GB2536738A

GB2536738A - Negative impedance converter-based impedance matching technique to achieve multi-feed multi-band antennas

Info

Publication number: GB2536738A
Application number: GB1513085.9A
Authority: GB
Inventors: Hu Sampson; Wan Liang
Original assignee: Smart Antenna Technologies Ltd
Current assignee: Smart Antenna Technologies Ltd
Priority date: 2015-03-25
Filing date: 2015-07-24
Publication date: 2016-09-28
Anticipated expiration: 2035-07-24
Also published as: GB201505058D0; GB2536738B; GB2536676A; GB2536676B; GB201513085D0

Abstract

Transceiver ports 3A-3C are matched to respective feed points 2A-2C of a compound antenna system 1A-1C by matching networks, at least some of the matching networks comprising a negative impedance converter (NIC) 5A,5B located between passive pre-matching 4A,4B and post-matching 6A,6B networks. The pre-matching networks 4 may be configured to selectively decouple the branches in the frequency bands of interest despite coupling between radiating elements. The post-matching networks 6 may decouple branches connected to a single transceiver port (figure 9). The pre-matching network may increase the real part of the antenna impedance and transform the imaginary part of the antenna impedance so that it increases from negative to positive in the frequency band of operation, the NIC then cancelling this reactance and the post-matching network converting the residual impedance to 50 Ohms.

Description

NEGATIVE IMPEDANCE CONVERTER-BASED IMPEDANCE MATCHING TECHNIQUE

TO ACHIEVE MULTI-FEED MULTI-BAND ANTENNAS

[0001] This invention relates to the provision of negative impedance converters in matching circuits for multiple-feed reconfigurable antenna systems to enable impedance matching across several different frequency bands.

BACKGROUND

[0002] Electrically small antennas can be generally classified as TM (transverse magnetic) and TE (transverse electric) mode antennas. For an electrically small antenna, which is widely used in wireless communication systems, the input impedance is considerably reactive with a small real part. It is therefore critical to match the antenna to the receiver or transmitter to maximise the total efficiency in the frequency range of interest.

[0003] Normally, an electrically small TM mode antenna can be characterised by or represented as a series connected combination of a resistor, a capacitor and an inductor.

Figures 1 and 2 show that, at low frequencies, the reactance can equally be represented by a series connected capacitor and inductor, with the capacitor playing a dominant role in the reactance. The resistor represents the resistance of the radiating element of the antenna.

[0004] There are two different ways to match a highly reactive antenna of this type. One approach is conventional passive matching, where a large series inductor Lex, is placed between the antenna and the signal port as a necessary component. However, the resistive loss that is introduced by the inductor Lext dramatically degrades the total efficiency. In fact, even with lossless inductors, the match is effective over only extremely small instant bandwidths because the reactive part of the electrically small antenna cannot be neutralised over a broad frequency band with passive components (the real part of the impedance is much smaller than the imaginary part). This is illustrated in Figure 3.

[0005] The other approach uses an NIC (negative impedance converter) in an attempt to cancel the reactance of the antenna. This is a type of non-Foster impedance matching, and is illustrated in Figure 4.

[0006] There is a relationship between antenna size and the realisable bandwidth as defined by the Chu limit (Chu, L. J.; "Physical limitations of omni-directional antennas"; Journal Applied Physics 19: 1163-1175; December 1948). The Chu limit gives the relationship between the radius of the circle that completely circumscribes an antenna and the Q of the antenna. However, McLean (McLean, J. S.; "A re-examination of the fundamental limits on the radiation Q of electrically small antennas"; IEEE Transactions on Antennas and Propagations; Vol. 44; No. 5; pp. 672-676; May 1996) redefined how the Q of an antenna should be calculated, and this is given in equation 1.1: 1 1 3 x a) where k is the wave number and a is the radius of a sphere that completely circumscribes the antenna as shown in Figure 5.

[0007] McLean's equation is a derivation from the original Chu limits equations. There has also been much research into ways of improving the gain of an antenna through the use of matching networks, but this is also bounded by the Harrington limits (Harrington, R. F.; "Effect of antenna size on gain, bandwidth and efficiency"; Journal of Research of the National Bureau of Standards -D. Radio Propagation; vol. 640; p. 12; 29th June 1959) on antennas as given in: G = + 2ka (1.2) [0008] The Chu limit can be related to the antenna bandwidth by rewriting the Q of the antenna as shown in equation 1.3: (1.3) where fe is the antenna centre frequency at resonance and af is the bandwidth of the antenna.

[0009] Comparing equation 1.1 with equation 1.3, it can be seen that reducing the radius of the sphere which translates to a physical reduction in the antenna size, the antenna bandwidth also reduces. The reduction in size means that the antenna radiation resistance also reduces, and this in turn leads to a reduction the antenna efficiency. From equation 1.2, it is clear that antenna gain is also proportional to the antenna size a.

[0010] These two fundamental limits on the antenna make it difficult to provide a small antenna with a low Q (wideband). However, more and more devices these days require smaller antennas and there is need for these antennas to still have wide usable bandwidths.

[0011] Passive matching networks help to match antennas, but because they involve resonating the reactive part of the antenna with passive elements, they only give a good match at specific frequencies. Away from the specific frequency, the antenna return loss decreases. This necessitates the use of multiple or reconfigurable matching networks to cover wide frequency bands. However, using non-Foster elements could help provide continuous wideband matching because unlike Foster elements, the slope of the reactance versus frequency of a non-Foster element is always negative as shown in Figure 4.

[0012] With these properties, non-Foster elements are able to cancel out completely the reactance of other elements and antennas because of the difference in slope and direction of rotation on the Smith chart.

[0013] One implementation of non-Foster elements is through the use of NICs (negative impedance converters). NICs were first proposed by Linvill (Linvill, J.G.; "Transistor negative-impedance converters"; Proc. IRE; vol 41, pp 725-729; 1953). The Linvill NIC consists of two transistors connected in a common base configuration. "Common base" or "common gate" refers to a specific input and output setup of a transistor in amplifier applications. In a Linvill type NIC, the RF input terminal is connected to the emitter or source of one transistor, and the RF output terminal to the emitter or source of the other transistor (in fact, since an NIC is normally a bidirectional device, it does not matter which terminal is used as the RF input and which as the RF output). The reactance to be inverted is connected between the two collectors or drains, and the base or gate of each transistor is connected to the collector or drain of the other transistor in the form of a feedback path. The emitters or sources form the two ports of the NIC. The circuit schematic of the NIC is shown in Figure 6.

[0014] A typical conventional Linvill type NIC matching arrangement is shown in Figure 7.

[0015] Earlier work by the present Applicant has investigated the use of negative components generated by NICs to cancel the input reactance of networks including an antenna and a subsequent pre-matching impedance transformer. However, this cancellation covers only a single continuous frequency band, which may not always be sufficient in today's multiple band communications environment.

BRIEF SUMMARY OF THE DISCLOSURE

[0016] Viewed from a first aspect, there is provided an antenna system comprising a plurality of antenna radiating elements each having an associated feed, at least one of the feeds being connected to an RF source or load by way of an active matching circuit comprising a pre-matching network, a negative impedance converter and a post-matching network.

[0017] The pre-matching network connects the negative impedance converter to the respective antenna feed, and the post-matching network connects the negative impedance converter to the RF source or load, which may be a transceiver port.

[0018] The pre-matching network may comprise a combination of capacitors and/or inductors to transform both a real part and an imaginary part of an impedance of the respective antenna feed. The negative impedance converter may be configured substantially to cancel the transformed imaginary part of the impedance of the antenna.

The post-matching network may comprise a combination of capacitors and/or inductors to transform a residual real part of the impedance of the antenna to match an impedance of the RF source or load.

[0019] In some embodiments, all of the feeds are connected to the RF source or load by way of an active matching circuit comprising a negative impedance converter. In these embodiments, active impedance matching is enabled for all of the feeds.

[0020] In other embodiments, at least one of the feeds is connected to the RF source or load by way of a passive matching circuit that does not include a negative impedance converter. The passive matching circuit may comprise a pre-matching network and a post-matching network as described in relation to the active matching circuit.

[0021] Where the RF load or source is a transceiver, the matching circuits may all be connected to a single transceiver port, or may be connected to different transceiver ports as required. In some embodiment, some matching circuits may be connected to one transceiver port, while other matching circuits may be connected individually to other transceiver ports.

[0022] Each of the radiating antenna elements and their associated matching circuits are configured to operate in a predetermined continuous frequency band. The predetermined continuous frequency bands may be selected so as to give appropriate coverage for desired applications, for example to provide coverage of two or more of DVB-H, GSM710, GSM850, GSM900, GSM1800, PCS1900, SOARS, GP51575, UMTS2100, WiFi, Bluetooth, LTE, LTA and 4G frequency bands.

[0023] The radiating antenna elements may be sized differently to each other and/or have different electrical sizes so as to provide a simple input impedance response at each frequency band to be matched. Because the radiating antenna elements are generally located close to each other, for example in a mobile handset or other portable device, the elements will tend to couple with each other to a greater or lesser degree during operation.

Antenna coupling can be a serious problem in known multi-antenna devices, since it makes it much harder to provide effective impedance matching. This is because such coupling can change the impedance of any given antenna in unpredictable ways, depending on which of the other antennas is operating at any given time.

[0024] The pre-matching networks have two main functions. The first is to decouple the antenna radiating elements over the frequency bands of interest at any given time. Typically, after decoupling, the input impedance after the pre-matching network in the relevant matching circuit is substantially independent of the matching networks connected after the pre-matching networks in the other matching circuits. The second function is to transform the antenna impedance to a level at which the negative impedance converter can easily cancel or substantially cancel the transformed reactance (the imaginary part of the impedance).

[0025] In order to enable the transformed reactance to be most effectively cancelled with the negative impedance converter, the transformed antenna impedance preferably has the following characteristics: i) the transformed real part should be higher than the real part of the impedance prior to transformation, and should be relatively flat across the frequency band of interest; and ii) the transformed imaginary part should increase monotonically from negative to positive over each frequency band of interest. In order to facilitate this, the antenna impedance of each antenna radiating element should be optimised for its associated frequency band by, for example, selecting or adjusting the physical size of the antenna radiating element. An antenna radiating element configured to handle a lower frequency band will be larger than an antenna radiating element configured to handle a higher frequency band, thereby helping to optimise the input impedances of the individual antenna radiating elements for their respective frequency bands.

[0026] The post-matching networks also have two main functions. The first is to match the impedance to the RE source or load (typically 50 ohms) after cancellation of the reactance in the negative impedance converter, or after transformation in a passive matching circuit. The second is to decouple the matching circuits from each other in embodiments where more than one matching circuit is connected to a single port, for example a single transceiver port.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: Figure 1 shows an electrically small antenna connected to a 50 ohm signal port; Figure 2 shows the antenna of Figure 1 represented as an equivalent series connected resistor, capacitor and inductor; Figure 3 shows the arrangement of Figure 2 provided with a passive impedance matching network, together with a plot of reactance against angular frequency; Figure 4 shows the arrangement of Figure 2 provided with a non-Foster matching network comprising a negative capacitance, together with a plot of reactance against angular frequency, Figure 5 illustrates an antenna circumscribed by a sphere of radius a; Figure 6 is a schematic of a conventional Linvill-type negative impedance converter (NIC); Figure 7 shows a conventional NIC arrangement for matching an antenna to a transceiver; Figure 8 is a schematic outline of a first embodiment; Figure 9 is a schematic outline of a second embodiment; Figure 10 is a variation of the embodiment of Figure 8; Figure 11 is a variation of the embodiment of Figure 9; Figure 12 is a more detailed schematic of a variation of the first embodiment; Figure 13 shows the return loss at each transceiver port of the embodiment of Figure 12; Figure 14 shows the total efficiency of the embodiment of Figure 12; Figure 15 is a more detailed schematic of a variation of the second embodiment; Figure 16 shows the return loss at the single transceiver port of the embodiment of Figure 15; Figure 17 shows the total efficiency of the embodiment of Figure 15;

DETAILED DESCRIPTION

[0028] Figure 8 shows a schematic outline of a first embodiment, in which a compound antenna comprising antenna radiating elements 1A, 1B and 1C has multiple feeds 2A, 2B and 2C. Feed 2A is connected to a transceiver port 3A by way of an active matching circuit comprising a pre-matching network 4A, a negative impedance converter network 5A and a post-matching network 6A. Feed 2B is connected to a transceiver port 3B by way of an active matching circuit comprising a pre-matching network 4B, a negative impedance converter network 5B and a post-matching network 6B. Feed 20 is connected to a transceiver port 3C by way of a passive matching circuit comprising a pre-matching network 4C and a post-matching network 6C. Transceiver port 3A is configured to handle a first frequency band A, transceiver port 3B is configured to handle a second frequency band B, and transceiver port 30 is configured to handle a third frequency band C. It will be appreciated that additional frequency bands can be accommodated by adding further antenna radiating elements, transceiver ports and matching circuits. While all embodiments will have at least one active branch comprising a pre-matching network, an NIC network and a post-matching network, some embodiments will comprise just active branches, and others may have one or more passive branches.

[0029] The antenna radiating elements 1A, 1B and 10, which will generally be close together, for example in a mobile handset or other portable device, will tend to couple with each other during operation. In order to address this problem, the pre-matching networks 4 (and, in some embodiments, the post-matching networks 6) are configured to selectively decouple the matching circuits or branches across frequency bands of interest. In other words, coupling between antenna radiating elements, which is often unavoidable, can surprisingly be made unproblematic by appropriate configuration of the pre-matching networks 4 and, in some embodiments, the post-matching networks 6.

[0030] The pre-matching networks 4 have two functions. One is to decouple the multi-feed antenna 1 over all of the interesting frequency bands. Typically, after decoupling, the input impedance after the pre-matching network 4 in one branch can be independent of the circuits connected after the pre-matching networks 4 in the other branches. The other function of pre-matching network 4 is to transform the antenna impedance to a proper level so that the NIC network 5 can cancel the transformed reactance. Typically, the real part of the antenna impedance should be transformed to a higher, relatively flat level across the relevant frequency band, and the imaginary part of the antenna impedance should be transformed so that it increases monotonically from negative to positive across the relevant frequency band. The post-matching network 6 also has two functions. One is to match the impedance after cancellation by the NIC 5 On an active branch) or the impedance after transformation On a passive branch) to the impedance of the transceiver port 3 (normally 50 ohms). The other is to decouple different branches when all branches are connected to a single transceiver port 3, as shown in Figure 9.

[0031] Figure 9 shows an alternative embodiment, with like parts labelled as for Figure 8.

The embodiment of Figure 9 is similar to that of Figure 8, except that all of the branches connect to a single transceiver port 3, rather than to separate transceiver ports 3A, 3B and 30.

[0032] Figure 10 shows a specific implementation of the embodiment of Figure 8 to cover low, middle and high frequency bands, with parts being labelled as for Figure 8. Antenna radiating element 1B and its associated matching circuitry 4B, 5B, 6B are configured for operation in a middle frequency band. Antenna radiating element 1C and its associated matching circuitry 40, 60 are configured for operation in a high frequency band. Antenna radiating element 1A has the largest size, with antenna radiating element 1B having a middle size and antenna radiating element 10 having the smallest size. The input impedance of each antenna radiating element 1A, 1B, 10 is optimised for its respective frequency band by appropriate adjustment of the pre-and post-matching networks 4 6. It will be noted that a mixture of active branches with NIC components 5 and passive branches with no NIC components may be provided in order to help fulfil desired bandwidth requirements.

[0033] Similarly, Figure 11 shows a specific implementation of the embodiment of Figure 9 to cover low, middle and high frequency bands, with parts being labelled as for Figure 9.

[0034] Figure 12 shows a more detailed implementation of the embodiment of Figure 8, comprising a multi-port NIC-based impedance matching circuit for a multi-feed antenna to cover multiple bands, with parts being labelled as in Figure 8. The embodiment of Figure 12 comprises first and second active branches, with no passive branch. Although the antenna radiating elements 1A, 1B are shown as a single component, this is merely a consequence of circuit diagram conventions. The multi-feed antenna will physically have different antenna radiating elements 1A, 1B.

[0035] Figure 13 shows the return loss at each transceiver port 3A, 3B of the Figure 12 embodiment. It can be seen that transceiver port 3A can cover the LTE low band (700MHz-960MHz), and transceiver port 3B can cover the GNSS band and the LTE middle and high bands (1.56GHz -2.7GHz). The isolation between the two transceiver ports 3A, 3B is mostly lower than -18dB. Figure 14 shows the total efficiency of the antenna system over the two continuous wide frequency bands after matching.

[0036] Figure 15 shows a more detailed implementation of the embodiment of Figure 9, comprising a single port NIC-based impedance matching circuit for a multi-feed antenna to cover multiple bands, with parts being labelled as in Figure 9. The embodiment of Figure 15 comprises first and second active branches, with no passive branch.

[0037] Figure 16 shows the return loss at the transceiver port 3 of the Figure 15 embodiment. It can be seen that the single transceiver port 3 can cover the LTE low band (700MHz-960MHz), GNSS band and the LTE middle and high bands (1.56GHz -2.7GHz) simultaneously. Figure 17 shows the total efficiency of the antenna system over the two continuous wide frequency bands after matching.

[0038] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0039] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0040] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

CLAIMS: 1. An antenna system comprising a plurality of antenna radiating elements each having an associated feed, at least one of the feeds being connected to an RE source or load by way of an active matching circuit comprising a pre-matching network, a negative impedance converter and a post-matching network.
2. The system of claim 1, wherein the RE source or load comprises at least one transceiver port.
3. The system of any preceding claim, wherein the pre-matching network comprises a combination of capacitors and/or inductors to transform both a real part and an imaginary part of an impedance of the respective antenna feed.
4. The system of claim 3, wherein the negative impedance converter is configured substantially to cancel the transformed imaginary part of the impedance of the respective antenna feed.
5. The system of claim 4, wherein the post-matching network comprises a combination of capacitors and/or inductors to transform a residual real part of the impedance of the antenna feed to match an impedance of the SF source or load.
6. The system of any one of the preceding claims, wherein the pre-matching networks are configured to decouple the antenna radiating elements over the frequency bands of interest at any given time.
7. The system of any preceding claim, wherein all of the feeds are connected to the RE source or load by way of a respective active matching circuit comprising a negative impedance converter.
8. The system of any one of claims 1 to 6, wherein at least one of the feeds is connected to the RE source or load by way of a passive matching circuit that does not include a negative impedance converter.
9. The system of claim 2 or any one of claims 3 to 8 depending from claim 2, wherein the matching circuits are all connected to a single transceiver port.
10. The system of claim 2 or any one of claims 3 to 8 depending from claim 2, wherein the matching circuits are all connected to different transceiver ports.
11. The system of any one of the preceding claims, wherein each of the radiating antenna elements and their associated matching circuits are configured to operate in a predetermined continuous frequency band.
12. The system of any one of the preceding claims, wherein the radiating antenna elements are sized differently to each other and/or have different electrical sizes.
13. An antenna system substantially as hereinbefore described with reference to or as shown in Figures 8 to 17 of the accompanying drawings.