GB2168538A - Mixed polarization panel aerial - Google Patents

Abstract

A rectangular conductive loop (10) composed of vertical segments (11, 12) and horizontal segments (13, 14) all half a wavelength long is mounted one quarter of a wavelength in front of a reflecting screen (15). The segments (11 to 14) are centrally fed by feeders (17) such that the feed phase advances 90 from segment to segment round the loop. A second conductive loop, in parallel with the first, may be provided, the outer tubes 19 for the feeders 17 being connected to the segments of the second loop. The horizontal radiation pattern is a cosine-squared pattern both for horizontally and vertically polarized components. Up to four such panel aerials may be mounted around a tower to provide omnidirectional or directional coverage for both polarizations. <IMAGE>

Description

SPECIFICATION Mixed polarization panel aerial The present invention relates to a mixed polarization panel aerial comprising a radiating structure spaced in front of a reflecting screen and a feed arrangement for the radiating structure. Such aerials are of particular use mounted on towers in groups, to achieve omnidirectional or specified mixed polarization coverage. They may operate at VHF, e.g.

Band II but the invention is not limited to any particular frequencies of operation. Furthermore, although reference has already been made to "a feed arrangement" and similar terminology is employed in the appended claims, this is for convenience of definition. As is well known, aerials are reciprocal devices and may be used either to transmit or receive. The present invention is not limited to transmission aerials.

The need for mixed polarization aerials arises in the United Kingdom, for example, because VHF radio was originally designed for horizontal polarization but vertical polarization has been added for the benefit of portable and car radio receivers. Moreover, in Band II, wideband aerials are required to cover the new frequency range of 88 to 108 MHz (about 20% bandwidth).

Mixed polarization panel aerials developed by several manufacturers successfully meet current requirements where omnidirectional coverage is required. A transmitter aerial may consist of a plurality of stacked tiers, each of which consists of three panel aerials looking in directions spaced at 1200. The radiating structure of each panel consists of crossed halfwave dipoles driven in phase quadrature and providing a horizontal radiation pattern falling to half amplitude at +60 .

In the United Kingdom, main VHF transmitters normally require omnidirectional radiation but many medium and low power transmitters require directional horizontal radiation patterns.

For these purposes, three panels per tier is not good enough. Four panels are typically required and the horizontal pattern of each panel should then fall to half amplitude at +45%.

A particular problem with mixed polarization aerials lies in the requirement to maintain the correct relationship between different polarization components, such as equality between vertical and horizontal polarized feed components. The known panel aerials cannot meet such requirements satisfactorily when designed for use four in a tier.

The object of the present invention is to provide an improved aerial which overcomes this problem. The aerial according to the invention is defined in the appended claims.

The invention will now be described in more detail, by way of example and with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a single panel aerial embodying the invention, Figure 2 is a diagrammatic side elevation of the panel aerial showing the electrical feed arrangement, Figure 3 comprises a series of explanatory diagrams, Figure 4 is a diagrammatic plan view of a tower aerial tier using four of the panel aerials, and Figures 5 to 8 show various horizontal radiation patterns.

Fig. 1 shows a single panel aerial comprising a square conductive loop 10 composed of two vertical conductor segments 11 and 12 and two horizontal conductor segments 13 and 14. These segments may be lengths of metal rod or tube conductively connected at the four corners of the square. The side of the square is half a wave length long at the mean design frequency for the aerial. The square conductive loop 10 is arranged in front of and parallel to a reflective screen 15 formed of wire mesh for example. The conductive loop 10 is supported by insulating rods 16 a quarter of a wave length away from the screen 15.

The conductive segments 11 to 14 are individually fed at their centres by four feeders 1 7. In the preferred arrangement (Fig. 2) each feeder 17 comprises a coaxial cable transmission line 18 passing from the centre of the screen 15 through a length of tube 19.

Before considering this feed arrangement in more detail, attention will be given to a different form of feed arrangement illustrated in Fig.

3(a) and which facilitates an explanation of the principle of operation of the aerial. In the arrangement of Fig. 3(a) the vertical conductive segments 11 and 12 are fed centrally from the ends of the two elements 20 and 21 of a centre driven half wave dipole (the drive to which is symbolized by a current generator 22). Similarly the horizontal segments 13 and 14 are driven by the elements 23 and 24 of a second half wave dipole.

Although the dipole elements are joined together by the conducting ring, the individual elements are independent of each other and it is therefore possible to consider the operation of one drive point alone, as shown in Fig.

3(b). Fig. 3(c) shows the expected current distribution. It can be seen that, in a direction normal to the plane containing the elements, the vertically polarized field will consists of the sum of the field components due to the currents of the dipole elements 23 and 24 and the currents on the vertical segments 11 and 12 of the conductive loop. In the horizontal direction in the plane of the elements, the field is due to the dipole elements 23 and 24 alone, because the contributions from the vertical segments 11 and 12 (spaced by half a wavelength in the said direction) cancel each other. The effect is to increase the directivity of the vertically polarized horizontal radiation pattern. However, the shape of the radiation pattern in the vertical plane is virtually unchanged from that of a single dipole.When both drives are applied as in Fig. 3(a), the horizontal and vertical polarized components in the horizontal plane are almost equal in magnitude.

Although the aerial is envisaged for use in VHF Band II, experiments have been carried out using a half size aerial operating in VHF Band Ill. Fig. 5 shows the horizontal radiation pattern measured at 850 MHz for such a panel aerial. It will be seen that the measured values approximate very closely to a cosinesquared pattern with half amplitude points at + 45 .

The input impedance of the arrangement illustrated in Fig. 3(a) is high and the impedance matching provided accordingly gives a limited bandwidth. Wideband performance can be obtained by using two drive points for each dipole, placed between the elements of the dipole and the corresponding conductive segments of the loop 10, where the impedance levels are low. To maintain the same current distribution as in Fig. 3(c), quarter wavelengths stubs are used in place of the central drive points and antiphase drives are applied to opposite conductive segments. The practical arrangement which results is as shown in Fig. 2 with the coaxial cables 18 connected through the tubes 19 and one cable 18 being half a wavelength longer than the other from a common feedpoint 25. For circular polarization, the feeds are arranged in phase rotation around the loop as symbolized in Fig. 3(d).

Fig. 4 is a diagrammatic plan view of an omnidirectional tower aerial comprising four panel aerials as in Figs. 1 and 2 mounted around a central tower 26 by support struts 27. Each panel aerial is represented solely by its reflecting screen 15 and conductive loop 10. It was found that the horizontal radiation pattern for this omnidirectional aerial was improved by adding diagonal conductive screens 27 extending outwardly from the junctions between the reflecting screens 15 and effectively partitioning the roots of the radiating beams of the four panel aerials. Fig. 6 shows the horizontal radiation patterns of fthe experimental aerial with such screens, at 200 MHz Although the panel aerials may be used very satisfactorily in an omnidirectional tower aerial, they are also particularly useful in creating directional patterns.Fig. 7 shows the horizontal radiation pattern at 200 MHz of just two panel aerials looking at 90" to each other and providing reasonably uniform coverage over nearly 1800. Fig. 8 shows a biased pattern obtained using all four panel aerials driven with unequal current magnitudes as indicated on the drawing.

As already explained with reference to Fig.

2, the two opposite conductive segments of a pair, such as the segments 13 and 14, are driven in antiphase using unequal lengths transmission lines 18. Considering the drive arrangement for both pairs of conductive segments, an additional quarter wave length of cable is required in the feed to one of the pairs of segments. This may give rise to some inequality between vertically and horizontally polarized components. In a tower aerial having a plurality of tiers of panel aerials, each tier as in Fig. 4, the feeds to the elements of the panels in successive tiers can be rotated 90" to give an effective advance in phase, this being compensated by adjusting the phases of the feeds to the tiers. This has the effect of averaging the differences between the horizontally and vertically polarized components and improves the input impedance of the whole aerial.

An alternative method of driving a single tier is to use OdB output-ratio couplers to feed the relevant elements of each panel, e.g. the direct output to feed the horizontal elements and the coupled output to give the quadrature feed to the vertical elements.

In the embodiment described with reference to Figs. 1 and 2 the square conductive loop 10 is electrically floating and derives mechanical support solely from the insulating rods 16.

In a modified structure it is possible to achieve a structure which is mechanically stronger and less likely to suffer from lightening. A second square conductive loop is added parallel to the loop 10. The distal ends of the tubes 19 are braized or otherwise bonded to the centre points of the segment of the second loop. Conductive bridges are provided between the two loops at the four corners thereof. The resulting structure has double shunts studs across the drive points to the loop 10. Both loops are rigidly supported by the tubes 19. Insulating rods 16 may be retained for further strength or may be found to be superfluous.

Claims (15)

1. A mixed polarization transmitting or receiving panel aerial comprising a radiating structure spaced in front of a reflecting screen and a feed arrangement for the radiating structure, wherein the radiating structure is composed of a plurality of conductive segments connected into a continuous conductive loop and the feed arrangement feeds into the segments with such relative phases that the radiating structure as a whole radiates with mixed polarization.

2. An aerial according to claim 1, wherein the conductive segments consist of pairs of opposite, parallel straight segments and the feed arrangement feeds into the two segments of each pair in antiphase.

3. An aerial according to claim 2, wherein the feed arrangement comprises dipoles indivi dual to the segment pairs, the ends of each dipole being connected to the segments respectively of the corresponding segment pair.

4. An aerial according to claim 1, 2 or 3, wherein there are four conductive segments in a rectangular loop.

5. An aerial according to claims 1 to 4, wherein the feed phases alter progressively round the loop from segment to segment.

6. An aerial according to claim 1, 2, 4 or 5, wherein the feed arrangement comprises transmission lines individual to the segments and feeding directly into the respective segments.

7. An aerial according to claim 6, wherein the said relative phases are established by the lengths of the transmission lines.

8. An aerial according to claim 6, insofar as dependent on claim 4, wherein the feed arrangement further comprises an OdB outputratio coupler with a direct output feeding one pair of elements and a coupled output feeding the other pair of elements.

9. An aerial according to any of claims 1 to 8, comprising a second continuous conductive loop parallel to the first said loop, the two loops being bonded together electrically t the junctions between the segments and the feed arrangement comprising outer conductors whose distal ends are electrically bonded to the centres of the segments of the second loop.

10. An aerial according to claim 9, wherein the said outer conductors are rigid tubes providing support for the two loops.

11. A tower aerial comprising a plurality of panel aerials according to any preceding claim mounted on a support to beam outwardly in mutually divergent azimuthal directions.

12. A tower aerial according to claim 11, wherein the said directions pertaining to adjacent panel aerials are at 90 to each other.

13. A tower aerial according to claim 11 or 12, comprising conductive screens extending outwardly from junctions between the reflecting screens of the panel aerials so as to partition the roots of the radiating beams of the panel aerials.

14. A tower aerial according to claim 11, 12 or 13, comprising a plurality of vertically stacked tiers of the panel aerials.

15. A tower according to claim 14, wherein adjacent tiers are fed 90O out of phase with each other and the feed arrangements within the tiers are such that all panel aerials having the same azimuthal direction are fed in phase.