GB2196796A - Antennas and antenna arrays - Google Patents

Abstract

An antenna comprises a support boom 2 and a generally cylindrical sheet-like radiating element 1 coaxially surrounding the boom and having a longitudinal, open ended slot. The feed line 5 for the antenna extends within the boom 2 and emerges transversely to connect to the two opposed edges of the slot. An array is also disclosed which comprises a single support boom and several radiating elements in predetermined angular orientations. <IMAGE>

Description

SPECIFICATION Antennas and antenna arrays This invention reiates to antennas and antenna arrays, and in particular, but not exclusively, to antennas and antenna arrays for emitting and/or receiving horizontally polarised radiation.

A horizontally polarised antenna is normally made from a dipole mounted horizontally. The dipole is usually fed by a coaxial line and supported by a boom at right-angles to it. A number of such dipoles are mounted on a central metallic support pole to produce a horizontally polarized antenna array. The spacing, orientation, method of feeding and distribution of antennas will produce a variety of radiating patterns in azimuth.

An alternative method of making a horizontally polarized antenna is by forming closed slots in flat and curved conducting surfaces, fed by coaxial or parallel lines. Commonly, a number of such closed slots are made in a single long cylinder, each occupying a specific length of the cylinder. Again, the distribution, orientation and excitation of the slots will produce arrays with different radiating patterns in the azimuth.

The former arrangement requires a large number of radiating dipoles to produce a worthwhile gain. The latter arrangement is very difficult to construct, adjust electrically and protect environmentally. The horizontal dimensions of both designs are large compared with electrical wavelength, resulting in excessive weight and wind loading.

According to one aspect of this invention, there is provided an antenna including an electrically conducting support element extending along a longitudinal axis, a radiating element of sheet-like form generally surrounding said support element and having two spaced edge regions defining an open-ended slot extending in a generally longitudinal direction, and electrical feed means connected across said slot.

By way of example only, two specific embodiments of the invention will now be described in detail, reference being made to the accompanying drawings, in which: Figure 1 is a perspective view of an embodiment of a single antenna for use in an array and incorporating a parallel feed; Figure 2 is a perspective view of an array comprising four antennas and incorporating a coaxial cable feed; Figures 3a and 3b illustrate the wiring harness for the array of Fig. 2 and a balun arrangement for the harness respectively.

The described embodiments of the invention are intended to operate as an omnidirectional antenna for fixed link telemetry for the 450-470 MHz band, although it will be appreciated that the invention can be applied to other applications and frequency bands.

The radiating element 1 comprises a flat sheet of conductive material, such as aluminium, rolled in the shape of a cylinder approximately 76mm in diameter and 380mm long and the edges of the sheet form a longitudinal parallel side gap extending along the whole length of the cylinder. The width "g" of the slot is selected in accordance with the impedance requirements of the feed and the operating frequency of the antenna. The parameters above have been chosen to provide an individual antenna impedance of 50 ohm.

Other combinations of length, diameter and width can also be chosen to provide pre-specified impedances in the operating frequency band, as will indeed be required for the examples of Fig. 1. The length of the cylinder is typically about one half of the :wavelength of the operating frequency, whilst the diameter is again selected in accordance with the desired operating frequency.

The central support boom 2 is made from conductive material such as brass, and is generally tubular in cross-section. A hole 3 is drilled through one side of the tube, facing the slot in the radiating element 1 centrally. The hole is of sufficient diameter to allow feeder wires 4 to pass through and be bonded to the radiating element 1. Bonding may be achieved by soldering or riveting. In Fig. 1 the single radiating element 1 is fed via a pair of parallel lines 5, held in position using PTFE spacers 6. The section of the feeder passing through the hole 3 must have thick insulating material to prevent arcing across to the boom 2.

A radiating element described thus, when mounted vertically, radiates a horizontally polarized wave; i.e. the electric field is exciusively in the horizontal plane. The maximum field occurs in the azimuth plane, decreasing steadily in elevation is a sinusoidal manner to zero pointing vertically upwards. In the azimuth plane, the field intensity has a maximum in the direction of the feed point; it decreases gently as a function of bearing. This variation is fairly small and the antenna may be said to be omnidirectional. A major advantage of the described arrangement is the lack of vertical electric components. Typical measured values of the vertical electrical component are in excess of 20 decibels below the horizontal component. This is a very useful feature, allowing polarization protection and re-use of the same frequency channels in closely located communication cells.

In modern antenna design, a coaxial cable feed arrangement is preferred, as detailed in Fig. 2. In this Figure the antenna comprises four radiating elements 1, which are disposed uniformly along a central conductive boom 2 at approximately 530mm intervals, the radiators being held in position using non-conductive spacers 7. The spacers are machined carefully to be bonded to the radiating ele ments 1 and the conductive boom 2; they have machined grooves fitted with '0 rings 8 which slide snugly in a protective shroud 9.

The shroud 9 is made from a non-conductive material such as GRP and provides environmental protection.

As shown in Fig. 3b the radiating elements 1 are fed through a coaxial feeder 10 and balun 11 cables. The feeder terminals are usually riveted centrally to either side of the slot of the radiating element 1. The outer of the feeder and balun cables are connected electrically to each other and soldered to the central boom 2. Short circuit stub 27 provides additional tuning and a DC short to drain any likely static charges that might collect on the radiators.

The antennas are mounted on the central boom at 90 degree angular intervals and stacked vertically with uniform spacing.

The four coaxial feeders 10 are combined together in the usual fashion, the upper pair onto coaxial cable link 12 and again to the main, larger diameter coaxial feeder 13. The feeder is finally terminated in a good quality connection 14. The feeder harness is thus totally isolated from the antenna radiating currents by virtue of the conducting boom 2, a feature usually absent from commonly available antennas.

The protective shroud 9 has a top cap 15 which prevents ingress of moisture into the antenna. A separate lightning spike 16 made from conductive material, is screwed to the top of the central boom 2 via a threaded insert 17, studding 18 and a nut 19. The lightning rod is detachable to minimise packing size. The enclosure is screwed to the cap at 20, and a spacer '0' ring 21 is employed to seal the connection and dampen any likely vibration.

The array includes a mounting stub 22 which is normally manufactured from conductive material; the stub 22 comprises a scaffold size tube 23 welded to a machined cylindrical block 24. Grub screws are provided to locate and lock the central boom 2 in position. A drain hole of sufficient diameter is also provided in the main block, to prevent the buildup of moisture inside the enclosure. The enclosure is screwed to the block at 25 and an '0' ring 26 is used to take up the slack between the shroud 9 and the block and dampen any likely vibration. The stub 22 is made sufficiently long to minimise interference between the antenna proper and the mounting structure.

The measured gain of a single radiator as illustrated in Fig. 1 is the same as a single dipole (0 dB). The array of four antennas as shown in Fig. 2 has a gain of 6dBd less 1dB feeder losses. The horizontal radiation pattern of the array is uniform with a variation of -1+ 1dB. The cross polarized field strength is 20dB lower than the horizontal component.

For most practical applications, the number of stacked radiators will be between 1 and 16. At low frequencies, one or two will be employed; at few GHz's, up to 16 radiators can be stacked and contained in a single enclosure.

The described arrangements provide a new antenna providing a horizontally polarized wave and a new method of mounting several such antennas on a conductive boom, thus providing additional gain. By choosing the correct number of elements and method of excitation, the pattern may be controlled to meet specific requirements. The described arrangements have applications across a wide frequency spectrum from 100 mHz and up to several GHz.

Typically, omnidirectional radiation is obtained through the use of four radiators on a boom, positioned at equal angular intervals, fed via coaxial feeder lines and coaxial baluns.

The specific example detailed has an operating frequency of 460 MHz with a frequency bandwidth of 20 MHz; omnidirectionality -1+1dB; a voltage standing wave ratio of 1.5:1; a gain of 5dBd and with DC static discharge paths provided. The example incorporates a robust method of construction, allowing many years of trouble-free operation.

An additional feature of the invention allows a lightning finial to be fitted at the top of the antenna. This invention provides horizontally polarized antennas of very slim-line form, thus providing significant reduction in weight and, more importantly, in wind loading.

The radiation pattern of a single antenna as described, is slightly directional in azimuth with a forward gain of three decibels. Omnidirectional radiation is obtainable by orientating a number of radiators at uniform angular intervals and feeding them uniformly. Conversely, a unidirectional pattern may be obtained by controlling the phase and amplitude of the signals fed to the various array members.

In the arrangement having one or more antennas and antenna arrays mounted on a single vertical boom, with one or several coaxial feeders extending downwards, a feeder can be connected to a single element operating on one frequency, connected to several similar elements to provide additional gain, or connected to two different types of radiators through a suitable duplexer to provide multifrequency operation.

Claims (7)

1. An antenna including an electrically conducting support element extending along a longitudinal axis, a radiating element of sheetlike form generally surrounding said support element and having two spaced edge regions defining an open-ended slot extending in a generally longitudinal direction, and electrical feed means connected across said slot.

2. An antenna according to claim 1, wherein said support element is of tubular form and said electrical feed means passes through said support element.

3. An antenna according to claim 1 or claim 2, wherein said radiating element is of generally cylindrical form, concentric with said support element.

4. An antenna array, comprising a plurality of antennas as defined in any of the preceding claims stacked end to end and wherein the respective support elements comprise a single support element.

5. An array as claimed in claim 4, wherein the slots defined by each radiating element are arranged at a predetermined angular displacement with respect to that of the or each adjacent radiating element.

6. An antenna substantially as hereinbefore described with reference to the accompanying drawings.

7. An antenna array substantially as hereinbefore described with reference to the accompanying drawings.