Realising personal satcoirn antennas by J. RI James The worldwide interest in personal mobile satellitle communications using constellations of low-orbiting satellites highlights the handset antenna as a component that has a critical effect on the overall system viability. The antenna design is, however, influenced by many factors and the purpose of this overview is to identify key design aspects, their interrelation and prospects for optimisation. Fundamental engineering design issues, difficulties and trade-offs are considered first, followed by a note of recent research and an outline appraisal of the power budget that should be achievable. The antenna requirements are application dependent and the future customer base is conjectured upon together with suggestions for alleviating the stringent antenna design constraints. 1 Introduction Designing a constellation of low-orbiting satellites with lowered orbits, higher radiated powers and complex beam shaping are obvious ways of making existing types of personal handset antennas practically realisable. The cost of such a system may well exceed existing expectations of what the user is prepared to pay and it makes good economic sense first of all to squeeze the maximum performance from the personal handset antenna. Antenna designers have already made significant creative contributions to niobile communication systems‘ and optimising the personal satcom antenna is now a priority task. At the time of researching this overview I was not able to identify the sort of antenna configuration that the major companies had settled on, although it seemed likely that they were ready with various options. As such my task has been all the more challenging because in the near future, you the reader will be able to judge the robustness of my arguments and prognosis. An obvious question is whether or not the requirements for personal satellite terminal antennas are much different from those for existing personal mobile antennas for cellular networks. I will therefore begin in the second Section with a scrutiny of the generic mobile personal antenna configuration from which we can deduce the fundamental engineering issues and problems that are, or are not, common to both cellular and satellite systems. It will be evident that the list of fundamental problems to solve is both extensive and diveise and, as is often the case in engineering, one is faced with the familiar task of trading one effect for another until the overall system requirement is satisfactorily met, which generally means allowing large enough operational margins. In the third Section I will expand on these fundamental problems to highlight the difficultiesand how the latter can be reduced by design and trading off one effect for another. My aim here is not only to summarise these design issues but to explain how they interact with one another and also relate to the various communication applications. Equipped with this information one is in a good position to appreciate in the fourth Section the sort of antenna designs that are available at a research stage and their strengths and weaknesses. Mobile antennas, and particularly those for the new personal satellite systems, are fairly recent developments stretching back no more than, say, a decade, although much of the basic antenna development, i.e. printed technology, has a longer history. Consequently, this overview is restricted to the most recent research and readers should refer to the various specialist antenna books for fuller background details on basic antenna design. In the fifth section I attempt, in this limited space, to sum up on what has been achieved, what remains to be optimised and what the prospect of further advances in design might be. I say ‘attempt’ because the amount of detail to sort through is enormous and, like all engineering system evaluations, it includes not only technical aspects, but also those concerning the customer, the environment, reliability, costs etc. I will note the foreseen applications for personal satellite systems; these are surprisingly extensive and open up further developments. In particular it seems that in future one need never be isolated from a communication network, irrespective of one’s location in the world, be it in the Amazon forests or the Arctic. Finally in the sixth Section I bring together the main points of the paper with critical comments as appropriate. 2 Fundamental engineering design issues A typical operational environment for a personal satellite terminal antenna is sketched in Fig. 1, which shows direct and multipath received signals and the proximity of the ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL APRIL 1998 73
Transcript
Realising personal satcoirn antennas by J. RI James
The worldwide interest in personal mobile satellitle communications using constellations of low-orbiting satellites highlights the handset antenna as a component that has a critical effect on the overall system viability. The antenna design is, however, influenced by many factors and the purpose of this overview is to identify key design aspects, their interrelation and prospects for optimisation. Fundamental engineering design issues, difficulties and trade-offs are considered first, followed by a note of recent research and an outline appraisal of the power budget that should be achievable. The antenna requirements are application dependent and the future customer base is conjectured upon together with suggestions for alleviating the stringent antenna design constraints. 1 Introduction
Designing a constellation of low-orbiting satellites with lowered orbits, higher radiated powers and complex beam shaping are obvious ways of making existing types of personal handset antennas practically realisable. The cost of such a system may well exceed existing expectations of what the user is prepared to pay and it makes good economic sense first of all to squeeze the maximum performance from the personal handset antenna. Antenna designers have already made significant creative contributions to niobile communication systems‘ and optimising the personal satcom antenna is now a priority task. At the time of researching this overview I was not able to identify the sort of antenna configuration that the major companies had settled on, although it seemed likely that they were ready with various options. As such my task has been all the more challenging because in the near future, you the reader will be able to judge the robustness of my arguments and prognosis.
An obvious question is whether or not the requirements for personal satellite terminal antennas are much different from those for existing personal mobile antennas for cellular networks. I will therefore begin in the second
Section with a scrutiny of the generic mobile personal antenna configuration from which we can deduce the fundamental engineering issues and problems that are, or are not, common to both cellular and satellite systems. It will be evident that the list of fundamental problems to solve is both extensive and diveise and, as is often the case in engineering, one is faced with the familiar task of trading one effect for another until the overall system requirement is satisfactorily met, which generally means allowing large enough operational margins. In the third Section I will expand on these fundamental problems to highlight the difficulties and how the latter can be reduced by design and trading off one effect for another. My aim here is not only to summarise these design issues but to explain how they interact with one another and also relate to the various communication applications.
Equipped with this information one is in a good position to appreciate in the fourth Section the sort of antenna designs that are available at a research stage and their strengths and weaknesses. Mobile antennas, and particularly those for the new personal satellite systems, are fairly recent developments stretching back no more than, say, a decade, although much of the basic antenna development, i.e. printed technology, has a longer history. Consequently, this overview is restricted to the most recent research and readers should refer to the various specialist antenna books for fuller background details on basic antenna design.
In the fifth section I attempt, in this limited space, to sum up on what has been achieved, what remains to be optimised and what the prospect of further advances in design might be. I say ‘attempt’ because the amount of detail to sort through is enormous and, like all engineering system evaluations, it includes not only technical aspects, but also those concerning the customer, the environment, reliability, costs etc. I will note the foreseen applications for personal satellite systems; these are surprisingly extensive and open up further developments. In particular it seems that in future one need never be isolated from a communication network, irrespective of one’s location in the world, be it in the Amazon forests or the Arctic.
Finally in the sixth Section I bring together the main points of the paper with critical comments as appropriate.
2 Fundamental engineering design issues
A typical operational environment for a personal satellite terminal antenna is sketched in Fig. 1, which shows direct and multipath received signals and the proximity of the
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,p ,I_I IT7
e direct radiation: t multipath; c thermal noise
Fig. 1 Personal-satcom-antenna electromagnetic environment showing proximity to person's head
antenna to the human body, which itself is in contact with the ground.
The ground, buildings etc., as well as the user, all contribute to the thermal noise background, as indicated in the Figure. The personal terminal will generally be held
Table 1 : personal mobile terminals
Fundamental antenna design issues arising in
0 Electrically small antenna constraints and/or constraints on antenna shape give rise to polarisation restric-tions, loss of radiation efficiency, narrow bandwidths and increased sensitivity t o rrianufacturing tolerances
0 Ekctrically small ground plane constraints due to compactness of handset case create pattern corruption .that varies with user's hand and body position
0 Input irnpedance matching, necessary for good t ra rism itter rad id ti on efficiency i? nd receiver si cj ria I-to- noise ratio on reception
0 Antenna bandwidth, generally dominated by the antenna input impedance frequency characteristic but also dictated by radiation pattern characteristics
0 Antenna radiation pattern, including sidelobes, hemispherical beam shape, nulls and polarisation speciticafion to be maintained over specitled bandwidth to reduce thermal noise and interference
0 Loss cf radiated power due to absorption in user's body impairs radiation efficiency and pattern characteristics
0 Time-varying effects on above parameters due to movement OF user's hand and body
0 Deployment of diversity techniques where possible to reduce multipath and time-varying effects
against the head in voice operation but'may well be required to receive initially when stored in a clothing pocket. Additionally, the person may well be seated or, perhaps, reclining. The provision of medium earth orbit (MEO) and low earth orbit (LEO) satellite systems makes it feasible to receive signals with low-gain, very broad beamwidth antennas compatible with compact handsets. Various operational frequencies around 1.5 to 2.5 GHz have been nominated but for the purposes of this paper it is noted that present and future personal satellite terminals are likelyZ to use various frequencies up to 3 GHz. From the user's point of view, continuous conversational speech operation is expected in the multitude of everyday open-air situations, with perhaps some lesser performance when the sky is obscured by sparse foliage, overhead wires, tall buildings etc. Clearly personal satellite terminals are advantageous in low-populated open countryside and network integration with cellular urban in-building systems is envisaged2. Most people are aware that portable handsets are fitted with an antenna which may need extending in operation but beyond this no other adjustments or angular positioning are expected. User- friendly, safe and reliable operation of the handset is thus the designer's operational benchmark.
Personal satellite terminals, like other communication equipment, are now increasingly configured in digital technology, which opens the door to integrated semiconductor chip subcomponents and associated reduction in handset size, weight and cost. Such techniques offer the designer immense signal-processing options and, with ingenious updatable software, the opportunity of giving the customer added-value with each new product version. Even so there is a limit to what in-box processing can achieve as regards the optimisation of the incoming signal with respect to noise: the antenna, as both a spatial and spectral filter, remains an indispensable and unique component of the personal terminal handset, facilitating the transition between waves in free space and confined circuits. The design and optimisation of personal terminal antennas thus demands an understanding of the convolved interactions taking place in this complicated electromagnetic environment and this fact is widely appreciated. What is seldom appreciated is the fact that each new handset that is envisaged usually commits the antenna designer to an entirely new programme of development since a previous antenna design can seldom be simply modified. The point is that a small change in the physical structure and environment of an antenna can invoke disproportionate changes in performance and this
A list of fundamental engineering design issues that arise in the design of an antenna mounted on or integral with some sort of portable handset in mobile communications is given in Table 1.
It is assumed here that both receive and transmit duplex operation is required, which will be the most common requirement for personal communication.
Since this paper focuses on satellite, as opposed to present-day terrestrial, mobile communications, one might hope for some easing of the constraints in Table 1
has previously been explained at len&h elsewhere'.
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but no really clear pitcture emerges. At present the trend' in choice of the frequency band seems to favour higher frequencies away from 1.5 GHz and above 2 GHz, in which case all physical dimensions are at least 33% electrically larger. It can be seen that for a given physical space occupancy both the antenna and handset can be electrically larger, thus alleviating' some of the restrictions listed in Table 1. Thus the bandwidth of the input impedance and the radiation pattern characteristics will be somewhat eased by the upward shift in frequency. The depth of penetration of radiation into the user's body will reduce and the latter will reflect a higher proportion of the incident radiation. Some benefit may arise due to lower power being absorbed but time-varying effects may now be more pronounced, thus increasing the necessity for some adaptive control of the antenna operation. There will certainly be a little more scope for diversity techniques given the increase in the handset's electrical dimensions. In summary, the trend to a slightly higher frequency of operation suggests no really significant easing of design constraints and some aspects may be more problematic. As such, Table 1 is taken as a good indication of design issues for personal satellite terminal antennas.
Some manufacturing and sales promotional considerations influencing the design of personal mobile antennas are listed in Table 2 and some of these are now creating significant design constraints. In particular, the interference levels that can be tolerated from electronic equipment are likely to be constrained even more in the future and likewise for the power absorbed by the user's body. Manufacturers are very conscious of regulations and have made much progress in reducing unwanted emissions and absorption, giving much benefit to the user.
3 Design difficullties and trade-offs
Aglance atTables 1 and 2 invites the question as to the best approach to the antenna design in what can only be described as a greatly constrained design environment. I do not know the answer to this question but my encounters with designers su,ggest a heuristic, iterative process starting with the choice of antenna type, followed by installation on the, handset case and finally system measurements and trials. Modern computer modelling methods are of increasing help at each step but no all- embracing computer model exists at present and is unlikely to do so into the foreseeable future. The various technical details involved in the overall design processes are well known' and will not be repeated here. However, I will highlight some of the major issues to emphasise how they interact with one another, giving rise to a trade-off in the design methods.
Generic antenna types and variants Simple wire dipoles and loops in free space are generic
dual antenna types, Fig.2, that relate respectively to electric and magnetic infinitesimal Hertzian dipoles5. When placed near a perfectly conducting ground plane the images of a dipole and a loop negate and reinforce, respectively, the field near the ground plane. There is no
Table 2: considerations having some influence on design of personal mobile antennas
Manufacturing and sales promotional
0 User-friendlyreliable operation allowing easy use with
0 Cost factors relating to ease of manufacture, spares
0 Exploitation of new materials that are lightweight,
minimum of moving parts to adjust
and after-sales service
non-breakable, non-toxic and do not generate intermodulation effects
0 Electromagnetic compatibility of components and layout to reduce reception of local interference and harmonics on transmit and local oscillator radiation
radiation absorbed in user's head and body 0 Safety and health hazards impose constraints on
rigid rule here but in general a loop antenna is less affected by the close proximity of the human body than a wire dipole, hence the use of loops at lower frequencies in pagers'.
For personal mobile terminals at higher frequencies, wire monopole antennas and their derivatives are commonly used since the antenna can be tilted away from the head. The wire monopole can be compacted by bending over the wire to form the inverted-L antenna6, but the input impedance and bandwidth are much lowered. Tapping into the wire along its length creates the inverted- F antenna (IFA)6 with increased input impedance, and replacing the wire with a flat strip increases the bandwidth. The polarisation characteristics depart from those of a
monopole and there is high cross-polarisation. The IFA in printed technology offers compactness and ease of manufacture but use of a thicker and higher permittivity substrate will transform the IFA into an elongated microstrip patch antenna which has pronounced magnetic antenna behaviour appropriate to operation near the human body; clearly the generic concept of electric and magnetic antenna behaviour ceases to be meaningful for the antenna variants such as the IFA and the precise behaviour near the human body can only be ascertained by measurement. Circularly polarised antennas, Fig.3, with hemispherical beam coverage, such as the quadrifilar helix antenna, are ideally used in personal satellite terminals when a clear view of the sky can be acquired, as in Global Positioning Systems, but such antennas are bulky. For personal satellite terminals a compact antenna with high , cross-polarisation and time-varying characteristics due to head movement may be acceptable in built-up areas where multipath signals are prominent.
However, an overriding consideration for satellite reception is that the incident radiation comes predominantly from the sky region and ideally a hemispherical beamshape is required since this much reduces thermal noise radiation from the ground and buildings, as sketched in Fig.1. The increased antenna gain generally demands a bulkier structure, as noted for the quadrifilar helix; thus size reduction techniques are clearly of greater significance in personal satellite terminals than in cellular mobile systems, where ample base station transmit power is available, giving adequate margins over thermal noise received by omnidirectional antennas.
Electrically small ground planes Antennas are usually measured, analysed, categorised,
etc, in a well defined electromagnetic environment which is either free standing in space, i.e. the wire dipole, or mounted on an infinite or electrically large perfectly conducting ground plane, i.e. the monopole. For personal handset installations the antenna must function on an electrically small box, which necessarily distorts the radiation characteristics and the input impedance; since the antenna counterpoise currents now run over the box, the antenna’s performance is sensitive to the presence of the user’s hands, not to mention the user’s head. Numerous interesting theoretical results for small ground plane effects, Fig.4, are given in the literatureis8. In principle the box currents can be impeded or held off in many ways but the latter invoke an increase in antenna and/or box size together with a narrowing of the bandwidth. The sleeve antenna mounted on a handset illustrates these propertiesg. With the revolutionary trends in integrated electronics, the size of a handset is governed only by the size of batteries, microphones, speakers, switch controls etc., and, with ever increasingly skilful packaging, handsets are capable of further size and weight reduction. This puts increasing pressure on antenna designers to find ways of isolating the ‘box’ from the antenna but there are clearly imponderable electromagnetic constraints to wrestle with.
The user? head problem This is a continuation of the electrically small ground
plane problem whereby, in addition to the hand effect, the user presses his or her head against the handset to engage the microphone and speaker. No concern seems apparent about radiation absorption in the user’s hand but this is not
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so for the head region, as noted in Table 2, since such power absorption in the head has become an emotive issue in itsetPo. International agencies and other authoritative sources emphasise" that the effects are well understood and at the power levels used there is no proven risk. Understandably this has generated much interest in developing accurate electromagnetic models of the head absorption process with a view to modifying the headset antenna design, generally at the expense of some deformation of the radiation pattern characteristics. Apart from the health aspects the proximity of the user's hands, body and head must increase thermal noise pickup to some degree but only practical system measurements can establish the relative significance of these effects on the system performance margin.
'Smart' handsets Established antenna diversity and adaptive-array
techniqueslZ demand extensive signal-processing power and modern electronics can now provide this in the small space offered by a personal communication handset. Naturally there is increasing interest in creating 'smart' handsets whereby the antenna system adapts itself for optimum reception in a multipath environment. To realise this facility, the antenna must be capable of spatial or frequency or polarisation discrimination of the incident signals or some combination of these discriminants. In practice, bandwid1 h restrictions generally rule out frequency discrimination and with the small electrical size of the handset space diversity is less useful than polarisation dis~rimination'~. With the use of higher frequencies approaching 3 GHz there is more scope for diversity antennas, as previously noted, and some 'smart' management of reception is realisable, presumably at modest extra equipment cost. Some spatial diversity options for cellular personal handsets are shown in Fig.5. To what extent such techniques increase the operational margin can only be finally ascertained with experimental trials in a given multipath scenario. Any increased margin could then be traded for some reduction in antenna sizes. Any options for satcom personal handsets are clearly more constrained and are discussed later.
4 Recent research
Research papers on novel mobile type antennas are legion and I have mainly selected some of the recent contributions over the past two or three years. It then seemed useful to sift out those papers that addressed some of the design issues listed in Table 1. Few if any of the papers considered more than one or two problematical aspects and how onti problem is traded for another, so any cited advancement needs final appraisal in a system sense and this will be cclnsidered later. This present Section highlights the advances cited under the following six sub- headings, putting the more importance aspects first.
Antenna size reduction A size-reduced circularly polarised antenna appears to
be the present design objective for the personal satellite
Fig. 5 Some spatial diversity options for personal handsets: (a) whip and planar inverted-F antennas; (b) printed patch and slot antennas; (c) quarterwave helix antenna and extendable inner helix antenna mounted on dielectric rod
terminal. Existing bulky types, such as the quadrifilar helii, the drooping crossed dipole, and the inverted-V drooping dipole, are described in Reference 14 and circularly polarised microstrip patch antennas are treated in Reference 15. Ways of making helical antennas either smallerI6 or of square-shaped windings at 1.8 GHz17 have been investigated and a printed quadrifilar helix antenna at 1-57 GHz'* has improved performance and is regarded as advantageous for GPS (Global Positioning System) marine applications. The numerical analysis of a circularly polarised patch antenna mounted on a handset at 2 GHzl9 confirms that the presence of the human head significantly perturbs the polarisation characteristics. This suggests that an antenna protruding vertically, such as the quadrifilar helii, will be less affected by the head than lower profile antennas; but simulated system calculationszo based on the early work of Kilgus21,22 indicate that the quadrifilar helii antenna is too large at L-band and more research on size reduction is needed.
It is of interest to ask if other configurations of twisted and bent wires could be designed to satisfy both the polarisation and small-size criteria and recent computational optimisation methods using genetic algorithm9 suggest that this might be so. A practical realisation of one such example is the monopole loaded loop antennaz4, which has a relevant performance over the
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band 1.4 to 2 GHz. It would seem only a matter of time before an optimal circularly polarised size-reduced antenna mounted on a handset is evolved but the practical realisation is fraught with fundamental physical limitations, as can be quantified as follows.
Given a source of radiation of free-space wavelengthh,, bounded by a spherical surface of radius a, then the field at a point (Y, 0, @) for @a can be represented by a sum of spherical transverse electric (TE) and transverse magnetic (TM) wave harmonics (Ref.5, Chap.7). The TE and TM field components are generated from the scalar functions yMand yE, respectively, where
k = 2n/$ and CoefficientsAA, An% , an;, a& characterise the radiation source. Other symbols have their usual meaning5. Low-gain communication antennas will be represented by a small number of coefficients, unlike narrow-beam antennas where the number is very high. Let the highest value of n be Nfor a given practically realisable source. Then
N 5 - 2m (2) a0
This well known approximate relation46 relates to the behaviour of the spherical Hankel function h(f (kr), whose imaginary part increases steeply in magnitude when k r a , thus escalating the antenna Q factor and its susceptibility to tolerances. The physical implications of this relationship are profound because if we attempt to miniaturise the antenna and at the same time to maintain the radiation pattern characteristics and hence the N value, the bandwidth will reduce and, worse still, the antenna will be tolerance-prone due to large oscillatory currents needed to generate the prescribed fields.
It can be readily shown that if
(3)
then the fields arising from eqn.1 are those of an antenna possessing perfect circular-polarisation (CP) characteristics. For a low-gain omnidirectional satcom CP antenna an N value of 1 (i.e. dipole) will suffice, which by eqn. 2 leads to an antenna bounded by a spherical surface of radius of about 2.5 cm at 2 GHz. Ignoring feeders, the antenna could be configured using a loop/dipole combination, similar to the concepts reported in References 23 and 24. So far this device has excellent CP characteristics and an acceptable Q and is fairly compact, but to realise it when mounted on the handset box is another matter. At 2 GHz the box will appear as an electrically small ground plane and the radiation pattern will be asymmetric in elevation, have impaired CP characteristics and may well be non-
omnidirectional, even in the absence of the user's head. A spherical harmonic representation of this mounted antenna will require higher order harmonics to represent the increased pattern detail and from experience I would say that one might expect an N around 4 at least, which from eqn. 2 gives a ,- 10 cm. This latter spherical surface, of diameter 2a=20 cm, now embraces the handset case as well as the antenna and this estimated sue is compatible with the space required by a handset-mounted quadrifilar helix at 2 GHz, i.e. a 12 cm long handset plus about 8 cm antenna length. A computational optimisation procedure as in Reference 23 will readily recompute the antenna/handset structure for a reduced antenna height giving the same pattern characteristic, but the reduction of a, eqn. 2, while maintaining Nwill predict that the configuration is unlikely to be practically realisable - it will be narrower band and almost certainly more tolerance prone. There would seem to be a research project here to see just how much a quadrifilar antenna can be size reduced when handset mounted in the close proximity to the head and yet be operable and manufacturable.
Minimising body absorption Research on absorption in the head and hand is based
mainly on numerical evaluation using linearly polarised antennas as used on mobile cellular handsets. Investigators have searched for field hot spots in the brain and eyes and have proposed numerous ways of minimising the absorption, generally without consideration of operational system margins. The absorption from a wire dipole, based on spherical wave expansions at 2 GHzZ5 and the finite difference time domain (FDTD) method at 900 MHzz6, has been described, while the advantages of a slot handset antenna have been presented using a modified method of moments computation at 1.8 GHzZ7. A rear mounted radiation-coupled dual-L antenna is also claimed in Reference 29 to give lower head absorption. The use of handset extensions and reflectors at 1.8-2 GHz has been investigated by measurementz8 and an adjustable screening device is now commercially available30 whereby the user adjusts the screen mechanically to suit operating conditions. All this research has given a greatly improved understanding of the extent of the absorption, the effect on the antenna's radiation pattep and options for minimising the absorption.
It seems very likely that a compromise situation can be reached for mobile cellular handsets given the higher signal-to-noise margins and the ever present multipath effects. For satellite personal handset antennas the situation is less clear if the circular polarisation purity is to be maintained in the presence of the head and with the already low operational margin.
Smart antenna progress Most of the research papers published on diversity
techniques and other adaptive methods pertain to cellular base-station antennas and cellular handset antennas. Examples are the finite element method (FEM) analysis of a four beam-switched planar array for a mobile terminal at 1.97 GHz3', investigations into propagation path
78 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL APRIL 1998
and methods for measuring mobile antenna diversity characteristic^^^. As already mentioned the satellite mobile personal antenna already has a size problem and severe circular polarisation constraints so a smart performance may well require some relaxation of these constraints in order to facilitate at least one more operational radia.ting element or discriminant. Apparently a two-el ement stacked phased-array antenna for satellite mobile personal operation, Fig.6, has been developed for a 14 GHz uplink and 2-5 GHz downlink system34 and sinnulations indicate that multipath problems are significantly reduced. The effect of the head and its movement clearly represents an additional complication and some simulated quantify the problem.
Other aspects The following innovation is associated with cellular
mobile antennas but may have some indirect influence on satellite personal handset antennas. The use of high- permittivity low-loss substrates to reduce the size of microstrip patch antennas at 900 MHz and 1.8 G H z ~ ~ has been demonstrated and similar size reduction is obtained at 880 MHz and 92C1 MHz using C-shaped patches on low- permittivity substrates". A loss of radiation efficiency in such techniques seems likely but no information was given in this respect. Dual-band operation with an inverted-F antenna37 has been achieved by a sandwich construction, while coupled Lsections create a dual-band L antenna around 900 MHz38. A somewhat similar coupling process in an inverted-F antenna together with a variable capacitor facilitates band swkching around 2 G H Z ~ ~ . An unusual mechanical arrangement is demonstrated at 1.9 GHz40 whereby a rectangular patch antenna can be rotated to tilt the beam away from the user's head and hence show superior gain performance to a whip antenna. Many novel antenna configurations continue to evolve for vehicle- mounted satellite antennas such as microstrip ring antenna^^^,^^, microstrip stacked antennas43 and cavity- backed helices44. These radiating configurations are generally wide-base squat devices and unsuitable for personal handsets.
5 System appraisal
The reader having r'eaching this point will be in no doubt as to the constraints surrounding the design of a personal satcom handset anlzenna. The reader may wonder about the perceived cominunication services that the handset might be required tla provide and how these impact on the antenna design. The following therefore summarises what is feasible and who might use it.
State-of-the-art From the abolre overview it is apparent that
recommended antenna designs exist to overcome most of the problems taken in isolation from one another but there is no clear-cui overall solution. The quadrifilar helix (QFH) antenna on a large enough ground plane has the desired radiation pattern shape and is currently used in
Fig. 6 multipath effects? (a) stacked quadrifilar helix antennas used in simulation; (b) principal multipath action
commercially available GPS personal handsets which, unlike the present speech system, will not be held against the head. In the above Section on 'antenna size reduction' a handset length of 12 cm was cited together with a realisable QFH antenna length of about 8 cm; doubtless the handset could be made smaller but this order of length is set by the need to speak and listen at the same time. Just how the QFH patterns would be perturbed by a 12 cm long compact handset is a question requiring an experimental evaluation but to further reduce the handset size will certainly aggravate the pattern perturbation. This QFH antenna will have at best an 8-10 dB operational margin over linear polarised antennas, such as wire monopoles and printed patches. This enhancement arising from the QFH is made up very approximately as follows:
Use of two-element phased array to reduce
0
0
- 3 dB for circular polarisation - 2 dB extra gain - 3 dB due to reduction of terrestrial thermal noise due to hemispherical pattern shape.
If the 8-10 dB enhancement can be relaxed somewhat then it may be possible to tolerate the additional pattern corruption and thermal noise due to the close proximity of the head and hand. This balancing act would thus trade the satcom link budget against operational constraints arising from the user requirement. Minimising head absorption
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Fig. 7
operation: (a) single band-switched antenna deployable by extension or folding; (b) stacked two-element phased array with band-switching and provision for tilt away from head
and smart control might well be constraints too many in this already over-constrained design. Furthermore, in some proposed systems the receive and transmit bands are widely separated so the antenna must have a switch facility. Further research will no doubt lead to some benefits and the merits of an antenna that can be mechanically extended clear of the head should be investigated. Fig. 7 shows sketches of likely options while Fig. 8 gives examples of publicised handsets.
Sketches showing likely dimensions of satcom personal handset quadrifilar helix entenna+ for 2 GHz
Who will use it? Business users with urgent messages who are out of
range of cellular base stations and have no conventional telephone available are likely to be an immediate customer base with a preference for slimline compact handsets. Then there are a multitude of potential users requiring some sort of security back-up, or perhaps an emergency distress communication line; the list of such user types is extensive and includes lone workers or stranded travellers in a remote terrain or at sea and isolated holiday makers. For these users a bulkier handset with a larger deployable antenna may well be acceptable in exchange for telephone access in an emergency.
Other types of users will be identified once a personal satcom communication system becomes available but the present commercial trends favour a flexible communicator rather than one that is customised for a particular application. For instance, a cellular handset that is satcom compatible relieves the user from carrying an extra box. Added to this is the on-coming demand for data handling
to a wide range of services. Receive-only satcom pagers might be another possibility. Finally, provision must be made to activate the ringing tone when on standby as a satcom receiver and this requires the link budget to be adequately maintained under typical user storage conditions.
My opinion from the above is that designers will not be given the option of creating different antenna designs and sizes to suit dedicated applications. The design task is likely to be a compact, all-purpose antenna system reflecting the design trade-offs outlined in the above Section on ‘state-of-the-art’.
(fax, Email, Internet) and multimode terminals for access
6 Possible solutions
a b
Fig. 8 for (a) Iridium and (b) Globalstar/GSM
Commercial publicity on proposed dual-mode handsets
At the start I said that it made good economic sense to see what additional performance could be squeezed out of the personal satcom antenna and I am sure the reader would like my opinion. In my experience speculation is often flawed by unforeseen developments and in this case it is I believe the future market which will no doubt surprise us in some way. I will therefore be brief and highlight the following:
e the antenna performance is constrained by fundamental physical properties and the perceived user operation the potential market is diverse and massive with large growth foreseen multirole, flexible operation is more likely to establish the market than dedicated equipment, at least initially
e adaptive control, spread-spectrum modulation, speech compression (vocoders) and other techniques may find use in increasing link margins and hence alleviate the antenna design problems to some extent
e
0
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a composite design, whereby a physically compact, circularly polarised antenna can be supplemented by some sort of pull-out extension which is clear of the head, i s a possihle compromise solution.
Only the blossoming of the personal satcom handset market will establis’n what the user wants, how much the user is prepared to pay and what can be practically realised. Whatever materialiases, the antenna designer’s role will have been vital and pivotal.
Acknowledgments
The deductions, opinions and speculation in this overview are necessarily mine but I would like to acknowledge gratefully discussions with Kenichi Kagoshima of NTT Japan, Gary Mauglian of I C 0 Global Communications (UQ, Chris Wilday of Nokia Mobile Phones (UK) and Andy Wilton of Motorola (UII).
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OIEE: 1997,1998
This is a revised version of an invited paper presented at the 10th IEE International Conference on Antennas and Propagation, Edinburgh, UK, 14th-17th April 1997.
TING SERIES The World Wide Web for scientists and engineers B. Thomas The World Wide Web has matured into a powerful tool for scientists, engineers and researchers. Far more than a collection of links and sites, today’s Web represents a sea change in the lives of professionals, providing new ways to communicate ideas and information. For Internet and Web users in the scientific and engineering community, this book provides strategies and essential information t o assist in moving beyond simply browsing the Web. Readers will discover how to conduct on-line research and publishing activities by following the author ‘s clear guidelines. Key points: e Essential tools and applications for accessing and navigating the Web e Web authoring and publishing, including basic and advanced HTML e Searching and researching the Web, including selected Web sites for 22 major technology
Series No. 502,356pp., paperback, 224 x 178mm, ISBN 0 85296 939 2,1998, f25 This book is published by the IEE. (A copublication with SPlE Press, USA) (Visit our website a t http://www.iee.org.uk/publish/books/www_s&e.html to access the full contents)
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