GB2251981A - Antenna for frequency agile transmitter - Google Patents

Abstract

To achieve a broad frequency bandwidth, especially for use with a frequency agile radio transmitter, an antenna may comprise a riser element 1 with a planar top loading element, 2, typically a capacitive disc, attached to the top of the riser element. However, the performance of such antennae, is limited by the necessary presence of inductive couplers between elements. Herein, a riser element 1 is coupled to a top loading element 2 which comprises folded portions e.g. a spiral and includes 1 or more electrical switchable bridging elements extending between different regions of the top loading elements. The bridging element provides direct electrical connections between the different regions so enabling the resonant frequency of the antenna to be switched more rapidly and efficiently. <IMAGE>

Description

ANTENNA SYSTEMS The present invention relates to antennas and in particular to compact antenna systems suitable for use with a frequency agile radio transmitter.

Frequency agile or frequency "hopping" radio equipment is widely used, particularly for military applications where security is important. In order to couple efficiently to a frequency hopping transmitter an antenna must have either a broad band match across the hopping frequency range or alternatively the ability to tune to the different hopping frequencies at a speed significantly faster than the hop speed. In many applications a further constraint on the design of the antenna results from the need to retain a low visual profile. Typically a reduced height antenna, that is an antenna with a height less than one quarter of its operational wavelength, is used. The construction of such reduced height antennas involves the use of reactive components and hence they in general have a comparatively narrow operational bandwidth.This effectively rules out the option of using a broad band match to the entire frequency range and makes it necessary to provide the antenna with some form of rapidly switchable tuning.

It is known to construct antennas with a planar top loading element mounted on the end of a rising element. In known designs the top loading element is typically formed from a capacitive disc. One example of such an antenna is. disclosed in US-A-4201989. Although this antenna is able to cover a relatively wide bandwidth its performance is limited by the need to include an inductive coupler between the top element and the rising element.

According to the present invention a frequency agile antenna system comprises a rising element and a top loading element, the top loading element being folded and including at least one electrically switchable bridging element extending between different regions of the top loading element, the bridging element being operable to provide a direct electrical connection between the different regions of the top loading element so changing the resonant frequency of the antenna system.

The present invention provides an antenna system which, is physically compact and capable of switching rapidly between different frequencies, making it ideal for use with frequency hopping transmitters.

Preferably the folded top loading element comprises a planar spiral and the at least one bridging element extends between radially adjacent arms of the spiral. The bridging element may comprise a reed relay but preferably comprises a PIN diode arranged when forward biassed to provide a short circuit at radio frequencies and when reverse biassed to provide an open circuit at radio frequencies. Preferably a connector for the bridging element is formed integrally with the top loading element.

Preferably the system includes a number of bridging elements arranged around the folded top loading element, the intervals between the bridging elements being such that as successive elements are switched the antenna is tuned to adjacent overlapping frequency ranges.

By using an antenna with a spiral configuration and by placing bridging elements at appropriate points along the spiral it is possible to provide overlapping coverage of a wide range of frequencies.

Preferably the control wires connected to respective bridging elements extend down the rising element and are would into a coil at the base of the rising element arranged to isolate the control wires from the RF feed to the antenna. Preferably the coil is wound concentrically with the rising element towards the bottom of the rising element. Preferably the antenna system is an end-fed monopole.

Preferably the rising element and top element are surrounded by an integral cover formed from anti-ballistic material.

A system in accordance with the present invention will now be described in detail with reference to the accompanying drawings in which: Figure 1 is a plan; Figure 2 is a side elevation; Figures 3a and 3b are diagrams showing alternative configurations for the top loading element; Figures 4a to 4d are diagrams showing the equivalent antennas; Figure 5 is a graph showing VSWR bandwidths; and Figure 6 is a diagram showing switching circuits for the The antenna can be tuned to any frequency, say fx, between and fH by placing an appropriate number of bridging elements to act as shorting links from position AA to xx.

Ideally for use with a frequency hopping transmitter or receiver the antenna system should provide continuous coverage of a band from, for example, 30-88MHz. However, all electrically short antennas tend to have narrow VSWR bandwidths. With the dimensions used for the antenna, the 3.1 VSWR band width is barely 3% at the lower frequencies.

Consequently continuous VSWR coverage from 30-88MHz is not possible with just one shorting link. Several shorting positions that are switchable are needed to enable the antenna to be tuned throughout the band. The appropriate shorting positions are found by the following procedure.

If for example, the antenna is tuned to the frequency fx by means of a shorting link at position xx, then the antenna VSWR plot would then be that shown by the solid line in Figure 5.

The centre frequency is fx, and the 3:1 upper and lower bandwidth edges are shown on fxL and fxH. A new shorting position YY can be found to tune the antenna to a new resonant frequency of fY. The VSWR plot is shown in dashed lined in Figure 5. If the position YY is chosen carefully, the new lower 3:1 bandwidth fYL can be set at the same point as the old upper 3:1 band frequency fxH. It can be seen that with just these two switch positions the antenna will now be able to cover the entire band from fXL to fYH. A similar exercise can then be carried out at position CC to extend the switched coverage from approximately 2.5m. The resonant frequency of this combination is around 30MHz. In order to change the resonant frequency either the height of the rising element or the top section length might be altered.As in practice the height is fixed, the antenna is tuned by using the bridging elements to change the effective length of the spiral antenna 3.

The spiral is shown in detail in Figure 3. When all the bridging elements are OFF the antenna is set at its lowest frequency of tune fL. If the spiral from AA to Z is unravelled the equivalent antenna shown in Figure 4a is formed. The variation of the length AA to Z effectively fixes the resonant frequency.

Moving outwards along the track from AA at the centre of the spiral position BB is reached. If a solid shorting link is put across BB the section from AA to BB effectively looks like a solid plate at radio frequencies. Also the remaining length of the spiral to Z has changed. If the spiral arm is unravelled from point BB then the equivalent antenna is that shown in Figure 4b. It can be seen that the length BB to Z is much shorter than the length AA to Z. This arm length is the dominant factor in determining the antenna resonance. As a result the antenna is now resonant at a higher frequency.

Again, travelling further to point CC and placing a shorter link there produces the equivalent antenna shown in Figure 4c The antenna is now tuned to an even higher frequency. The highest frequency f is obtained when the entire spiral from AA to Z is shorted out. The equivalent antenna is shown in Figure 4d.

The antenna can be tuned to any frequency, say fx, between and fH by placing an appropriate number of bridging elements to act as shorting links from position AA to xx.

Ideally for use with a frequency hopping transmitter or receiver the antenna system should provide continuous coverage of a band from, for example, 30-88MHz. However, all electrically short antennas tend to have narrow VSWR bandwidths. With the dimensions used for the antenna, the 3.1 VSWR band width is barely 3% at the lower frequencies.

Consequently continuous VSWR coverage from 30-88MHz is not possible with just one shorting link. Several shorting positions that are switchable are needed to enable the antenna to be tuned throughout the band. The appropriate shorting positions are found by the following procedure.

If for example, the antenna is tuned to the frequency fx by means of a shorting link at position xx, then the antenna VSWR plot would then be that shown by the solid line in Figure 5.

The centre frequency is fx, and the 3:1 upper and lower bandwidth edges are shown on fxL and fxH. A new shorting position YY can be found to tune the antenna to a new resonant frequency of fY. The VSWR plot is shown in dashed lined in Figure 5. If the position YY is chosen carefully, the new lower 3:1 bandwidth fYL can be set at the same point as the old upper 3:1 band frequency fxH. It can be seen that with just these two switch positions the antenna will now be able to cover the entire band from fXL to fYH. A similar exercise can then be carried out at position CC to extend the switched coverage from fXL to fCH using 3 switches. The above procedure can be extended throughout the desired band to give, in this example, continuous switched coverage from 30MHz through to 88MHz.

When the antenna is tuned to the resonant frequency fx it has a VSWR better than 3:1. The usable portion of the frequency spectrum is considered to be between fXL and fXH which are the 3:1 VSWR bandwidth edges shown in Figure 5.

This bandwidth is fairly narrow and heavily influences the design of the antenna.

All electrically short antennas tend to have very low radiation resistances leading to narrow VSWR bandwidth. As the antenna height is only 0.5m the radiation resistance on a large ground plane is typically 10 ohms or less. Ordinarily this would put the antenna VSWR worse than 3:1. Increasing the height would improve the match but this would take the antenna above the desired maximum height of 0.5m. Various frequency matching techniques could be used to improve the VSWR but in practice such techniques are so frequency sensitive that they produce an unacceptable reduction in the VSWR bandwidth.It is found that these problems can be overcome by designing the antenna to take account of the fact that in general it will not be operating in free space but rather will be mounted near an edge of the vehicle looking like an electrically small ground plane significantly increases the antenna's radiation resistance to an improved value. In practice therefore the antenna is designed to be mounted within 300mm from the edge of the ground plane.

The bandwidth of the antenna may be further improved by introducing a small series loss component. The sacrifice of just one dB of power by installing a small series resistor in the base feed provides a worthwhile increase in operational bandwidth.

As already noted, the antenna system of the present invention is intended for use with a frequency hopping transmitter or receiver and so is required to tune to different frequencies at high speed. The key to faster tuning is the use of electronic switches. These, unlike relays, can be switched very rapidly (typically with a switching period of 10 or 20 microseconds) and do not wear out after a large number of operations. The electronic device used to provide switching is the PIN (or alternatively NIP) diode. The device is a P-N junction that has been separated by a very thin and highly doped intrinsic layer. This diode has unique properties at radio frequencies. When the diode is forward biassed it readily allows RF to pass through it, looking almost like a short circuit.When the diode is reversed biassed the intrinsic layer effectively becomes a low value capacitor with a typical capacitance of two PicoFarads. At the operational frequencies of the system described above this effectively looks like an open circuit and the diode switch is OFF.

The antenna in this example is designed to be used with 50-100 watt transmitters. Consequently diodes capable of handling both high power and the RF voltages must be used.

High power PIN diodes typically need 1V at 100mA to forward bias them and 100V and 10 x 10-6 A to fully reverse bias then.

However, the RF voltages present in the antenna system influence the final bias level. When the diode is fully turned on the recovery time of the diode is long enough to ensure that the diode stays shorted during RF negative voltage cycle. However, when reversed biassed a different effect occurs. If the positive RF voltage cycle is greater than the negative bias voltage then the diode behaves like a lossy resistor. The recovery time in this state is short enough to keep the diode lossy. Experiments have shown that the reverse bias voltage must be greater than the peak RF voltage. The RF voltage levels present on the antenna system require a reverse bias voltage of at least 400V to ensure the diodes stay OFF.

A further problem which is addressed is the RF de-coupling of control lines 5 which carry control signals to the PIN diodes of the bridging elements 4. These control lines 5 once they leave the antenna system must be isolated from RF effects. If RF power at high levels appears on the lines then the lines become part of the antenna radiating system. The effects are two-fold. Firstly, large levels of RF power can severely harm the switching drive units and then eventually filter into various electronic systems, disrupting them. Secondly, the lines become part of the antenna matching system, degrading the match or at least making the VSWR performance of the antenna unpredictable.

As a result it is necessary to provide an RF filter. However, the use of, for example, discrete LC components is impractical due to the volume needed for all the conductors. Instead, all the control lines are wound together to form a solid wire and then wound to form one single inductor. In practice this can easily be implemented using screened multi-core cable which is then fed down the rising element 1 for connection to the source of control signals at the base of the antenna. The cable is wound in a coil around the base of the rising element 1 to provide the necessary high impedance at RF frequencies. This makes possible direct connection of the conductor to the ground plane. The high parallel shunt impedance of the coil has negligible effect on the antennas natural impedance.At the same time the screening braid physically contains all the control lines in a neat package and the insulation on the outer of the cable allows a compact inductor to be wound. Connecting the coil in this manner also allows the screening braid to be used as the earth return from the DC control lines. When the control lines emerge from the bottom of the earthed coil they are found to be acceptably RF decoupled. If further RF decoupling is required this can be achieved by tying each control line to earth via a modest value of capacitor.

The above arrangement allows the control lines to be taken safely away from any end fed antenna system without affecting the general performance of the antenna. In practice, the control lines receive control signals from a transceiver which is arranged to tune to the antenna appropriately at the same time as it "hops" from frequency to frequency.

An alternative version of the antenna may employ conventional relays in place of PIN diodes in applications where frequency agility is not of prime importance.

Other configurations for the top loading element such as a zig-zag conductor, may be used instead. Such a configuration is shown in Figure 3b.

Although described above in relation to an end-fed monopole the present invention is applicable to other antenna types including dipoles and shunt-fed antennas.

Claims (10)

1. A frequency agile antenna system comprising a rising element and a top loading element, the top loading element being folded and including at least one electrically switchable bridging element extending between different regions of the top loading element, the bridging element being operable to provide a direct electrical connection between the different regions of the top loading element so changing the resonant frequency of the antenna system.

2. A system according to claim 1, in which the folded top loading element comprises a planar spiral and the at least one bridging element extends between radially adjacent arms of the spiral.

3. A system according to claim 1 or 2, in which the bridging element comprises a PIN diode arranged when forward biassed to provide a short circuit at radio frequencies and when reversed biassed to provide an open circuit at radio frequencies

4. A system according to any one of claims 1 to 3, in which a connector for the or each bridging element is formed integrally with the top loading element.

5. A system according to any one of the preceding claims, including a number of bridging elements arranged around the folded top loading element, the intervals between the bridging elements being such that as successive elements are switched the antenna is tuned to adjacent overlapping frequency ranges.

6. A system according to any one of the preceding claims, in which control wires connected to respective bridging elements extend down the rising elements and are wound into a coil at the base of the rising element arranged to isolate the control wires from the RF feed to the antenna.

7. A system according to claim 6, in which the coil is wound concentrically with the rising element towards the bottom of the rising element.

8. A system according to any one of the preceding claims, in which the rising element and top element are surrounded by an integral cover formed from anti-ballistic material.

9. A system according to any one of the preceding claims, in which the antenna is an end-fed monopole.

10. An antenna system substantially as described with respect to the accompanying drawings.