Abstract:
A multifilar antenna comprises n spaced antenna filaments, where n is an integer greater than 1; a matching circuit for matching the characteristic impedance of the antenna to that of a transmitting and/or receiving apparatus; a weighting circuit for applying gain and phase adjustments to signals passed to or from the n filaments; switch means associated with at least some of the filaments for selectively altering the electrical length and/or interconnections for the filaments; means for detecting electrical properties of the multifilar antenna with respect to the frequency, polarization and/or direction of propagation of a signal to be received or transmitted by the multifilar antenna and/or impedance matching of the antenna; and control means, responsive to the detecting means, for controlling the operation of the matching circuit, the weighting circuit and the switch means to adjust the properties of the multifilar antenna to suit better a current signal to be received or transmitted.

Description:
FIELD OF THE INVENTION 
   This invention relates to adaptive multifilar antennas. 
   BACKGROUND OF THE INVENTION 
   In fields such as mobile telephony and communication, it is being proposed that radio frequency transceivers operating in different frequency bands, and providing different services, should be integrated into single consumer devices. 
   For example, in order to improve the coverage area in which a mobile telephone can be used, a satellite system transceiver, a terrestrial transceiver and a domestic cordless telephone transceiver might be integrated into one hand-held unit. An alternative example is a dual service telephone operating at 1800 MHz in the user&#39;s home country but having the capability of operating at 900 MHz in other countries under a so-called roaming arrangement. 
   The electronics needed to achieve this aim are rapidly becoming smaller and lighter. A remaining problem area for multi-frequency, multi-system operation, however, is the antenna. 
   In order to operate as described above, an antenna should be able to work at different frequencies and with different types of base station. For example, one service may use terrestrial base stations and another may use orbiting satellites. This means that if the handset antenna is typically used in a vertical position (with the handset held next to the user&#39;s head) then for one service the antenna should have a radiation pattern substantially omnidirectional in azimuth and for the other service it should have an approximately hemispherical radiation pattern. 
   SUMMARY OF THE INVENTION 
   To cater for the different pattern and frequencies in use, it has been proposed to employ at least two distinct antennas within a common volute. 
   In a first aspect, the invention provides an adaptive multifilar antenna comprising:
     n spaced filaments, where n is an integer greater than 1;   at least one filament group having a predetermined plurality of the filaments coupled together in a fixed phase relationship;   a weighting circuit operable to apply phase adjustments to signals passed to and/or from the n filaments and/or filament group;   detecting means operable to detect at least one electrical property of the multifilar antenna with respect to the frequency, polarisation and/or direction of propagation of a signal to be received or transmitted by the multifilar antenna and/or impedance matching of the antenna; and   control means, responsive to the detecting means, operable to control the operation of the weighting circuit to adjust the properties of the multifilar antenna to suit better a current signal to be received or transmitted.   

   In another aspect, this invention also provides an adaptive multifilar antenna comprising:
     n spaced antenna filaments, where n is an integer greater than 1;   at least one filament group having a predetermined plurality of the filaments coupled together in a fixed phase relationship;   a matching circuit for matching the characteristic impedance of the antenna to that of a transmitting and/or receiving apparatus;   a phasing circuit for applying respective gain and phase adjustments to signals passed to and/or from the n filaments and/or filament group;   switch means associated with each filament for selectively altering the electrical length and/or interconnections of the filaments;   means for detecting electrical properties of the multifilar antenna with respect to the frequency, polarisation and/or direction of propagation of a signal to be received or transmitted by the multifilar antenna and/or impedance matching of the antenna; and   control means, responsive to the detecting means, for controlling the operation of the matching circuit, the phasing circuit and the switch means to adjust the properties of the multifilar antenna to suit better a current signal to be received or transmitted.   

   In the invention, the phase and/or gain relationships for signals from individual filaments of a multifilar antenna, and optionally also with the electrical length and/or interconnection pattern of the filaments, can be varied automatically in order to improve (or possibly to optimise, within the resolution of the adjustment system) the properties of the antenna for a particular signal to be received or transmitted. The automatic variation may be applied identically to predetermined groups of individual filaments. 
   For example, in embodiments of the invention, at least one of the above parameters could be varied to provide the best received signal level, the best signal to noise ratio, or the best signal to (noise plus interference) ratio and/or the best VSWR. 
   The adjustments will generally lead to a change in the antenna&#39;s frequency response and radiation pattern (shape and polarisation). It may not matter to the adjustment system what that change is quantitatively; the system may simply measure the output and make adjustments so as to improve the handling of the current signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: 
       FIG. 1  is a schematic diagram of a quadrifilar helical antenna (QHA); 
       FIG. 2  is a schematic diagram of an antenna interface circuit; 
       FIG. 3  is a more detailed schematic diagram of one possible implementation of the antenna system of  FIG. 2 ; 
       FIG. 4  is a more detailed schematic diagram of another possible implementation of the antenna system of  FIG. 2 ; 
       FIG. 5  is an enlarged view of an alternative for the portion of  FIG. 3  enclosed in dotted lines; 
       FIG. 6  is an enlarged view of an alternative for the portion of  FIG. 4  enclosed in dotted lines; and 
       FIG. 7  is a plot comparing the diversity performance of differently configured QHAs. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , a QHA comprises four helical elements  10  . . .  40  and eight radial elements  50  . . .  120 . (In other embodiments six, for example, angularly spaced helical elements could be used). It will also be noted that not all the radial elements  50  . . .  120  will be present in all antenna configurations. 
   The helical elements are intertwined as shown in  FIG. 1 , and are disposed about a longitudinal axis of the antenna by 90° with respect to one another. Four of the radials  50  . . .  80  are disposed on the top and four  90  . . .  120  on the bottom of the volute, connecting the helical elements and forming two bifilar loops. The antenna is fed on one set of radials  90 ,  110  with 90° phase difference between the two feeds. 
   The radials  50  . . .  80  at the top end of the antenna with respect to the feeds (which in this example are at the bottom) may be shorted in pairs or may be open-circuit depending on the resonant length of the helical elements and the required response. 
   The QHA is described in the following references:
     [1] Kilgus C. C., “Multielement, Fractional Turn Helices”, IEEE Transactions on Antenna and Propagation, Vol.AP-16, pp. 499-500, July 1968   [2] Kilgus C. C., “Resonant Quadrifilar Helix”, IEEE Transactions on Antenna and Propagation, Vol.AP-17, pp. 349-351, May 1969   [3] Kilgus C. C., “Resonant Quadrifilar Helix Design”, The Microwave Journal, December 1970.   

   The antenna&#39;s radiation pattern mode (hemispherical or other) depends on the phase combination used on the two or four feeds. The exact shape of the antenna&#39;s radiation pattern in each mode depends on the pitch and dimensions of the helices. In the axial mode it has a shape varying from hemispherical to cardioid depending on the dimensions of the structure. The polarisation is circular with a very good axial ratio inside the 3 dB angle. 
   In other embodiments, the multifilar antenna arrangement can also be used for diversity purposes. The different filaments can be used to provide space diversity between generally uncorrelated received signals. The effect of weighting the gain and/or phase can affect both the shape and the polarisation of the radiation pattern. This effect can benefit the transceiver in two ways. Firstly, the pattern shape and the polarisation are matching the direction and the polarisation of the incoming signal to try to optimise or improve the criterion ratio (S/N or S/(N+I), and secondly the structure can be used for polarisation diversity using the resulting pattern of different filaments or pairs of filaments. 
     FIG. 1  shows an antenna which has a generally cylindrical volute (i.e. circular in plan). Other volute shapes such as those having elliptical or rectangular plans or a truncated cone shape are also suitable for use in the present invention. 
     FIG. 2  is a schematic diagram of an antenna system comprising an adapted QHA  200  and an antenna interface circuit. 
   In  FIG. 2 , the four elements of the QHA  200  are connected separately to an adaptive matching circuit  210 . (In the configuration shown in  FIG. 2 , the antenna is in a receive mode, but it will be clear that signals could instead be supplied to the antenna, in a transmit mode, by reversing the direction of signal propagation arrows in FIG.  2 .) The adaptive matching circuit  210  is under the control of a matching controller  220 , which in turn is respective to a system controller  230 . 
   Received signals from the adaptive matching circuit are supplied to four respective variable weighting circuits W 1  . . . W 4 . Each of W 1  . . . W 4  comprises a variable phase delay and optionally, a variable gain stage, all controllable by the system controller  230 . 
   An alternative which is described in more detail below is to combine diametrically opposite pairs of elements ( 10 ,  30  and  20 ,  40 ) with fixed 180° weights at RF so that the antenna has only two feeds (each relating to a respective diametric pair) and therefore requires only two weighting circuits W 1 , W 2  and two transceivers  400  and  450 . 
   In the embodiment of  FIG. 2 , the outputs of the four variable weighting elements W 1  . . . W 4  are combined by an adder/weight combiner  240  to form a composite signal. This composite signal is then stored in a store  250 . A sensor  280  examines the signal (e.g. the level of the signal to (noise plus interference) ratio) and passes, this information to the controller which in turn adjusts the weighting factors of the weighting elements W 1  . . . W 4 , the matching circuit  210  and the switch elements  290 ,  300  to improve or possibly optimise the parameter sensed by the sensor  280 . The optimisation information can be used to optimise or improve the quality of the stored signal, which is then passed to the demodulator  260 . The information is also used to adjust the antenna system to receive the next incoming signal. 
   In each element of the QHA, there is a switch  290  capable of isolating a portion of the element remote from the feed point. The switch could be, for example, a PIN diode switch. Similarly, a switch  300  is capable of shorting or isolating pairs of the elements at the end remote from the feed point. 
   The operations performed by the switches  290  and  300 , under the control of a switch controller  310 , can change the response and radiation pattern of the antenna. In particular, by isolating a section of each element, the electrical length of the elements is made shorter and so the frequency of operation will be higher. Again, these operations are carried out under the control of the system controller to improve or possibly optimise operation with a particular signal frequency, polarisation and direction of propagation. 
   Alternatively, or additionally, the antenna element may be caused to have several resonant modes by the inclusion of one or more antenna traps. This causes the antenna to be resonant (and therefore have increased gain) at more than one operating frequency. 
     FIG. 3  is a more detailed schematic diagram of one possible implementation of the antenna system of  FIG. 2 , which also shows operation to improve or optimise the VSWR during a transmission operation and S/N+I during a receive mode. (Incidentally, when S/N+I is improved by adapting the antenna matching in a receive mode, this has an indirect side-effect of tending to improve the VSWR. Also, when the pattern mode, polarisation and direction are improved by adjusting for the best or an improved S/N+I, this similarly has a corresponding improving effect in a transmit mode.) 
   In  FIG. 3 , the operation of the weighting elements W 1  . . . W 4  is carried out at baseband in a digital domain, as is the operation of the adder/weight combiner  240 . 
   The output of the adaptive matching circuit  210  is supplied to a quadrature downconverter  400  comprising an intermediate stage  410  where a local oscillator signal is mixed with the radio frequency signal, an amplifier  420  and a further stage of mixing with a local oscillator signal with a 0° and 90° phase relationship to generate two demodulated outputs I and Q. These are both converted to digital representations by A/D converters  430  before being stored in a RAM  440 . This process is replicated for each of the elements of the QHA. Similarly, for the transmit side, an output from the RAM  440  is passed to a quadrature modulator  450  before being routed via the adaptive matching circuit  210  to the respective antenna elements. A VSWR detector  460  operates in a transmit and/or receive mode to detect the standing wave ratio of the antennas. The output of this is stored in the RAM  440 . 
   The RAM is connected to a digital signal processing (DSP) unit  470  which combines the digital representations of the signals stored in the RAM  440  in respective proportions and using respective phases (i.e. performs the operation of the weighting blocks W 1  . . . W 4 ), detects and optimises the selected parameter such as signal-to-noise ratio, sends control signals to the adaptive matching circuits to change from one frequency band to another or to overcome de-tuning effects, and also controls the switch controller  310  and in turn the switches  290 ,  300  within the helical elements. 
   One appropriate DSP algorithm is for the transmitter to send packet header, reference or training symbols, which are known to the receiver. Any disturbance to the received signals during the reception of the training symbols is a measure of N+I and can be reduced by trial and error (repeated combining of the digital representations stored in the RAM  440 ), direct matrix inversion of the associated correlation matrix or by iteration approaches such as so-called LMS or RLS algorithms. However, even if known training symbols are not available, a measure of the disturbance to the signal can be made by error detection algorithms applied to the received symbols. 
     FIG. 4  is a more detailed schematic diagram of an alternative implementation of the antenna system of FIG.  2 . This implementation has a quadrature downconverter  400 ′ which operates in the same way as the downconverter  400  of FIG.  3 . Similarly, it has a quadrature modulator  450 ′ which operates in the same way as the modulator  450  of FIG.  3 . 
   The operation at baseband of the implementation shown in  FIG. 4  is also similar to that of  FIG. 3  in that the downconverted signals are converted into the digital domain and stored in a RAM  440 ′. The data in the RAM is processed by a digital signal processing unit  470 ′ and the DSP  470 ′ is operable to cause changes in the adaptive matching circuit  210 ′ and in the antenna switches  290 ′,  300 ′ and  310 ′. 
   However, the operation of a circuit of  FIG. 4  differs significantly from that of  FIG. 3  in that the weighting operation is performed at RF in weighting blocks  500  which are coupled in the signal path from the individual antenna elements to the quadrature downconverter  400 ′. 
   In  FIG. 4 , the weighting block  500  is coupled directly between the adaptive matching circuit  210 ′ and a combiner  240 ′ which operates to additively combine the outputs of the respective weighting circuits W 1 , W 2 , W 3 , W 4  contained in the weighting block  500 . 
   The output of the combiner  240 ′ is fed into a single quadrature downconverter  400 ′. Thus, unlike the implementation shown in  FIG. 3 , only one downconverter  400 ′ is required. Similarly, only one quadrature modulator  450 ′ is required. 
   This alternative implementation has two main advantages. Firstly, since only one downconverter  400 ′ and one modulator  450 ′ is required, there is a resultant cost saving in the manufacture of the transceiver. 
   Secondly, since most of the noise in the received signal is introduced by the receiver, there is a fourfold decrease in the noise added by the receiver section since the signal passes through only one (instead of four) downconverters  400 ′. As a further subsidiary advantage, since the signal from all four antenna elements is subjected to the same noise in the single downconverter  400 ′, it is not necessary to apply gain weightings. Thus the weighting circuits W 1 , W 2 , W 3 , W 4  may be arranged only to apply phase adjustments to the signals received by the antenna elements. This simplifies their construction and therefore also has cost and reliability advantages. 
   In order to optimise the weightings, a slightly different approach may be taken to that used with the implementation of FIG.  3 . It will be noted that in the implementation of  FIG. 3 , the stored data may be iteratively processed with different weighting applied to the data until an optimal or at least improved result is obtained. However, in the implementation of  FIG. 4 , the data stored in the RAM  440 ′ already has weighting applied to it and in fact the signals from each of the elements of the antenna have already been combined by the combiner  240 ′. Thus, in order to find the correct weighting, the weighting are adjusted dynamically during reception of a signal (for example a training sequence). By storing data representing the known weighting settings against data representing the quality of the received signal, it is possible to determine which weighting gives the best reception and/or transmission characteristics. Thus the principles are similar but in the first case ( FIG. 3 ) the weighting optimisation may occur “off line” whereas in the implementation of  FIG. 4 , the weighting optimisation occurs “on line” during reception of a signal. 
   As mentioned above, the number of weighting blocks (and in the case of the embodiment shown in  FIG. 3 , of up and down converters) may be reduced by coupling together predetermined antenna elements. This has the advantage of reducing further the complexity of the circuit and therefore its cost. 
   In the preferred embodiment using a quadrifilar helical antenna as shown in  FIG. 1 , the predetermined groups of antennas are two groups containing the diametrically opposite pairs of elements  10 ,  30  and  20 ,  40  respectively. 
   The Table below shows the diversity correlation coefficient matrix for each of the elements. The figures have been derived from complex coefficients produced empirically. It will be noted that in the table below, the diametrically opposite pairs of elements have correlation coefficients in excess of 0.7. 
                                                               TABLE 1                   Diversity parameters for four elements of the QHA            Correlation                       coefficient       matrix   Element 10   Element 20   Element 30   Element 40                    Element 10   1.00   0.13   0.75   0.14       Element 20   0.13   1.00   0.17   0.76       Element 30   0.75   0.17   1.00   0.20       Element 40   0.14   0.76   0.20   1.00                    
Thus, although the grouping of elements is described below in connection with two pairs of elements, on a more general level, the predetermined groups of elements may be groups of elements which are each correlated to within 0.6, preferably 0.7 and more preferably 0.8 or better.
 
   For the quadrifilar helical antenna described below, the pairs of elements are coupled in pairs with a 180° phase shift. This may be achieved using fixed combiners or baluns B 1 , B 2  as shown in  FIGS. 5 and 6 . 
   Looking particularly at  FIG. 5 , it will be noted that the components shown in that Figure can be used to replace the components shown within the dotted outline on 
   FIG.  3 . This allows the circuit in  FIG. 3  to only have two up and down converters  400 ,  450  which reduces cost. Although  FIG. 5  does not show an adaptive matching circuit  210 , this could be included. 
     FIG. 6  shows the equivalent modification for the circuit of FIG.  4 . Similarly, the adaptation of  FIG. 6  could include an adaptive matching circuit  210 ′. 
   The circuits of  FIGS. 5 and 6  could also include provision for structure switches  290 ,  300  or  290 ′,  300 ′ respectively. 
   The grouping of elements in this way may produce a slightly reduced diversity gain compared to the earlier described circuit in which all four elements are independently adjusted. 
   However,  FIG. 7  shows a comparison of the performance of a QHA having four independently adjusted elements and a QHA in which the elements are combined into two pairs, against a standard QHA (which has been normalised to the 0 dB level). It will be seen that the diversity gain penalty for using the grouped configuration is only about 1 dB in areas of deep shadow with high multipath and that there is an advantage in situations where the signal is not significantly decorrelated between elements (for example, in environments where there is a direct line of sight between the base station transceiver and the antenna). 
   Thus it will be seen that the optimal solution will usually be separate control of each element  10  . . .  40 . However, a very satisfactory compromise may be reached between cost and performance by carefully selecting elements (for example according to their diversity correlation coefficient, however measured) and combining these elements with suitable fixed phase shifts to provide a reduced number of antenna feeds.