor 22, Fig. 4, of a coaxial feeder 24 itself forms the lower element of a dipole aerial which is connected to a ground plane 9 through a reactive network 10. The coaxial feeder 24 is coiled up over a region 26 to form an inductor, and thus provide isolation at radio frequencies of the bottom of the dipole from the ground plane 9. The reactive network and the coiled region of the coaxial feeder may be enclosed in a screening tube placed below and connected to the ground plane (Fig. 5, not shown). Such an arrangement can be used on vehicles and the like, to provide physical protection. In a sixth embodiment, connection of external equipment to the feed point 8, Fig. 6, of a dipole aerial is by means of a twin-wire or twin-strip transmission line 30, one conductor 32 of which provides the lower element 5 of the dipole, and the other of which is connected to the other element 6. The conductor 32 of the line 30 is connected to the ground plane 9 through a reactive network 10, and isolation at radio frequencies is provided by coiling up both conductors of the line over a region 34 to form a bifilar inductor. The sizes of the two conductors of the line may be varied along their lengths to improve matching. The reactive network and the transmission line may be arranged in a screening tube below the ground plane (Fig. 7, not shown). Where no ground plane is available, such as on the mast of a ship, two dipole aerials of the kind shown in Fig. 4, or in Fig. 6, may be combined to form a balanced pair. The two aerials are placed back-to-back in line, and are connected to a common feeder (Figs. 8, 9, not shown). In a tenth embodiment, two dipole aerials of the kind described with reference to Figs. 1 to 7 are spaced apart on the ground plane by a distance equal to a quarter-wavelength at the mid-band frequency (Fig. 10, not shown). The aerials are connected to a common feeder through a phase shifter which provides a 90 degrees phase difference between the signals fed to them. The arrangement may be extended to include more than two aerials, and the phase shift between adjacent elements may be made to change with frequency so as to maintain the directivity constant over a wide frequency band. An eleventh embodiment (Fig. 11, not shown) is similar to the tenth, but utilizes two (or more) balanced pairs of aerials similar to the kind described with reference to Figs. 8, 9. In a twelfth embodiment, a single dipole aerial which is of the kind described with reference to Figs. 1 to 7, and which is driven, is associated with one or more similar aerials which are made parasitic by short-circuiting their feeder terminals (Fig. 12, not shown). It is stated that the directivity of the arrangement can be altered by adjusting the reactance/frequency characteristics of the reactive networks associated with the parasitic members. A similar arrangement is described which utilizes balanced pairs of aerials similar to the kind described with reference to Figs. 8, 9 (Fig. 13, not shown).