GB2066580A - Antenna - Google Patents

Abstract

An antenna for use at UHF or low microwave frequencies, and for receiving signals from satellites, has a vertical monopole element (2) mounted on a ground plane disc (1), and a tap element (4) extending from a 50 OMEGA tapping point (5) on the monopole to a insulated feed point (6) on the ground plane disc (1). The position of the tapping point (5) and the other dimensions of the antenna elements are chosen for maximum reduction of spurious responses coupled with optimum impedance matching. A capacitive loading disc (3) is mounted on the monopole (2) to remove the overhead null in the response of a conventional monopole and to reduce the physical height of the antenna. The antenna may be connected via a lambda /4 line to a preamplifier mounted directly beneath the ground plane disc (1) and be provided with a radome. The antenna may be used as part of a three section tuned filter, fig. 3, (not shown). <IMAGE>

Description

SPECIFICATION Antenna This invention relates to an antenna primarily intended for operation at UHF or low microwave frequencies, and to an antenna assembly including the combination of such an antenna and a preamplifier.

An antenna for receiving signals from satellites should have an omnidirectional response pattern, and where the antenna is to be used on a small boat cr vehicle t should be a relatively small size.

Known antennas in this field include a simple vertical half-wave dipole and a quarter-wave monopole mounted above a plurality of horizontal or inclined ground plane elements. Disadvantages of these known antennas and of the monopole in particular are that impedance matching to the receiver is relatively critical, often necessitating the use of a balun with its associated losses, and that due to their use of simple half- or quarterwave elements they have relatively high spurious resonances. A further disadvantage is that it is often difficult to connect a pre-amplifier close to the antenna without creating earth-loop and matching problems.

It is an object of this invention to provide an antenna in which these disadvantages are at least to some extent overcome.

According to this invention an antenna operable at a wavelength A comprises a ground plane, and elongate element electrically connected at one of its ends to the ground plane and extending generally perpendicularly from the latter, the said elongate element being less than ;t/4 in length, and a tap element electrically connected to the elongate element at a tapping point. Preferably, the tap element extends generally parallel to the elongate element and terminates at an insulated feed point on the ground plane. To reduce the length of the elongate element, a capacitive loading element, for example a disc, may be mounted on the distal end of the elongate element. This also has the effect of eliminating a null in the response pattern of the antenna in the direction of the elongate element.

However, introduction of the capacitive loading element requires other measures to be taken to achieve a resistive impedance at the feed point.

Such measures may include the provision of a matching stub at the feed point, but a preferable, simpler solution is to adopt a comparatively small diameter for the elongate element, typically less than 0.004R, depending on the size of the loading disc. The smaller the diameter of the elongate element, the greater Is the inductive contribution at the feed point to cancel out the capacitance of the loading disc. The reduction in the length of the elongate element has advantages in for example ship-borne equipment where minimum wind resistance is an important requirement. The extent to which the size can be reduced depends on the size of the loading disc which may be typically 0.05R to 0.2A.The length of the elongate element can be reduced to A/8 or less in this way whilst still maintaining relatively good matching characteristics.

The ground plane is preferably constituted by a conductive disc of at least three times the diameter of the capacitive loading disc. This arrangement has the advantage that it provides a continuous screen so that a preamplifier or other metallic bodies can be more easily mounted beneath the ground plane without substantially affecting the matching and response pattern of the antenna. The continuous ground plane disc also serves as a convenient physical base for mounting the end of a feed cable and allows the designer considerable freedom in choosing the position of the feed point.

A further advantage arising from the relatively good matching and spurious response characteristics of the antenna is that it can be made part of a bandpass filter comprising the antenna, the feed cable and the input components of a masthead pre-amplifier. The filter is preferably a 3-section filter in which the first tuned section is the antenna itself, the second tuned section is a length of feed cable which has an effective elcctrical length of A14, and the third tuned section is, for example, a tuned microstrip line connected to an initial amplifying device in the preamplifier. An antenna with less predictable and poorer matching characteristics, such as is commonly encountered in the prior art, would severely affect the performance of such a filter.

The invention will now be described by way of example with reference to the drawings in which: Fig. 1 is a perspective view of an antenna in accordance with the invention; Fig. 2 is a diagrammatic section of an antenna assembly including the antenna of Fig. 1; and Fig. 3 is a simplified diagram of a feeder and a portion of a pre-amplifier printed circuit board.

An antenna intended primarily as a shipborne antenna for receiving data signals from a navigation satellite is shown in Fig. 1, and has a frequency of operation of approximately 400 MHz.

For receiving signals from a satellite, the antenna is mounted in the orientation as shown in Fig. 1, and has a radiation pattern which extends laterally to all sides of the antenna as well as directly overhead.

The antenna comprises a horizontal ground plane disc 1 of diameter 240 mm, at the centre of which is mounted a vertical elongate monopole element 2 of length 85 mm (i.e. less than A/8). A horizontal capacitive loading disc 3 of diameter 65 mm (i.e. approximately 0.9A) is mounted at the distal end of the element 2. The joints at each end of the element 2 are electrically conductive.

The feed from the antenna is taken via a tap element 4 which lies parallel to the element 2 and has a horizontal portion which is connected to the element 2 at a tapping point 5. The tapping point 5 is in the top half of the element 2, and in this example is approximatley 60 mm from the ground plane disc 1. The tap element 4 extends from the tapping point 5 to a feed point generally indicated at 6 where the lower end of the tap element is insulated from the ground plane disc 1 by a small low-loss nylon stand-off insulator or bolt 7. A connection to a coaxial feeder cable 8 passes through a hole in the ground plane disc, and the screen of the cable 8 is soldered to the underside of the disc.

The loading disc 3 has the effect of reducing the length of the element 2 required for resonance at 400 MHz. However, it introduces a capacitive reactance at the feed point 6 which is cancelled out by the inductance of the element 2. A substantially resistive feed point impedance at reasonance is obtained if the element 2 is of relatively small diameter, preferably less than 0.04A and typically 2 mm. The element 2 may be screw-threaded to obtain accurate and reliable positioning of the tapping point 5 and the discs during assembly, these components being mounted on nuts screwed to the required positions.

The position of the tapping point is chosen to provide a 50Q output impedance at resonance.

However, all the dimensions given above are interrelated and represent a compromise between a number of requirements such as good matching to 50Q over a narrow band, good reduction of spurious responses below 1 600 MHz, relative compactness, and a substantially uniform response pattern without an overhead null. Any one of the dimensions may be varied to obtain an improvement in one of these characteristics, but at the same time the other dimensions must be altered. An antenna having the dimensions given above can have a S.W.R. (Standing-Wave Ratio) match of better than 1.2:1 over a proportional frequency band of 1.5% of 400 MHz, with no significant spurious resonances below 1 600 MHz.

One disadvantage of choosing a relatively small diameter for the monopole element2 is that the mechanical stability of the antenna is impaired.

This is overcome by inserting a cylindrical polystyrene foam support 9 between the two discs, as shown by dotted lines in Fig. 2, which largely prevents bending or vibration of the element 2. Other low-loss materials may be used for the support.

Fig. 2 shows the antenna of Fig. 1 mounted inside a weatherproof plastics cover or radome which has a conical top half 10 and a dish-shaped lower half 11. The radome also houses a preamplifier 12, which, since it is beneath the ground plane disc, can be mounted relatively close to the feed point without substantially affecting the response pattern of the antenna. Amplified signals from the pre-amplifier are fed to a coaxial output terminal 13 in the base of the radome. To avoid earth loop prcbiems, the case of the pre-amplifier is mounted on a ground plate 14 which is separate from the ground plane disc 1.

The assembly shown in relatively compact compared with most known antennas used for shipborne UHF satellite navigation systems, and is also resistant to corrosion by salt water spray.

Both the antenna and the pre-amplifier are coated with varnish before being fitted into the radome.

The superior matching and spurious response characteristics of the antenna enable it to be used as part of a three section tuned filter, the second and third sections of which are shown in Fig. 3.

Referring to Fig. 3 the cable 2, a connector 15, and the short length 1 6 of conductor on the preamplifier printed circuit board 1 7 form a tuned 50R transmission line of effective length ,I/4.

This line forms the second section of the filter. The third section comprises a printed microstrip line 18 which, together with the input capacitance of a field-effect transistor 1 9 and of a trimmer capacitance 20 constitutes a tuned 1/4 line. the position of the conductor 1 6 on the line 1 8 is chosen to match to 50Q. The printed circuit board 1 7 has a ground plane on its reverse side to which the lower end 21 of the line 1 8 is connected via a plated-through hole.

The antenna described above can be manufactured relatively cheaply since it is compact, it can be easily fabricated to the required accuracy, and has relatively few connections. The provision of a tuned 3-section filter input and the close proximity of the pre-amplifier to the antenna feed-point gives relatively good spurious response characteristics and allows a receiving system including the antenna to have a relatively good noise performance. It should be noted that the feed-point is short-circuited to tne ground plane disc 1 at d.c. resulting in a reduction of signal interference due to precipitation static.

Claims (14)

1. An antenna operable at a wavelength A comprising a ground plane, an elongate element electrically connected at one of its ends to the ground plane and extending generally perpendicularly from the latter, the said elongate element being less than .1/4 in length, and a tap element electrically connected to the elongate element at a tapping point.

2. An antenna according to claim 1 wherein the tap element extends from the tapping point to an insulated feed point on the ground plane.

3. An antenna according to claim 1 or claim 2 including a capacitive loading element at the distal end of the said elongate element and extending laterally thereof.

4. An antenna according to claim 4 wherein the loading element is a disc electrically connected to the distal end of the elongate element and lying in a plane generally parallel to the ground plane.

5. An antenna according to claim 4 wherein the ground plane comprises a disc of a diameter D which is at least three times the diameter d of the loading disc.

6. An antenna according to claim 4 or claim 5 wherein the diameter d of the loading disc is in the range 0.05A to 0.2A.

7. An antenna according to any of claims 3 to 6 wherein the elongate element is a rod having a diameter less than 0.004.

8. An antenna according to any preceding claim wherein the tapping point is between respectively the mid-point and the distal end of the elongate element.

9. An antenna according to any preceding claim wherein the tap element, over the greater part of its length, extends generally parallel to the elongate element.

10. An antenna assembly comprising the combination of an antenna according to claim 2 or any claim dependent thereon and a pre-amplifier mounted on the other side of the ground plane from the elongate element and having an input connected to the feed point by a feeder having an effective electrical length in the region of A/4.

11. An antenna assembly according to claim 10 wherein the pre-amplifier input is a tap on a tuned input line.

12. An antenna assembly according to claim 11 wherein the tuned input line includes a microstrip element on a printed circuit board.

13. An antenna constructed and arranged substantially as herein described and shown in the drawings.

14. An antenna assembly constructed and arranged substantially as herein described and shown in the drawings.