GB1469156A - Passive antenna element - Google Patents

Abstract

1469156 Passive aerial element HARRIS CORP 8 May 1974 20368/74 Heading H4A A passive aerial element comprises at least three electrically conductive arms spaced from each other, and extending radially and circumferentially relative to a common axis, the inner ends of the arms being rotationally displaced about said axis relative to each other by a given angle to achieve a given rotational phase progression about the common axis of received circularly polarized radio waves. The element also comprises means to control the phase relation of waves to be radiated therefrom, including means for interconnecting at least one pair of either inner or outer arm ends so that electrical signals therein are interchanged with a relative phase change dependent upon the rotational phase relation between the interconnected arm ends. The embodiments described all feature four-start spiral elements, which may each be formed by a printed circuit process from a copper layer or a plastics substrate. A number of such elements may be arranged on a parabolic contour and collimate circularly polarized waves from a focally disposed horn-type primary radiator (Fig. 1, not shown). The direction of the emergent beam may be steered by suitably operating the phase control means of the various elements of the array. A planar lens array may also be constructed using a number of the elements. In this case the primary horn is mounted behind the array, and the collimated beam emerges from the front of the array (Fig. 2, not shown). In one embodiment four like spiral arms with respective outer terminals T1, T2, T3, T4, Fig. 6, have their inner terminals connected to a network of switching diodes. Each outer terminal is connected to a respective bias switch, by means of which positive, negative or zero voltages may be applied. For example, when the terminal T2 receives a positive voltage from switch 102, and the terminal T4 a negative voltage from switch T4, no voltage being applied to the terminals T1, T3, the diode pairs B, D, E, F, G, H, are conductive and the diode pairs A, C are non-conductive. The inner terminals of the spirals T2, T4 are thus effectively connected. The relative phase of the group of transmitting currents for this condition may be termined 0 degrees, or reference phase condition. When terminals T1, T3 are respectively positive and negative, with zero bias on T2, T4, only diode pairs B, D are non-conductive and the relative phase of the transmitting currents is 180 degrees. When T1, T4 are positive and T2, T3 are negative, only diode pairs F, H are non-conductive, and the relative phase of the transmitting currents is 90 degrees. When T1, T2 are positive and T3, T4 are negative only diode pairs E, G are non-conductive, and the relative phase of the transmitting currents is 270 degrees. The spiral arms are configured so that they define an element for operation with received waves of left-hand circular polarity, and the reradiated waves are of like polarity. If operation with right-hand circularly polarized waves is required, suitable phase control means must also be provided at the outer ends of the spirals, and in one embodiment, as shown in Fig. 7, said means comprises additional independently operable switching diodes. The inner ends of the spirals are connected to a network of switching diodes fulfilling the same purpose as the network of Fig. 6. An inner bias control means is connected to the outer end of each spiral through a quarter wavelength line, for RF isolation. The outer end of each spiral is also connected through a D.C. blocking capacitor to a respective switching diode A<SP>1</SP>, C<SP>1</SP>, E<SP>1</SP>, G<SP>1</SP>, the other side of which is earthed, and which may be rendered conductive or non-conductive by a respective bias control means 110A, 110C, 110E, 110G. Between each pair of switching diodes is a further switching diode which is connected to them by quarter wavelength sections of a common transmission line TL. These further diodes B<SP>1</SP>, D<SP>1</SP>, F<SP>1</SP>, H<SP>1</SP> have respective bias control means 110B, 110D, 110F, 110H, for rendering them conductive or non-conductive to earth. Diodes A<SP>1</SP>-H<SP>1</SP> may be selectively operated so that a right hand circularly polarized wave is re-radiated with a relative phase change of 0, 90, 180 or 270 degrees.