Antenna Design for Applications not defined and not standardized in
Rice. 3.14. Antenna dimensions
9 0 5 . The directivity factor (DPC) of the antenna, defined as the ratio of the power radiated in the direction of the maximum of the pattern, to the value of the power flux density averaged over all directions, depends on the type of the radiation pattern.The gain of the antenna is uniquely related to the directivity factor, which is defined as the product of the directive factor and the antenna efficiency. Typically, gain is measured in decibels relative to the gain of an isotropic antenna (dBi). An isotropic antenna is an antenna that provides the same radiation in all directions.
Another important indicator of antennas is the type of polarization. Polarization is linear (horizontal and vertical) and elliptical, in a particular case circular. In Bluetooth communication networks, antennas omnidirectional in the horizontal plane with a gain of (0-5) dBi will find application.
It should be noted that for antennas, the principle of reciprocity applies, according to which the same antenna can be used both as a transmitter and as a receiver.
In Bluetooth applications, microstrip and printed antennas, which are a metal conductor of one form or another, located above a grounded substrate, can be widely used. Such an antenna can be successfully combined with printed circuit board, on which the microwave stages of the transceiver are located. The transceiver is connected to the antenna at a certain point. At this point, the signal is diverted to the receiver and power is supplied from the transmitter.
Some Bluetooth applications may use directional antennas. Below are short descriptions and antenna specifications for Bluetooth systems from some manufacturing companies.
Company AntennasRange Star
P/N100903
Vertically polarized antennaBluetooth(TM)/802.11b
|
Table 3.11. |
|
|
frequency range |
2400-2483 MHz |
|
Maximum Gain | |
|
Polarization |
Linear |
|
Width radiation patterns |
omnidirectional |
|
Switched power | |
|
Feed point impedance | |
|
Dimensions |
22.0 x 12.7 x 0.8 mm |
Rice. 3.13. External view of the antenna 100903
The 100903 antenna is a vertically polarized antenna with an operating frequency range of 2400-2483 MHz. It is well suited for integration into access points, table and wall mounted devices, mobile phones, PC cards, PDAs and other Bluetooth applications. This is a reliable, simple and no tuning antenna. The appearance of the antenna, design and radiation pattern are shown in fig. 3.13, 3.14 and 3.15 respectively. The main characteristics are shown in Table 3.11.
Rice. 3.15.
AntennaBluetooth(TM)/802.11b- P/N100930
100930 is a built-in antenna for Bluetooth and 802.1 lb systems with an operating frequency range of 2400-2483 MHz. It can be integrated into access points, table and wall mounted devices, PC cards, and other Bluetooth devices. The appearance of the antenna, design and radiation patterns are shown in Fig. 3.1673.19. The main characteristics are summarized in table 3.12.
[With. 3.16. External view of the antenna 100930
Company AntennasKOSANT
KOSANT manufactures miniature microstrip antennas for Bluetooth. The main types of antennas and their characteristics are given in table 3.13.
Table 3.13. Main types and characteristics of KOSANT antennas
Rice. 3.18. Radiation pattern in the azimuthal plane
|
Frequencyrange (MHz) |
2400-2500 |
|||||||
|
Gain (dBi) | ||||||||
|
Polarization |
Linear |
Linear |
Linear |
Linear |
Linear |
Linear |
Linear |
Linear |
|
Impedance (P) | ||||||||
Example appearance and radiation patterns for these antennas is shown in fig. 3.20 -5- 3.22.
Rice. 3.20. Antenna appearance
Rice. 3.19. Antenna dimensions
not. 3.21. Radiation pattern in the azimuthal plane
Rice. 3.22. Directional pattern in the elevation plane
Note that the transceiver has these antennas mounted on a grounded shield.
3.8. Debugging and development aids for Bluetooth-based products
To simplify the understanding of technology, development and debugging of products based on it, Ericsson offers several special tools, each of which is focused on a specific range of users, developers and integrators. These tools help reduce the cost, streamline, and speed up the development of Bluetooth devices.
Starter Kit -Bluetooth® Starter Kit
The design of the set is shown in Figure 3.23.
Rice. 3.23. Bluetooth Starter Kit
Description
The Bluetooth Starter Kit provides a low cost and fully functional development environment for voice and data applications.
based on the Bluetooth module from Ericsson Microelectronics. The kit allows budding Bluetooth wireless technology developers to build Bluetooth applications, saving development time and reducing costs.
The Starter Kit provides a flexible design environment for engineers to familiarize themselves with the technology and begin development work. It demonstrates the key features of Bluetooth wireless technology, allowing developers to create integrated applications for breadboard products.
Set contains motherboard with matched connectors and power circuits, as well as a daughter board with a full Bluetooth module located on the board. With this set, you can get all the features you need to implement wireless technology bluetooth.
The kit is used to develop applications based on the host device, and also provides the core Bluetooth software, including the corresponding application programming interfaces (Application Programming Interface - API).
Development kit -Bluetooth™ Development Kit
The design of the board is shown in fig. 3.24.
Rice. 3.24. Bluetooth Development Kit
Description
approved by special working group Bluetooth as a Blue Unit, the Bluetooth Development Kit from Ericsson Microelectronics simplifies, speeds up and reduces the cost of developing Bluetooth applications.
It provides a complete and flexible development environment where engineers can integrate an open wireless standard into a range of digital devices. Providing access to all hardware interfaces, the set is used to develop both embedded and standalone applications. The suite includes software and hardware debugging capabilities to make the design process as quick and easy as possible.
Upgrade kitBluetooth - Bluetooth Upgrade Kit
The design of the product is shown in fig. 3.25.
Rice. 3.26. Bluetooth Application & Training Tool Kit
Rice. 3.25. Bluetooth Upgrade Kit
Description
The Upgrade Kit allows Bluetooth Development Kit owners to upgrade their Bluetooth Development Kit functionality. Several different upgrade kits are available depending on your needs.
VersionR1A
Upgrade Kit version R1A provides multi-point (multi-point) communication. An organized network supports up to seven slave devices, as well as master / slave switching. This version supports the OBEX and TCS protocols.
VersionR3B
Upgrade Kit R3B is required to test the Blue Unit. This set complies with Bluetooth v.l.Ob specifications. And can be used to test the Blue Unit according to the Bluetooth v.1.1 specification.
This version supports the OBEX and TCS protocols.
Toolkit for application and learning -Bluetooth Application &Training Tool Kit
The design of the product is shown in fig. 3.26.
Description
The Bluetooth Application & Training Tool Kit is designed for schools and universities and is a low cost, convenient tool for practical exercises when learning about Bluetooth wireless technology. It was developed by Ericsson Microelectronics and allows university students to both theoretically and practically study the Bluetooth short range radio communication system.
Rice. 3.27. Connecting the module to a computer using a USB connection
The hardware consists of a module that can be easily connected to a computer using a USB connection (fig. 3.27), which ensures that the full data transfer rate is used. A well-defined application programming interface (API) provides access to various layers of the protocol stack.
3.9. Power saving modes for Bluetooth devices
Access points based on Bluetooth technology will enable new generations of mobile devices to transmit large amounts of voice and data. Usually voice applications Bluetooth is powered by small battery packs. At the same time, data transmission systems can operate from network sources. In the first case, the economical mode of operation is most relevant. An effective way to save power is to reduce the amount of time the Bluetooth transceiver is active. Bluetooth Baseband specifications provide for three main ways to operate in economy mode:
1. If the slave device does not need to participate in the piconet, but still needs to be synchronized, it can be put into the "PARKING" (Park) mode. This mode is suitable for slave devices that need to communicate with the master from time to time. Devices in this
mode, can request to exit Park mode from the master by transmitting a periodic beacon signal (beacon) transmitted by the master. The intervals between beacon signals can be several seconds.
The Sniff mode is suitable for devices that need to communicate with the master periodically at a predetermined frequency. In this mode, there is no guarantee that devices will be serviced at every periodic request. Sniff mode saves battery consumption by reducing request traffic. Sniff intervals can last up to several seconds.
The "PAUSE" (Hold) mode is appropriate when the device can sometimes suspend call traffic. The device can enter Hold mode for a predetermined period of time to process another task, for example, to participate in the operation of another piconet, when nothing needs to be transmitted for a certain period of time, naturally saving energy.
In addition, if the master communicates with known (earlier discovered) devices, then when establishing communication, you can skip the request procedure. If at the same time the slave device is in the “call waiting” mode (Page Scan), then the call waiting time will be only a few tens of milliseconds. This is especially important if the master device is running on battery power and the slave device, which is constantly on call waiting, is powered by the mains. In this case, the power consumption of the device will be reduced.
In order to select the correct economical baseband mode, the hardware designer must consider the bandwidth, response time (or latency) and power requirements of each particular application. The longer the device remains idle, the greater the power savings. One of the limiting factors that determines how often a device needs to communicate is the condition of clock synchronization between the master and slave devices participating in the piconet. Bluetooth specifications require that a device operating in normal mode within a piconet (in this mode, it can be accessed at any time) operates with a clock that provides a stability of 20 pps. To maintain piconet synchronization, the master must provide synchronization messages at least every 225 ms. This defines the maximum period between switching on in normal mode.
The use of power-saving operating modes not only reduces the power consumption of Bluetooth devices, but also increases the reliability of the piconet by reducing interference from other wireless devices. Each Bluetooth piconet uses 79 frequency channels. Collisions between different piconets or between Bluetooth piconets and other wireless devices operating in the same frequency region will be reduced by keeping Bluetooth devices passive most of the time, ie. when using power saving modes. Thus, in this case, the two most important resources are saved - the frequency band and the energy of the power source.
Wireless devices are very convenient - you do not need to worry about wires anymore, but you must clearly understand that communication "over the air" has its own certain limitations in radius. Moreover, the cheaper, for example, the Bluetooth adapter that you purchase for your computer, the less you can move away from it in order to get a stable connection. Of course, some expensive devices do not always give out nice results. Today we will talk about how to amplify the Bluetooth signal and how realistic it is.
general information
The article describes some methods that involve disassembling the adapter, replacing its parts or modifying it by soldering, which may not be suitable for everyone. If you are not electronics savvy, not very nimble with a soldering iron, or your device is under warranty, then please avoid these methods.
Complementing the adapter
The simplest, but not the most effective way to increase the speed of Bluetooth, can be considered the addition of an adapter with a reflector, which will direct the signal in a certain direction, rather than amplify its distribution to all 360 degrees.
You can try to make such a reflector from a beer can by cutting off the top of it and making a few more cuts: from top to bottom and then from it a little to the sides, as if slightly separating the bottom of the can.
The Bluetooth adapter is mounted in the center with whatever you like and connects to the computer with a USB adapter.
Something similar can be built from cardboard with foil glued to it.
Another option that might work is to cut off just the top of the can, then make a cutout for the case closer to the bottom of the can, and insert the adapter in with the side that the antenna is on. Then again, we fix it with a method convenient for you and connect it through an extension cord.
Modifications
And now we'll talk about methods that involve physical modification of the adapter itself. In cheaper ones, you are unlikely to find an external antenna, which, in fact, is their problem.
We open the case, if possible, and look for the SMD antenna that is soldered to the board - you will need to unsolder it, only very carefully, without overheating the part.
Next, we solder the SMA connector in place of the antenna, before that we removed everything superfluous: we do not touch the part into which the antenna is screwed, but at the other end we cut off the edge, separate the screen and the cores, strip them, tin and solder.
If you have any doubts about where exactly to solder, then it is best to contact the amateur radio forums.
Now we connect to what we got, an antenna that can be safely twisted from the old Wi-Fi.
If you have a more expensive device already with an external antenna, but you are still unhappy with the signal, then the Hyper gain antenna can save the situation - buy it, cut off the adapter for connection and share the screen with the core.
Now quite often they release smartphones, phones or communicators with a built-in Wi-Fi adapter. And the range of Wi-Fi is about a hundred meters, but phones that are equipped with Bluetooth transmit and receive files only at a distance of no more than ten meters. If you have USB-blutooth for your computer, as well as a phone with bluetooth, but you would like to achieve an increase in the reception range. All this is quite possible, but usb-bluetooth needs to be improved.
Well, let's start. We disassemble the bluetooth adapter for the computer, after that you need to debug the bluetooth case and very carefully inspect the adapter board.
In all adapter models, at the end of the board there is a copper contact that looks like a spiral, in the photo it is number 1. This spiral is a bluetooth antenna, and an additional homemade antenna will be soldered to it.
We need a single-core copper wire with a diameter of 0.4 to 0.8 mm. The wire is covered with varnish insulation, and you do not need to get rid of it completely. We twist the wire as shown in the photo, then we will process the tip of the copper wire with rosin, then with tin. The same procedure must be carried out with a copper spiral in bluetooth, do not overheat the adapter board, do all the work very carefully.
Then, you need to make a hole in the case itself for the bluetooth adapter, at the exit point homemade antenna. Now very carefully close the board in the case. So the upgraded bluetooth is ready, which gives an increase in the reception range of 4 times.
To further increase the reception range, you can take a stranded sufficiently long wire that will be covered with insulation, you need to strip the tip and attach it to the antenna, the second tip can be attached to a small carnation driven into the wall.
BlueTooth Planar Antenna Systems cell phones
V. Kalinichev, A. Kurushin, V. Nedera
BlueTooth Planar Antenna Systems in Cell Phones
The issues of using planar microstrip antennas in the Bluetooth wireless local communication system are considered. The designs and methods of analysis of a planar ceramic antenna are considered, taking into account losses in ceramics. For the numerical analysis of the antenna in the case, the HFSS program was used. For specific handset calculations were made: current distribution over the surface of a metal, top-coated with a dielectric, phone case, radiation patterns for different orientations of a cell phone. An overview of serial Bluetooth antennas is given, as well as recommendations for installing these antennas in the housing.
Introduction
Increasing the speed of information exchange contributed to the development wireless systems communications at the "home" level. Personal computers and laptops, cell phones, CD and MP3 players, digital cameras and video cameras and many others digital devices(Fig. 1), often connected to each other and to desktop computers, created the problem of their connection.
Figure 1. Short-range communication system using Bluetooth wireless technology
The cable has become inconvenient - you need to connect often, the dimensions of the cable itself with connectors are almost larger than the device itself, and so on. Against this background, the relevance of wireless local technologies WLAN (Wireless Local Area Networking) has sharply increased, providing contactless connection of the device to the disk of the host computer.
As a result, a system was proposed and began to develop rapidly. wireless communication Bluetooth (Fig. 1). In the radio frequency spectrum, it has 79 channels in the 37 MHz band (approximately 2 MHz each) in the 2.4465-2.4835 GHz band.
essence Bluetooth standard in equipment electronic devices transceivers operating at a frequency of 2.45 GHz, with a range of up to 10 m and an information transfer rate of up to 1 Mbps. The possibilities of using these devices are truly endless. Wireless headphones, mice, keyboards, connection mobile phones and laptops, the exchange of information between pocket computers - just do not list.
The Bluetooth system operates in the authorized 2.45 GHz band (ISM - Industry, Science, Medicine band), which allows you to freely use Bluetooth devices around the world. The technology uses frequency hopping (1600 hops/s) with spread spectrum. During operation, the transmitter jumps from one operating frequency to another according to a pseudo-random algorithm. To separate the receiving and transmitting channels, time division is used (Fig. 2). Synchronous and asynchronous data transfer is supported and integration with TCP/IP is provided. Time slots are synchronized for the transmission of packets, each of which is transmitted on its own radio frequency.
Figure 2 Alternate communication between instrument A and instrument B
The power consumption of Bluetooth devices should be within 0.1W. Each device has a unique 48-bit network address compatible with the standard local networks IEEE 802.
The basic principle of building Bluetooth systems is the use of frequency hopping spectrum spreading (FHSS - Frequency Hop Spread Spectrum). The entire frequency range of 2.402 ... 2.480 GHz allocated for Bluetooth radio communication is divided into N frequency channels. Bandwidth of each channel is 1 MHz, channel spacing is 140…175 kHz. Frequency shift keying is used to encode packet information.
For the USA and Europe, N = 79. The exceptions are Spain and France, where 23 frequency channels are used for Bluetooth. Channels are changed according to a pseudo-random law with a frequency of 1600 Hz. Constant frequency interleaving allows the Bluetooth air interface to broadcast information over the entire ISM band and avoid interference from devices operating in the same band. If a this channel is noisy, then the system will switch to another, and this will continue until a channel free of interference is found.
The simplicity of the structure contributed a lot to the rapid start of the Bluetooth system. It consists of a radio module-transceiver, a communication controller (aka a processor) and a control device that actually implements the upper-level Bluetooth protocols, as well as an interface with a terminal device. Moreover, if the transceiver and the communication controller are specialized microcircuits (integrated or hybrid), then the communication control devices are implemented on standard microcontrollers, signal processors, or its functions support central processing units powerful terminal devices (for example, laptops).
In addition, Bluetooth devices use integrated circuits used in other applications, since the 2 GHz microwave band has been mastered quite well, and embedded in Bluetooth technical solutions in and of themselves are not particularly novel. In fact, the modulation scheme is widespread, the frequency hopping spectrum spreading technology is well developed, and the power is low.
The key to the success of Bluetooth technology is the radio transceiver. Low price and low power were the primary considerations in both the implementation of the interface specifications (short aerial radio link) and the design of the transceiver. Bluetooth technology makes it possible to create a single-chip transceiver by combining RF circuitry and digital stream processing circuitry on a single silicon chip.
Bluetooth transceiver
The Bluetooth transceiver can be divided into three functional blocks (Fig. 3). The radio unit contains RF up and down converters, baseband IF, channel filter, modulator/demodulator and frequency synthesizer.
Figure 3. Basic elements of a Bluetooth transceiver
The radio unit converts the FM signal at a frequency of 2.45 GHz into a bit stream and vice versa. The antenna is a very important element of the system. The antenna must be omnidirectional and have a gain of 0 dBi, the presence of the user must not affect the propagation of the signal. Due to the small wavelength at 2.45 GHz, the size of the antenna is limited to a few cm. Currently, flat or PIFA antennas are most commonly used, but even smaller E-type designs on a ceramic substrate have been proposed. The antenna is complemented by a bandpass filter that separates the 2.45 GHz frequency from the ISM band.
In order to realize simple and stable receivers and non-coherent detection, Bluetooth uses binary frequency shift keying (FM, FSK), with a Gaussian pulse around the frequency hop, at a rate of 1 Mbps. The area of such a signal is BT = 0.5, where B is the band, T is the pulse duration, with a modulation index from 0.28 to 0.35 and a pulse duration of 1 μs. FM eliminates the need for AGC, which is difficult to operate when switching frequencies and when data arrives at irregular time intervals. The front end of the RF receiver consists of a downconverter, a channel bandpass filter, and a frequency detector.
The channel filter allocates a bandwidth of 1 MHz, and it has rather high selectivity requirements. Because the ISM band must be shared with other systems in the band (which may include other Bluetooth systems), steps must be taken to prevent instrument interaction. Typically, a Bluetooth receiver is built with downconversion (that is, when the image channel falls into the IF band). For decoupling a number of working Bluetooth systems, the blocking factors for mirror channel should be 20, 30 and 40 dB for the first, second and third adjacent channels.
Due to the nature of the operation of the Bluetooth system, the technical requirements for intermodulation are more stringent than for the sensitivity of the receiver.
To cover a distance of 10 m with an output power of 0 dBm, the receiver sensitivity P min = -70 dBm is sufficient. Taking into account the noise level at the receiver input of -114 dBm (in a noise band of 1 MHz) and the requirement at the output of the receiving path K m = 21 dB, to ensure the maximum information transmission error rate BER = 0.1%, we obtain that the noise figure is 13 dB . This value is calculated from the sensitivity formula
P min = -174 dBm + NF + 10lgB + a + K m , (1)
where -174 dBm is the thermal noise power (kTB) in a 1 Hz band in normal temperature; NF - noise figure, dB; B - frequency band before the demodulator, 1 MHz; a - response threshold, a = 3 dB; K m - coefficient depending on the type of modulation.
Compared to the noise figure achieved to date, which is well below 13 dB, this seems to be a rather poor value. However, this low requirement allows the use of cheap lossy components and provides protection against interfering signals (crosstalk in the substrate and power wiring).
Bluetooth Receiver Dynamic Range Calculation
The upper limit of the dynamic range can be estimated from the level of the 3rd order intermodulation distortion product, if we assume that there are 2 signals at the input with the frequencies of two adjacent channels.
Two signals with frequencies f 0 + D f and f 0 + 2D f produce a third-order intermodulation distortion product P IM3 in the considered radio channel with frequency f 0 . The power level of the product P IM3 depends on the input interfering power P in and the non-linear parameter of the entire receiver - the third-order intercept point IP 3 - and is equal to:
P IM3 = 3P in - 2IP 3 [dB]. (2)
The distortion-free dynamic range is determined from the condition that distortions of linear and non-linear origin equally affect the distortion in the demodulator and equally degrade the detection of the own signal. This means that in order for the BER not to exceed the same value of 0.1% that was set when determining the sensitivity, it is necessary that the received signal power be 3 dB above the noise level (which corresponds to the receiver sensitivity Pmin). Therefore, IP3 = -16 dBm in expression (2) was obtained, assuming that the PIM3 intermodulation product is equal to the receiver sensitivity, the two interfering signals have a power of 0 dBm, and the interference is present at a distance of 1 m.
Combining the IP3 value = -16 dBm with the receiver sensitivity Pmin = -70 dBm, from (1) and (2) we get that the distortion-free dynamic range (SFDR) of the Bluetooth receiver should be equal to
SFDR = 2/3(IP 3 - (P min + 3 dB)) = 50 dB. (3)
The transmitter block is also quite simple. Binary GFSK modulation is obtained by direct modulation of the FM local oscillator. Additional phase upconversions are therefore not needed. The baseband signal is filtered with a Gaussian filter so as to keep the 1 MHz spectrum width required for FM systems operating in the 2.45 GHz ISM band. Gaussian envelope modulation does not impose high requirements on the linearity of the transmitter output stage; economical class C amplifiers can be used here.
The power of the Bluetooth transmitter is about 0 dBm (maximum power up to 20 dBm can be used). For power levels greater than 0 dBm, closed loop power control is applied.
Calculating the range of a cell phone in a Bluetooth system
It is known that the power of the radio signal at the reception point P n is equal to:
where P is the power radiated by the transmitter; G m - maximum gain of the transmitting antenna; A eff.m - maximum effective area receiving antenna (proportional to the geometric area of the antenna); F(,) - function of the transmitting antenna radiation pattern; F"(",") - function of the receiving antenna radiation pattern.
From this formula, you can get the maximum radio range, provided that the antennas are pointed at each other,
where P n.min - receiver sensitivity, in our case P n.min = 10-10 W (-70 dBm).
Substituting into formula (4) the transmitter power P = 10-3 W, G m = 0.5, A eff.m = 25 10 -6 (5 by 5 mm), we get r m = 3 m.
This value approximates the requirements of the Bluetooth system, and may Starting point calculation of the antenna geometry, since the rest of the characteristics are determined by the standard for the transceiver chip.
Antennas for Bluetooth (overview of manufacturers and solutions)
Several firms such as Hitachi Metals, Murata, Yocowo, Antek Wireless, Centurion and others already produce a wide range of antennas that are used in cellular telephony and specifically designed for Bluetooth systems using ceramic materials with good high frequency properties.
Hitachi Metals has released "E-Type Electrode Configuration" antennas (Figure 4) well suited for Bluetooth applications. The space required for the new antenna is very small (15x3x2mm), it is not sensitive to the location of peripheral parts, it can be made as a high-performance Bluetooth crystal antenna, and it is easy to use.
Figure 4. View of the Hitachi Metals antenna for Bluetooth
Antek Wireless Inc. has developed a new 2.4 GHz antenna of an original design that provides efficiency that exceeds virtually any project specification, is small, and can be installed in almost any device. The antenna is applicable for various applications such as wireless video transmission, audio equipment, headphones, modems, mobile computers, portable phones and other portable handheld devices using the Bluetooth, IEEE 802.11 and HomeRF protocols.
Centurion International has developed internal antenna PIFA or variations of a flat antenna for use in laptop computers using Bluetooth technology. The new antenna enables computer manufacturers to develop portable devices that easily communicate with portable phones and messaging systems, connect to the Internet on high speeds data transmission.
Murata Manufacturing Co. started production and sale of built-in dielectric antennas for laptops using Bluetooth technology (Fig. 5). The module dimensions of the new G2 series are 15x5.8x7.0 mm.
Figure 5. Chip antenna ANCG22G41 Murata
Miyazaki Matsushita Electric Industrial Co. Ltd. Introduces an ultra-compact antenna for Bluetooth devices. The antenna is made on a ceramic base and has dimensions of 5x1.2x1.2 mm. It is the smallest antenna in the Bluetooth industry. The characteristics of the antenna are as follows: operating frequency 2.4 GHz, gain -2 dBi, voltage standing wave ratio (VSWR) 2.0.
Figure 6. Ceramic antenna in a cell phone case (photo)
TDK Corp. manufactures two small 7mm by 7mm half-wave antennas for use in Bluetooth products. The CANPB0715 antenna has a gain of -5 dBi and the CANPB0716 antenna has a gain of 3 dBi. Most other small antennas are quarter wave. Their use is possible only in larger mobile devices, such as laptops, where grounding is carried out to the body of the device. Mobile phones required the development of half-wave antennas.
Figure 7. 3D view of a Bluetooth antenna in a metallized cell phone case (drawing in HFSS)
E-type Antenna Configuration
Previously, antennas had two basic configurations: the F-type reverse single-ended antenna and the flat antenna.
An inverted F antenna has one side open and the other side grounded to reduce size, but the open side is subject to the influence of the ground electrode. Therefore, a large area is required to realize antenna properties in a given space, and care must be taken when designing the arrangement of peripheral components.
In addition, the flat antenna is highly sensitive (high gain) and has strong directional properties, making it unsuitable for Bluetooth applications where omnidirectionality is required.
The type of antenna developed by Hitachi Metals has the unique advantages of an F-type reverse antenna, but includes ground electrodes on both sides and a central, cone-shaped electrode is added. In other words, the new E-Type Electrode configuration invented by Hitachi Metals can be further miniaturized and does not significantly impact nearby ground electrodes. The smaller the antenna, the less the housing affects its parameters.
The analysis of all antenna designs for the Bluetooth system given above makes it possible to identify the main antenna parameters included in the antenna specification, on the basis of which it is possible to choose a method for designing a cell phone with such an antenna.
Technical requirements for Bluetooth system antenna:
- operating frequency band: 2400…2500 MHz;
- average gain: -3 dBi;
- input impedance: 50 Ohm;
- VSWR: 3 or less.
In the process of designing an antenna system, it is necessary to:
- calculate the matching structure between the filter input and the feed point of the microstrip antenna;
- optimize the ground surface (sometimes called a counterweight), that is, find the optimal filling of the inner surface of the phone case with conductive areas. Nowadays, this is often implemented by painting individual parts of the case with conductive paint.
The goal of antenna design is to obtain the required radiation pattern (RP) and good matching in the operating frequency band.
Analysis of the generalized structure of a planar antenna
A review of the existing antennas for the Bluetooth system shows that they have metal shapes of complex configuration deposited on one or more sides of a three-dimensional substrate, most often ceramic with high permeability (Fig. 8). Therefore, we can say that each of these forms is a resonator. It is known that the dimensions of the antenna are related to the operating frequency. If we assume that the antenna resonates along the longer side, then the length of the antenna can be estimated using the following simple formula:
where f r - given resonant frequency; is the relative permittivity of the substrate material. This formula does not take into account the effect of the antenna substrate width and substrate thickness on the resonant frequency, but this effect is usually negligible. Formula (1) reflects the physical nature of the printed antenna (Fig. 9) as a half-wave resonator, which is formed in the space between the top conductor and the ground plane of the antenna. For example, at a frequency f r = 2.5 GHz and = 34 (ceramics) from (1) we have A ~= 10.3 mm.
Figure 8. YCE-5207 Bluetooth Antenna Geometry in AutoCAD
Figure 9. Bluetooth antenna (top view) designed in AutoCAD
The length of the antenna can be at least halved (when operating at the same frequency) if one end is grounded. In this case, you get the so-called inverted F-antenna (PIFA), which is a quarter-wave resonator, one end of which is grounded and the other open (idle). PIFA (Fig. 3) is driven by a coaxial line at a point where the input impedance of the antenna is close to 50 ohms. So the PIFA length can be roughly estimated as
For an antenna tuned to the same frequency f r = 2.5 GHz and = 34, we get a ~= 5.1 mm, which already takes up much less space than in the previous case. The actual size of the antenna can be even smaller due to the effect of the edge near field concentrated at the open end of the resonator.
The size of the E-antenna, since it is rolled up on both sides, can be roughly estimated as
Since the antennas for the Bluetooth system are in a semi-enclosed shield of complex shape, the performance of the antenna system may differ significantly from the performance calculated by theoretical formulas. In this case, the antenna parameters (the dimensions of the conductors and the distance between them in height) can be optimized using one of software packages, modeling electromagnetic structures (Fig. 10).
Figure 10. Near field in a cell phone (in the field of the HFSS program)
Note that the advantage of the small size of the PIFA antenna is achieved by reducing its emissivity (radiates only one edge), moreover, PIFA antennas are usually narrow-band.
Numerical Methods for Designing Planar Antennas
Antennas are the basic building blocks of all radio communication systems and use free space as a carrier medium. They are used to interface a transmitter or receiver in free space.
Antennas have a number important parameters, of which the gain, radiation pattern, bandwidth, and polarization are of greatest interest.
The modern design of cell phone antennas (Fig. 11) is based on the simulation of electromagnetic phenomena on a computer, using as initial data the results obtained on the basis of preliminary calculations and heuristic considerations.
Figure 11. View of the Bluetooth antenna in the cell phone case
When creating a model, it must be remembered that the geometry must correspond to the actual position of the antenna during operation, that is, such that the housing is in a vertical position (or at a slight angle). In this case, the flat antenna is in the edge-on position.
Features of miniature ceramic antennas
The ceramic antenna is made on a substrate with a high dielectric constant. Material with high permeability also has high losses.
Therefore, the calculation of such antennas must be carried out using programs that fundamentally take into account losses in ceramics. Such a program is the HFSS program.
In order to successfully install a flat antenna into the structure of a cell phone handset, it is necessary to carry out computational studies that would show the dependence of the characteristics of the antenna system on certain elements of the phone's structures.
We note the following features of microstrip antennas:
- microstrip antennas are narrower than helical ones;
- microstrip antennas easily implement circular polarization, compared to the predominantly vertical polarization of helical antennas;
- microstrip antennas have a more uneven radiation pattern in the azimuth plane than helical and vibrator ones, due to their asymmetry about the vertical axis.
As already noted, a ceramic antenna is a 3D structure, on the surface of each side of which metal conductors of a certain shape are deposited. This design may have one or more excitation points. At these points, an excitation voltage is applied to the antenna, which induces radiation currents in the structure. The excitation points can be connected by a balancing transformer (balun).
In addition to the excitation points, the printed antenna may have ground points (connection to the ground plane). The currents induced in this complex structure shape the radiation pattern and implement other characteristics of the antenna necessary to establish communication with personal computer or other device.
Since, as a result of electrodynamic calculation, it is possible to determine the distribution of currents in the system, their analysis can serve as the basis for upgrading the antenna.
In the process of designing an antenna, it is necessary, first of all, to obtain an input impedance close to 50 Ohms, since in this case it will be possible to match the antenna with a low-noise input amplifier and a power amplifier of the transmitting path with less loss.
For example, if the return loss of the antenna (parameter 20 log |S 11 |) is about -20 dB, this indicates that the antenna will work with good coordination with the surrounding space in the operating frequency range. The value of -20 dB indicates that the generator power will be absorbed almost without reflection by the antenna, which in turn is loaded with free space. The antenna is a transformer between the output of the power amplifier (or the input of the low-noise amplifier) and free space, the impedance of which for a plane wave in the far field can be considered equal to 377 ohms.
The next requirement is the radiation characteristics, which determine the ability of the antenna to radiate in different directions. When designing and calculating antennas, they are usually interested in the sections of the radiation pattern in two mutually perpendicular planes: azimuth and elevation. Azimuthal RP determines the ability of the antenna to radiate in the horizontal plane, elevation RP - in the vertical. Both patterns are important for a cellular phone, but the former defines omnidirectionality and is more relevant for field emission evaluation. The directivity parameters of a printed antenna or its modifications must be no worse than those of existing helical-whip antennas.
Calculation of the radiation characteristics of a Bluetooth antenna
The table shows the results of modeling an antenna in a housing using the exact geometric dimensions of a particular design. The table shows that the parameters of the calculated design differ significantly from the measured matching parameters (Fig. 16). Therefore, we analyze the reasons for these differences.
Table. The power radiated by the antenna, directivity, gain and magnitude in the absence of losses in the substrate (dielectric tangent = 0). The rated power of the generator at the input (port) is 1 W
| F Frequency | P izl Radiated power, calculation, W (calculated sum of powers through radiation planes) |
D Directivity, dB (calculation for HFSS) | G Gain, dB = P izl / P nom | S 11 HFSS calculation | 20 logS 11 dB |
| 2 | 0,07 | 3,47 | -7,8 | 0,96 | -0,5 |
| 2,2 | 0,15 | 2,87 | -5,4 | 0,92 | -1 |
| 2,4 | 0,3 | 2,5 | -2,7 | 0,83 | -2 |
| 2,6 | 0,47 | 2,6 | -0,6 | 0,73 | -3 |
| 2,8 | 0,08 | 2,8 | -8,3 | 0,96 | -0,4 |
| 3 | 0,02 | 3,8 | -12,3 | 0,99 | -0,2 |
The biggest fundamental difference between the calculated and real design lies in the parameters of the substrate. Thus, the calculation data given in the table correspond to the idealized case of the absence of losses in the ceramic substrate. In this idealized lossless case, we find the connection of the table parameters.
The rad is calculated by the HFSS program over the entire radiation boundary. All the power that has passed through the walls that mark the boundary of the far field is summed up and gives this P rad.
If the substrate and conductors are lossless, then all the power that came to the antenna is radiated, that is, P rad. = P ant, and this power, which came to the antenna and then radiated, is determined, in turn, by the mismatch:
P izl \u003d P ant \u003d P nom (1 - | S 11 | ²), (7)
where Pnom is the rated power of the generator. Based on HFSS, it is set to 1 W.
At a frequency of 2 GHz, in accordance with the table, from (7) we have
P ant \u003d 1 (1 - | 0.96 | ²) \u003d 0.07W,
which corresponds to the calculated value P izl in the table.
Antenna gain is, by definition,
Substituting (7) into (8), we obtain, on a logarithmic scale,
G \u003d 10lg (1 - | S 11 | ²) + D. (9)
For a frequency of 2 GHz, we have the antenna gain
G \u003d 10lg (1 - | 0.96 | ²) + 3.47 \u003d -7.8 dB.
So, we have shown the connection of the antenna parameters for the case without losses in the substrate.
Let us rewrite (7) in the following form:
Analyzing the HFSS calculation, we see that at a frequency of 2 GHz and at other frequencies, the antenna gain is poor, and, most importantly, there is an antenna mismatch (Fig. 12). Experiment shows, however, that the antenna gain is much higher, even without the inclusion of matching circuits. What's the matter? It turns out, oddly enough, the presence of losses in the ceramic substrate helps to match the antenna and improve the performance of a small antenna, compared to a conventional antenna, the dimensions of which are commensurate with the wavelength. Indeed, by increasing the losses to tg = 0.1 (of course, unrealistically large), by calculation on HFSS, we obtain the matching dependences shown in Fig. 13.
Figure 12. Frequency response of a Bluetooth antenna with ceramic parameters = 34, tg = 0 (lossless). It can be seen from the figure that the agreement is poor.
Figure 13. Bluetooth Antenna Frequency Response at Ceramic Parameters = 34, tg = 0.1 (at 2 GHz)
In order to investigate the efficiency of the antenna as a function of losses, we calculate the dependences of the characteristics of the antenna in the housing on the losses in the ceramics. Ceramics has losses, and calculations show that if we assume that there are no losses, then the antenna has poor matching, if there are losses, matching improves.
The power P izl is calculated numerically by the program as the sum of the powers incident on all radiation boundaries. This power is less than the rated power of the generator, and is only a part of it.
Since in this case we have losses, they are defined as the power difference between the lossless case, formula (7), and the value of P rad. The equality P izl = P ant is no longer valid, these powers differ by the power of losses in the substrate:
P izl \u003d P ant - P absorption. (eleven)
Substituting (11) into formula (8), we obtain that the antenna gain, taking into account losses in ceramics, is found by the formula
which can be represented in the form
|S 11 |² \u003d 1 - Ktg - G / D, (13)
where K * tg \u003d P deep / P nom, K in the general case is not equal to 1.
It can be seen from (13) that |S 11 |² decreases with increasing losses, and one can understand why matching with the antenna is achieved easier for the case of lossy ceramics.
Figure 14. Bluetooth Antenna System Elevation Pattern
Figure 15. Azimuthal radiation pattern of a cell phone with a Bluetooth antenna system
Calculations show that the influence of the user's body on the radiation pattern of a small antenna is much less than on the RP of the main cell phone antenna. The same can be said about the reverse effect of the radiated power of the Bluetooth antenna on the human body.
Experimental study of a planar antenna
Experimental tuning of the antenna can be performed according to the matching criterion and according to the RP criterion. On fig. 16 shows the measured frequency response of parameter S11 plotted on a Smith chart.
Figure 16. Antenna input impedance measured with a network analyzer in the housing
These experimental measurements were made on the HP8632 circuit meter.
Experimental bias measurement resonant frequency antenna system when shielding the antenna with a screen showed that the resonant frequency drift when the antenna was inserted into the housing was 50 MHz.
Conclusion
The article discusses the features of modeling a microstrip antenna in a Bluetooth system designed for wireless local communication. The Bluetooth system in a cell phone is considered. main feature operation of the antenna system - the operation of the antenna in a heavily metallized housing, that is, with a large counterweight. Therefore, to calculate the currents induced by the antenna on the housing surface, it is necessary to use the analysis program in 3D representation. Such a program is HFSS. In this case, the modeling of the antenna, together with other housing elements, is an essential part of the entire antenna and tube design process.
The features of the modeling process are demonstrated using the Yocowo YCE-5207 patch antenna, which is a combination of a rectangular metal pad and a microstrip line on ceramics with a large dielectric constant of rather complex shapes. The results of a specific analysis are presented as frequency characteristics reflection coefficient, case currents, near field and DN. The influence of the elements of the tube body on the radiation pattern in the far zone is shown. Both external and internal antenna mounting options are considered.
Literature
- Jennifer Bray, Charles Sturman. Bluetooth: connect without cables. Prentice-Hall, 2001. 495 p.
- Balanis C.A. Antenna Theory: Analysis and Design, Wiley & Sons. 2nd edition. 1997.
- Fujimoto K. and James J.R. (editors). Mobile Antenna Systems Handbook. 2nd edition. Artech house. 2001. 710 p.
- Kessenikh V., Ivanov E., Kondrashov Z. Bluetooth: principles of construction and operation // Chip News. 2001. No. 7. S. 54–56.
- Kalinichev V., Kurushin A. Microstrip antennas for cell phones // Chip News. 2001. No. 7. S. 6–12.