GB2286291A

GB2286291A - Microstrip patch antenna

Info

Publication number: GB2286291A
Application number: GB9400736A
Authority: GB
Inventors: Vince Fusco
Original assignee: Queens University of Belfast
Current assignee: Queens University of Belfast
Priority date: 1994-01-15
Filing date: 1994-01-15
Publication date: 1995-08-09
Anticipated expiration: 2014-01-15
Also published as: GB2286291B; GB9400736D0

Abstract

A microstrip patch antenna comprises one or more active devices 1, 2 located on or adjacent to the patch P. Oscillator functions in the antenna are provided by applying one or more independent dc bias voltages to one or more of the active devices such that each active device forms part of an associated oscillator for which the patch acts as a resonating load. At least one reference oscillator is provided either by means of a further active device or by means of an external oscillator so that frequency locking occurs between the oscillators. The active devices may comprise Gunn-diodes or MESFETS. <IMAGE>

Description

ACTIVE MODE CONTROL FOR MICROSTRIP PATCH ANTENNAE.

The present invention relates to micros trip patch antennae and, in particular, to a method for allowing the radiation modes of a microstrip patch antenna to be controlled.

Research has been carried out over the past twenty years into the design and application of passive planar microstrip radiators such as the rectangular patch. These antennae have many advantages such as light weight, planarity and low cost.

More recently work has started on the integration of active devices into the patch antenna body, leading to a new area of research on active patch antennae. The integration of active devices leads to new applications and characteristics for patch antenna. It is now possible to realise oscillator functions and amplifier functions directly on the patch as described in Chang, K., Hummer, K.A., Klein, J.L., Experiments on injection locking of active phased arrays and spatial power combiners, IEEE Trans., 1989, MTT-37, (7), pp 1078-1084 and Robert, B., Razhan, T., Papermile, A., Compact Active Patch Antenna, ICAP '93 INT Conference on Antennas and Propagation IEE Conf. Pub. No. 370, April, Edinburgh, 1993, pp 307-310, respectively. This in turn leads to new systems which are ideal for exploration in spatial power combining phased array radar systems and for new high volume consumer and commercial applications in the automotive industry as described in Lowbridge, P., Low cost millimeter wave cruise control, Electronic Product Design, November, 1992, pp 23-27. Ongoing research work by the inventor has revealed a new and interesting phenomena which has implications for the use of active micros trip antenna systems and which forms the basis of this invention.

Known methods of controlling the radiation modes of a microstrip patch antenna involve the use of an external oscillator coupled to a phase shifter. The outputs from the oscillator and the phase shifter are connected at suitably located feed points on the patch so that resonant radiation modes are established in the antenna. The phase of the phase shifter output, which is usually for example 900 or 1800, determines the radiation mode in the patch.

An object of the present invention is to provide a method whereby a microstrip patch antenna, operated either as a single element or as part of an array, can have its mode of radiation controlled by a dc signal. The potential for dc controlled radiation mode variation means that simple methods for polarization control and beam steering become feasible.

According to the invention there is provided a method for providing oscillator functions in a microstrip patch antenna comprising the application of one or more independent dc bias voltages to respective one or more active devices located on or adjacent to the patch such that each such active device forms part of an associated oscillator for which the patch acts as a resonating load, at least one reference oscillator being provided either by means of a further such active device or by means of an external oscillator so that frequency locking occurs between the oscillators.

According to the invention there is also provided a micros trip patch antenna comprising one or more active devices located on or adjacent to the patch, means for applying independent dc bias voltages to respective such active devices, each such active device being capable of forming part of an associated oscillator for which the patch acts as a resonating load and at least one reference oscillator provided either by means of a further such active device or by means of an external oscillator so that in use frequency locking between the oscillators occurs.

The active devices may comprise any suitable two or three terminal nonlinear active devices such as Gunn diodes, MESFETs or the like. Clearly any combination of such devices may be used. The active devices should be appropriately terminated to realise such oscillation conditions. The patch body may have any suitable shape or dimensions but is preferably of substantially rectangular form. The patch dimensions may be selected to give the required mce reasonance. The active devices may be formed or mounted at or adjacent any suitable patch location. In the case of a rectangular patch, the active devices are preferably located at or adjacent the patch periphery, most preferably along the same edge or along two orthogonal edges.

Using the method and/or antenna of the invention, the electric field distribution of an antenna may be varied by altering the dc bias voltage applied to one or more of the active devices while maintaining the bias voltage applied to the reference oscillator, whether active or passive, constant. The method and/or antenna of the invention may be used to switch between the available modes of oscillation and/or as a means of rotating the electric field in the patch continuously or discretely in response to the dc bias control. Thus, the method and antenna of the invention allow active mode control for micros trip patch antennas with lowcost electronic polarization and beam steering capabilities.

In the preferred embodiment, the patch is rectangular and two active devices are located along its length. An alternative embodiment employs the use of one active device located on the patch short edge and one active device located on the patch long edge.

Preferably, the reference oscillator is provided by means of a further active device. However, for achieving a high stability phase reference, an external oscillator or signal from a frequency sweeper may be more advantageous.

The invention therefore provides a new method of operation of a micros trip patch antenna incorporating active nonlinear elements. The method using injection locking effects between oscillators in order to control signal phasing and hence antenna radiation modes leads to dc controlled variable polarization and dc controlled variable antenna for field patterns. The invention is of potential generic significance for modern microwave and millimetre wave systems.

The invention will now be described further, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 is a schematic plan view of a rectangular patch of a microstrip patch antenna of the invention; Fig 2(a) is a three dimensional plot of the electric field distribution or strength of an antenna according to the invention radiating in the To1 mode, at an oscillator frequency of 10.5 GHz; Fig. 2(b) is a two dimensional plot showing the electric field strength variation along the L and W dimensions of the patch of Fig. 1 with the antenna radiating in the T01 mode (1800 phases difference between the oscillators) at an oscillator frequency of 10.5 GHz; Fig. 3 is a two dimensional plot similar to Fig. 2(b) but with the antenna radiating in the T11 mode (00 phase difference between the oscillators) ; and Fig. 4 is a two dimensional plot similar to Fig. 2(b) but with a 450 phase difference between the oscillators.

Referring to Figure 1, a microstrip patch antenna of the invention comprises a rectangular patch P having a width W and a length L. Two active devices 1, 2 are located along the length of the patch P. A further optional active device 3 may be located on the side L of the patch. The active devices may comprise any two or three terminal nonlinear active devices such as a Gunn diode, MESFET or the like similar device and are appropriately terminated to realise oscillation conditions. The patch body acts as a resonating load whose dimensions have been selected for fundamental mode reasonance.

By connecting a dc bias onto device 1, the patch antenna can be made to oscillate and power will be radiated in the fundamental mode with one half wavelength field variation along the L direction and zero field variation along the W direction. This variation of the electric field strength E in the L and W directions of the patch is shown in Figure 2(a) and represents the Tol mode with an oscillator frequency of 10.5 GHz. With the dc held constant on device 1, a separate dc bias is applied to device 2. When sufficient dc bias is applied to device 2 it too will oscillate. Both devices 1 and 2 thus act as oscillators operating concurrently. If the frequencies of oscillator 1 and oscillator 2 are similar then frequency locking will occur and both oscillators will operate at the same frequency but with a definite fixed phase shift. Such frequency locking is described in Kurokawa, K., Microwave Solid State Oscillator Circuits in Microwave Devices Device Circuit Interactions, Ed M.J. Howes, D.V. Morgan, Wiley, 1976, pp 209-265.

Referring to Fig. 2(b), the field variation is shown for an oscillator frequency of 10.5 GHz and with a 1800 phase shift 1, 2 between the oscillators 1 and 2 giving the T mode.

If then the bias onto only one of the active devices, say 2, is allowed to vary while the other is held constant, frequency pushing of oscillator 2 will tend to occur.

However, since the frequency of oscillator 2 is fixed (locked by oscillator 1) then the phase between the signals produced by oscillator 1 and oscillator 2 will vary. Using this injection locking approach, a continuously variable phase variation under dc bias control is obtained. Thus, any phase shift between the oscillators in the continuous range 0 to 1800 can be achieved. Unlike conventional phase shifting schemes, no external componentry PIN diodes or the like is required.

Referring to Figure 3, when the dc controlled phase shift between the oscillators is zero, 2 = "0, the patch can be operated in the T11 mode with one half cycle of field in both the L and W directions. At intermediate values; the patch field can be rotated away from the T01 mode. Thus, for example, for a phase shift of A1 - '!2 = 450, the field is rotated away from the Tol mode shown in Fig. 2(b) and is as shown in Figure 4.

In summary, from an antenna/radiation mode perspective the dc bias on one of the active devices, say 2, is used to control the phase shift between the oscillators such that for a 1800 phase shift the patch operates in the Tol mode, and, as the phase shift is reduced the patch field is rotated away from the Tol mode until at a 0 phase shift the patch operates in the T11 mode.

Thus, it is possible to switch the mode of operation in an agile fashion between the T01 and T11 mode and also to rotate the electric field in the patch under dc bIas control.

This means that active agile polarization of the locally radiated generated signal can occur. Therefore, lowcost electronic polarization and beam steering are feasible in that there is no longer a requirement for external phase shifters as is current practice.

By replacing active device 1 or 2 by active device 3 as shown in Figure 1 and by controlling the dc bias until the phase difference between the two signals is 900, circular polarization can be achieved without the need for a 900 hybrid circuit as is current practice. By using a phase difference of other than 900 elliptical polarization becomes possible.

Finally it is also possible to remove one of the active devices 1, 2 or 3 and replace it with a signal from a frequency sweeper. This signal can then be used as a high stability phase reference. The operation of the antenna will be as described above.

It will be appreciated that the present invention is not intended to be restricted to the details of the above described embodiment. For example, the antenna may be operated either as a single element or as part of an array; any suitable patch shape and dimensions may be used and any number of active devices may be used either alone or in combination with passive devices. The active and passive devices may be located at or adjacent or fed to any desired feed point on the patch, and may operate at any suitable frequency.

Claims

1. A method for providing oscillator functions in a microstrip patch antenna comprising the application of one or more independent dc bias voltages to respective one or more active devices located on or adjacent to the patch such that each such active device forms part of an associated oscillator for which the patch acts as a resonating load, at least one reference oscillator being provided either by means of a further such active device or by means of an external oscillator so that frequency locking occurs between the oscillators.

2. A microstrip patch antenna comprising one or more active devices located on or adjacent to the patch, means for applying independent dc bias voltages to respective such active devices, each such active device being capable of forming part of an associated oscillator for which the patch acts as a resonating load and at least one reference oscillator provided either by means of a further such active device or by means of an external oscillator so that in use frequency locking between the oscillators occurs.

3. A method or an antenna as claimed in any preceding claim wherein one or more of the active devices comprise any suitable two or three terminal nonlinear active devices.

4. A method or an antenna as claimed in claim 3 wherein one or more of the active devices comprise Gunn-diodes or MESFETs.

5. A method or an antenna as claimed in any preceding claim wherein the patch dimensions are selected to give the required mode resonance.

6. A method or an antenna as claimed in any preceding claim wherein the patch is of substantially rectangular form and the active devices are located at or adjacent the patch periphery.

7. A method or an antenna as claimed in claim 6 wherein the active devices are located along the same edge or along two orthogonal edges of the patch.

8. A method or an antenna as claimed in claim 6 wherein two active devices are located along the longer edge of the patch.

9. A method or an antenna as claimed in claim 6 wherein one active device is located on the patch short edge and one active device is located on the patch long edge.

10. A method or an antenna as claimed in any preceding claim wherein the reference oscillator comprises an active device.

11. A method or an antenna as claimed in any of claims 1 to 9 wherein the reference oscillator comprises an external oscillator or signal from a frequency sweeper.