mobile / wireless communications

Types of IMT

2008-07-16T23:44:00.000-07:00

The ITU standardized 5 groups of 3G radio access technologies.They are as follows :-

IMT-DS :-The directr spread technology comprises wideband CDMA (W-CDMA) systems .This technology specified for UTRA-FDD and used by all european providers and the japanese NTT DoCoMo for 3G wide area services . To avoid complete confusion ITU'S name for the technology is IMT-DS ,ETSI called it UTRA -FDD in the UMTS context ,and the technology used is called W-CDMA .Today , standardization of this technology takes place in 3GPP .Details of it will be further provided.

IMT-TC :- Intially ,this family member called time code ,contained only UTRA-TDD
systems which uses time-division CDMA (TD-CDMA).Latr on, the chinese proposal ,TD -synchronous CDMA (TD-SCDMA) was added.Both staandards have been combined and 3GPP fosters the development of this technology.It is unclear when and to what extent this technology will be introduced.The initial UMTS installation are based on W-CDMA.

IMT-MC :- Cdma2000 is a mutli-carrier technology standardized by 3GPP2(third generation partnership project 2, 3GPP2 ,2002 ),which was formed shortly after 3GPP to represent the second main stream in 3G technology version cdma2000 EV-DO has been accepted as the 3G standard.

IMT-SC :- The enhancement of the US TDMA systems, UWC-136, is single carrier technology originally promoted by the universal wireless communications consortium.It is now integrated into 3GPP efforts .This technology applies EDGE , among others, to enhance 136 standards.

IMT-FT :- As frequency time technology ,an enhanced version of the cordless telephone standard DECT has also been selected for applications that do not require high mobility.ETSI is responsible for standardisation of DECT.

UMTS & IMT-2000

2008-06-21T04:31:00.000-07:00

A lot has been written about 3 rd generation networks(3G) in last few years.Let us see how did this start in many countries around the world ? ? ?..First of all International Telecommunicaiotns Union (ITU) made a request for appropriate radio transmission technologies(RTI ) for the internal Mobile communications (IMT) 2000 program (ITU, 2000) .IMT-2000 ,formerly called future public land mobile communications systems( FPLMTS), tried to establish a common world wide communication system alowed for terminal and user mobility , supporting the idea of universal personal telecommunications (upt).Within this context . ITU has created several recommendations for FPLMTS systems e.g: network architecture for FPLMTS systems ,Requirements for the radio interface for FPLMTS frame work for services supported by M.816.
The number 2000 in IMT-2000 should indicate the start of the system(year 2000 + x) and the spectrum used around 2000 MHz .IMT-2000 includes different environments such as indoor use ,vehicals ,satellites ,pedestrian's .The world radio conference bands that should be available worldwide for the new IMT-2000 systems recommendations ITU-R.Within these bands, two times 30 Mhz have been reserved for mobile satellite services.
The ITU frequency allocation together with examples from several regions that already indicate the problem of world wide common frequency band.
The ITU frequency allocation together with examples from several regions that already indicate the problem of world wide common frequency band .In Europe some parts of the ITU frequency bands for IMT- 2000 are already allocated for DECT . The remaining frequencies have been split into bands for UTRA-FDD (uplink: 1920-1980 MHz,downlink: 2110-2170 Mhz ) and UTRA -TDD (1900-1920 MHz and 2010-2025 MHz )The technology behind UTRA-FDD and TDD will subsequently be explained in much more details as the form the basis of UMTS.Currently , no other systems is planned for IMT-2000 in Europe. More bandwidth is available in CHINA for Chinese 3G systems TD-SCDMA or possible other 3G systems.

Digital Audio Broadcasting

2008-04-23T00:21:00.000-07:00

Todays analog radio system still follows the basic principle of frequency modulation invented back in 1993.In addition to audio transmission ,very limited information such as the station identification can accompany the programs.Transmission quality varies greatly depending on multi-path effects and the interferance.The fully digital DAB systems doen not only offer sound in a CD like quality ,it is immune to interferance and multipath propagation.
DAB systems can use single frequency network(sfn) i.e all the senders transmitting the same radio program operate at same frequency .Today ,different senders have to use different frequencies to avoid inteferance although they are transmitting the same radio program.Using an SFN is very frequency efficient , as a single radio station only needs one frequency throughout the whole country.Additionally , DAB transmission power per antenna is orders of magnitude lower compared to traditional FM stations.DAB uses VHF and UHF frequency bands(depending on national regulations) e.g:- the terrestial tv channels 5 to 12 (175- 230 Mhz ) or the L-band .The modulation with 192 to 1536 carriers so called ensemble within DAB channel of 1.5 Mhz.

Example Broadcast disk

2008-04-12T07:35:00.000-07:00

Let us assume that the broadcast sender is a radio station transmitting information about the radio conditions (block A ),the weather report (block B),the latest events in town and the menu to access these and other topics in addition to music.The sender can now assume ,knowing something about the importance of the data blocks,the block D is the most important to enable access to the other information.The second important block is A ,then B and finally C.A possible broadcast disk for this scenario could now look as follows.

DADBDADBADADDDDBDC............

It is now the receiver's task to cache the data blocks to minimize access delay as soon as a user needs a specific type of information.Again ,the receiver can only optimize caching if it knows something about the content of the data blocks.The receiver can store typical access patterns of a user to be able to guess which blocks the user will access with a high probability.Caching generally follows a cost-based strategy: what are costs for user? if if the data block has been requested but is currently not available in the cache.

Cyclic Repetition of Data

2008-04-01T20:55:00.000-07:00

A broadcast sender of data does not know when a receiver starts to,listen to the transmission .While for radio or television there is no problem,transmission of the other important information,such as traffic or weather conditions ,has to be repeated to give receiver a chance to receive this information after having listened for a certain amount of time.
The cyclical repetition of data blocks sent via broadcast systems is often called a broadcast disk according to the project in Acharya od data carousel,eg according to the DAB or DVB standatrds.The sender repeats the three data blocks A,B and C in a cycle.Using a flat disk , all blocks are repeated one after the another.Every blog is transmitted for an equal mount of time,the average waiting time for receiving a block is the same for A,B and C.Skewed disks favour one ofr more data blocks by repeating them once or several times.Thsi raises the probability of receiving a repeated block ,if the block was corrupted the first time.
Finally ,multi-disks distribute blocks that are repeated more often than others evenly over the cyclic pattern.This minimizes the delay if a user wants to access .

Broadcast systems

2008-03-29T00:46:00.000-07:00

Uni-directional distribution systems or broadcast systems are an extreme version of asymmetric communications systems .Quite often ,bandwidth limitations ,differences in transmission power ,or cost factors prevent a communication system from being symmetrical.Symmetrical communication systems offer the same transmission capabilities in both communication directions ,i.e the channel characteristics form A to B are as same as from B to A .
Examples of symmetric communications services are the plain old telephone service (POTS) or GSM ,if end-to-end communication is considered .In this case ,it does not matter if one mobile station calls the other or the other way round,bandwidth and delay are the same in both services .
This symmetry is necessary for a telephone service ,but many other applications do not require the same characteristics for both directions of information transfer.Consider a typical client /server environment.Typically ,the clients needs more data from server than the server needs from the client .Today's prominent example is world wide web .Millions of users download the data using there browsers form the web servers .A user only returns information to server from time to time .Single requests for new page with typical size of several hundreds bytes result in responsese of upto some 10 kbytes on average.
A television with a set-top box represents more extreme scenario .While high resolution video stream requires several mbits/s, a typical user returns some bytes from time to time to switch between channels or return some information for television shopping
Finally ,todays pagers and radio work completely one-way.These devices can only reveive information backto e.g,the radio station .Typically the telephone system is used for this purpose.
A special case of asymetrical communication systems are unidirectional broadcast systems where typically a high bandwidth data stream exists from one sender to many receivers.The problem arising from this is that the sender can only optimize transmitted data for the whole group of the recivers and not fro individual users .

Multiplexing

2008-03-14T21:06:00.000-07:00

Multiplexing is not only a fundamental mechanism in communication systems but also in everyday life.Multiplexing describes how several users can share a medium with minimum or no interferance.One example ,is highways with several lanes.Many users (car drivers)use the same medium (highways) with hopefully no interferance(accidents).Thid is possible due to provision of several lanes (space division multiplexing )seperate traffic .In addition ,different cars use the same medium (i.e the same lane) at different points in time(time division multiplexing).
While this sample example illustrates our everday use of multiplexing ,the following examples deal with the use of multiplexing and assignment of a medium to users (traffic regulations)

In extc, computer-networks multiplexing (short muxing) is a term used to refer to a Process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in electronics, multiplexing allows several analog signals to be processed by one analog-to-digital converter (ADC), and in telecommunications, several phone calls may be transferred using one wire. In communications, the multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side.
A device that performs the multiplexing is called a Multiplxer (MUX), and a device that performs the reverse process is called a De-multiplexer (DEMUX).
The two most basic forms of multiplexing are time division multiplexing (TDM) andfrequency division multiplexing (FDM).
In optical, FDM is referred to as wave Division multiplexing (WDM).
Variable bit rate digital bit streams may be transferred efficiently over a fixed bandwidth channel by means of statisics, for example packet mode communication. Packet mode communication is an asynchronousmode time-domain multiplexing, which resembles but should not be considered as time-division multiplexing.
Digital bit streams can be transferred over an analog channel by means of code division multiplexing.
In wireless communication, multiplexing can also be accomplished through alternating Polarization (horizontal/vertical or clockwise/counterclockwise) on each adjacent channel and satellite, or through phase multiarray antennacombined with a multiple input multiple output comm (MIMO) scheme.

Signals

2008-03-11T10:17:00.000-07:00

Signals are physical representation of data .Users of a communication system can only exchange data through the transmission of signals.Layer 1 of the ISO/OSI basic reference model is responsible for the conversion of data, i.e bits into signals and vice versa.
Signals are functions of time and loaction.Signal parameters represent the data values .The most intresting type of signals for radiotransmission are peroidic signals, especially sine waves as carriers.

GSM NETWORK

2008-03-05T06:31:00.000-08:00

Technical details of GSM

2008-03-05T06:17:00.000-08:00

GSM is a cellular networkwhich means that mobile phones connect to it by searching for cells in the immediate vicinity. GSM networks operate in four different frequency ranges. Most GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated.
The rarer 400 and 450 MHz frequency bands are assigned in some countries, notably Scandinavia, where these frequencies were previously used for first-generation systems.
In the 900 MHz band the uplink frequency band is 890–915 MHz, and the down link frequency band is 935–960 MHz. This 25 MHz bandwidth is subdivided into 124 carrier frequency channels, each spaced 200 kHz apart. TDM is used to allow eight full-rate or sixteen half-rate speech channels per radio channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate is 270.833 kbit/s, and the frame duration is 4.615 ms.
The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.
GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 5.6 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called half rate (5.6 kbit/s) and full rate (13 kbit/s). These used a system based upon linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.
GSM was further enhanced in 1997 with the enhanced full rate (EFR) codec, a 12.2 kbit/s codec that uses a full rate channel. Finally, with the development of umts, EFR was refactored into a variable-rate codec called AMR-narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.
There are five different cell sizes in a GSM network—macro, micro, pico, femto and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen meters; they are mainly used indoors. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.
Cell horizontal radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometers. The longest distance the GSM specification supports in practical use is 35 KM (22 mi). There are also several implementations of the concept of an extended cell, where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.
Indoor coverage is also supported by GSM and may be achieved by using an indoor picocell base station, or an indoor repeator with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed when a lot of call capacity is needed indoors, for example in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of the radio signals from nearby cells.
The modulation used in GSM is (GMSK), a kind of continuous-phase frequency shift-keying. In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low pass filter prior to being fed to a frequency modulator, which greatly reduces the interferance to neighboring channels (adjacent channel interference).
Interference with audio devices
This is a form of RFI, and could be mitigated or eliminated by use of additional shielding and/or bypass capacitors in these audio devices. However, the increased cost of doing so is difficult for a designer to justify.
It is a common occurrence for a nearby GSM handset to induce a "dit, dit di-dit, dit di-dit, dit di-dit" output on PAs, wireless microphones, home stereo systems, televisions, computers, cordless phones, and personal music devices. When these audio devices are in the near feild of the GSM handset, the radio signal is strong enough that the solid state amplifiers in the audio chain act as a detector. The clicking noise itself represents the power bursts that carry the TDMA signal. These signals have been known to interfere with other electronic devices, such as car stereos and portable audio players. This also depends on the handset's design, and its conformance to strict rules and regulations allocated by the FCC in part 15 of FCC rules and regulations pertaining to interference with electronic devices.

GSM

2008-03-05T06:08:00.000-08:00

Global System for Mobile communications (GSM: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world. Its promoter, the GSM ASSOCIATION, estimates that 82% of the global mobile market uses the standard. GSM is used by over 2 billion people across more than 212 countries and territories. Its ubiquity makes international roaming very common between mobile phone ops, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signalling and speech channels are digital quality, and thus is considered a second generation mobile phone system. This has also meant that data communication was easy to build into the system.
The ubiquity of the GSM standard has been advantageous to both consumers (who benefit from the ability to roam and switch carriers without switching phones) and also to network operators (who can choose equipment from any of the many vendors implementing GSM. GSM also pioneered a low-cost alternative to voice calls, the short mess service (SMS, also called "text messaging"), which is now supported on other mobile standards as well.
Newer versions of the standard were backward-compatible with the original GSM phones. For example, release 97 of the standard added packet data capabilities, by means of general packet radio service (GPRS). Release '99 introduced higher speed data transmission using enhanced data rates for GSM (EDGE).

Spectrum

2008-03-01T22:32:00.000-08:00

Spread spectrum:-
As the name suggest ,spread spectrum techniques involve spreading the band width needed to tansmit the data - which does not make sense at the first sight.Spreading the bandwidth has a several advantages.The main advantage is the resistance to narrow band interferance .Idealized narrowband signal from a sender of user data .The sender now spreads the signal ..i.e converts the narrow band signal to broad band signal.The energy neededto transmit the signal is the same , but it is now spread over a large frequncy range.The power level of the spread signal without losing data .Depending on the generation and reception of the spread signal, the power level of the signal can even be low as background noise.This makes it difficult todistingiush the user signalfrom background noise and thus hard to detect
During transmission , narrowband and broad band interferance add to signal.The sum of interferance and user signal is received .The receiver now knows how to de-spread the signal,converting the spread signal into narrowband signal again,while spreading the narrowband interferance and leaving the broadband interferance.The receiver can reconstruct the original data because the power level of the user signal is high enough ,ie the signal is much stronger than the reamining interferance.The folowing sections show how spreading can be performed.
The seperation of channels ,CDM is now used insted of FDM .This application shows the tight coupling of CDM and spred spectrum .Spreading the narrowband signal is acheived using a special code.

About

2008-03-01T22:19:00.000-08:00

This blog is about mobile & wirless communication ,the working and basic requirements in mobile communications.Please fell free to put your suggestions or you can mail me @ rocky1171986 at gmail.com. Feel free to ask and comment...i lll be giving important topics on the subject as per university of mumbai.In this site some precise news and develpoments will also be put up, all will be in updates sections.All the updates will be done soon.
Regards,
Rocky

Updates

2008-03-01T22:15:00.000-08:00

I will be putting the updates regarding mobile communications subject for semester 7 ,IT & com sci. I am in the process of scaning some important questions which ill be soon uploading!!!!!!!

CHAPTER NO :-1
INTRODUCTION
QUE 1:- Explain the need of wireless communication with its application ?

OUE 2:- mobile & wireless devices .

CH NO :- 2
WIRELESS TRANSMISSION

OUE 1: - Explain suitable waveforms BFSK without abrupt phase changes which belong to CPM scheme

QUE 2: - SPREAD SPECTRUM , cellular systems (if possible )

Applications of wireless technology

2008-03-01T08:46:00.000-08:00

Security systems:
Wireless technology may supplement or replace hard wired implementations in security systems for homes or office buildings .They are also used to keep a track on visitors to the nearby area or as per the wish and settings.

Television remote control:
With the help of wireless communications a user can be able to switch television channels by just sitting on his place

Mobile telephone:
A latest gizmo which helps us to communicate with the desire unit/person anywhere ,it is the device which has made communication much easier.These instruments use radio waves to enable the operator to make phone calls from many locations world-wide. They can be used anywhere that there is a cellular telephone site to house the equipment that is required to transmit and receive the signal that is used to transfer both voice and data to and from these instruments.

Bluetooth:
One of the device which actual works on radiation ,helpful in sending and receiving data .The only drawback of this unit is the range is limited...

Basic Introduction on wireless communication

2008-03-01T08:42:00.000-08:00

The term "wireless" has become a generic and all-encompassing word used to describe communications in which electromagnetic waves or RF (rather than some form of wire) carry a signal over part or the entire communication path. Common examples of wireless equipment in use today include:
Professional LMR (Land Mobile Radio) and SMR (Specialized Mobile Radio) typically used by business, industrial and Public Safety entities
Consumer Two Way Radio including FRS (Family Radio Service), GMRS (General Mobile Radio Service)
Amateur ("Ham") and Citizens band ("CB") radios
Consumer and professional Marine VHF radios
Cellular telphones and pagers: provide connectivity for portable and mobile applications, both personal and business.
Global Positioning System (GPS): allows drivers of cars and trucks, captains of boats and ships, and pilots of aircraft to ascertain their location anywhere on earth.
Cordless computer peripherals: the cordless mouse is a common example; keyboards and printers can also be linked to a computer via wireless.
Cordless telephone sets: these are limited-range devices, not to be confused with cell phones.
Satellite television: allows viewers in almost any location to select from hundreds of channels.
Wireless networking (i.e. the various flavors of unlicensed 2.4 GHz WiFi devices) is used to meet a variety of needs. Perhaps the most common use is to connect laptop users who travel from location to location. Another common use is for mobile networks that connect via satellite. A wireless transmission method is a logical choice to network a LAN segment that must frequently change locations. The following situations justify the use of wireless technology:

To span a distance beyond the capabilities of typical cabling,
To avoid obstacles such as physical structures, EMI, or RFI,
To provide a backup communications link in case of normal network failure,
To link portable or temporary workstations,
To overcome situations where normal cabling is difficult or financially impractical, or
To remotely connect mobile users or networks.

Polarization

2008-03-01T08:39:00.000-08:00

The "polarization" of an antenna is the orientation of the electric field (e-plane) of the radio wave with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation. It has nothing in common with antenna directionality terms: "horizontal", "vertical" and "circular". Thus, a simple straight wire antenna will have one polarization when mounted vertically, and a different polarization when mounted horizontally. "Electromagnetic wave polarization filters" are structures which can be employed to act directly on the electromagnetic wave to filter out wave energy of an undesired polarization and to pass wave energy of a desired polarization.
Reflections generally affect polarization. For radio waves the most important reflector is the re-ionosphere signals which reflect from it will have their polarization changed unpredictably. For signals which are reflected by the ionosphere, polarization cannot be relied upon. For line of sigth for communication for which polarization can be relied upon, it can make a large difference in signal quality to have the transmitter and receiver using the same polarization; many tens of dB difference are commonly seen and this is more than enough to make the difference between reasonable communication and a broken link.
Polarization is largely predictable from antenna construction but, especially in directional antennas, the polarization of side lobes can be quite different from that of the main propagation lobe. For radio antennas, polarization corresponds to the orientation of the radiating element in an antenna. A vertical omni-directional wi-fi antenna antenna will have vertical polarization (the most common type). An exception is a class of elongated waveguide antennas in which vertically placed antennas are horizontally polarized. Many commercial antennas are marked as to the polarization of their emitted signals.
Polarization is the sum of the E-plane orientations over time projected onto an imaginary plane perpendicular to the direction of motion of the radio wave. In the most general case, polarization is elliptical (the projection is oblong), meaning that the antenna varies over time in the polarization of the radio waves it is emitting. Two special cases are linaer polariation (the ellipse collapses into a line) and circular polarization (in which the ellipse varies maximally). In linear polarization the antenna compels the electric field of the emitted radio wave to a particular orientation. Depending on the orientation of the antenna mounting, the usual linear cases are horizontal and vertical polarization. In circular polarization, the antenna continuously varies the electric field of the radio wave through all possible values of its orientation with regard to the Earth's surface. Circular polarizations, like elliptical ones, are classified as right-hand polarized or left-hand polarized using a "thumb in the direction of the propagation" rule. Optical researchers use the same rule of thumb, but pointing it in the direction of the emitter, not in the direction of propagation, and so are opposite to radio engineers' use.
In practice, regardless of confusing terminology, it is important that linearly polarized antennas be matched, lest the received signal strength be greatly reduced. So horizontal should be used with horizontal and vertical with vertical. Intermediate matchings will lose some signal strength, but not as much as a complete mismatch. Transmitters mounted on vehicles with large motional freedom commonly use circularly polarized antennas so that there will never be a complete mismatch with signals from other sources. In the case of radar, this is often reflections from rain drops.

Transmission and Reception

2008-02-24T09:11:00.000-08:00

All of the antenna parameters are expressed in terms of a transmission antenna, but are identically applicable to a receiving antenna, due to reciproicity. Impedance, however, is not applied in an obvious way; for impedance, the impedance at the load (where the power is consumed) is most critical. For a transmitting antenna, this is the antenna itself. For a receiving antenna, this is at the (radio) receiver rather than at the antenna. Tuning is done by adjusting the length of an electrically long linear antenna to alter the electrical resonance of the antenna.
Antenna tuning is done by adjusting an inductance or capacitance combined with the active antenna (but distinct and separate from the active antenna). The inductance or capacitance provides the reactance which combines with the inherent reactance of the active antenna to establish a resonance in a circuit including the active antenna. The established resonance being at a frequency other than the natural electrical resonant frequency of the active antenna. Adjustment of the inductance or capacitance changes this resonance.
Antennas used for transmission have a maximum power rating, beyond which heating, arcing or sparking may occur in the components, which may cause them to be damaged or destroyed. Raising this maximum power rating usually requires larger and heavier components, which may require larger and heavier supporting structures. This is a concern only for transmitting antennas, as the power received by an antenna rarely exceeds the microwatt range.
Antennas designed specifically for reception might be optimized for noise rejection capabilities. An "antenna sheild" is a conductive or low reluctance structure (such as a wire, plate or grid) which is adapted to be placed in the vicinity of an antenna to reduce, as by dissipation through a resistance or by conduction to ground, undesired electromagnetic radiation, or electric or magnetic fields, which are directed toward the active antenna from an external source or which emanate from the active antenna. Other methods to optimize for noise rejection can be done by selecting a narrow-bandwidth so that noise from other frequencies is rejected, or selecting a specific radiation pattern to reject noise from a specific direction, or by selecting a polarization different from the noise polarization, or by selecting an antenna that favors either the electric or magnetic field.
For instance, an antenna to be used for reception of low frequencies (below about ten MHz ) will be subject to both man-made noise from motors and other machinery, and from natural sources such as lightning. Successfully rejecting these forms of noise is an important antenna feature. A small coil of wire with many turns is more able to reject such noise than a vertical antenna. However, the vertical will radiate much more effectively on transmit, where extraneous signals are not a concern.

Practical Antennas

2008-02-24T09:03:00.000-08:00

Although any circuit can radiate if driven with a signal of high enough frequency, most practical antennas are specially designed to radiate efficiently at a particular frequency. An example of an inefficient antenna is the simple Hertzian dipole antenna, which radiates over wide range of frequencies and is useful for its small size. A more efficient variation of this is the half-wave dipole, which radiates with high efficiency when the signal wavelength is twice the electric length of the antenna.
One of the goals of antenna design is to minimize the reactance of the device so that it appears as a resistive load. An "antenna inherent reactance" includes not only the distributed reactance of the active antenna but also the natural reactance due to its location and surroundings (as for example, the capacity relation inherent in the position of the active antenna relative to ground). Reactance diverts energy into the reactive field, which causes unwanted currents that heat the antenna and associated wiring, thereby wasting energy without contributing to the radiated output. Reactance can be eliminated by operating the antenna at its resonat frequency, when its capacitive and inductive reactances are equal and opposite, resulting in a net zero reactive current. If this is not possible, compensating inductors or capacitors can instead be added to the antenna to cancel its reactance as far as the source is concerned.
Once the reactance has been eliminated, what remains is a pure resistance, which is the sum of two parts: the ohmic resistance of the conductors, and the radiation resistance. Power absorbed by the ohmic resistance becomes waste heat, and that absorbed by the radiation resistance becomes radiated electromagnetic energy. The greater the ratio of radiation resistance to ohmic resistance, the more efficient the antenna.

Basic antenna models

2008-02-24T08:57:00.000-08:00

There are many types of antennas below mentioned are few basic :-
The isotropic radiator is a purely theoretical antenna that radiates equally in all directions. It is considered to be a point in space with no dimensions and no mass. This antenna cannot physically exist, but is useful as a theoretical model for comparison with all other antennas. Most antennas' gains are measured with reference to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator).
The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Since this is the simplest practical antenna, it is also used as reference model for other antennas; gain with respect to a dipole is labeled as dBd. Generally, the dipole is considered to be omin-directional in the plane perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. Variations of the dipole include the folded dipole, the half wave antenna, the ground plane antenna, the whip, and the j-pole.
The Yagi-uda antenna is a directional variation of the dipole with parasitic elements added with functionality similar to adding a reflector and lenses (directors) to focus a filament light bulb.
The random-wire antenna is simply a very long (greater than one wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might have a random impedance that varies nonlinearly with frequency.
The horn is used where high gain is needed, the wavelength is short (microwave) and space is not an issue. Horns can be narrow band or wide band, depending on their shape. A horn can be built for any frequency, but horns for lower frequencies are typically impractical

More on Antennas

2008-02-24T08:45:00.000-08:00

A dipole has a uniform or omni-directional radiation patern in one plane and a figure eight pattern in other two planes .This type of antenna can only over cum envoirnmrntal challenges by boosting the power level of signal.Challenges could be mountains, valleys ,buildings,etc.
An antenna is a transducer designed to transmit or recive elecrto-magnetic waves. In other words, antennas convert electromagnetic waves into electrical currents and vice versa. Antennas are used in systems such as radio and T.V broadcasting, point-to-point radio communication,WLAN, radars, and space stations. Antennas usually work in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies for short distances.
Physically, an antenna is an arrangement of conductors that generate a radiating electrnic magnetic feild in response to an applied alternating voltage and the associated alternating electric current, or can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. Some antenna devices (parabolic antenna,horn antenna) just adapt the free space to another type of antenna.
Thomas A Edison used antennas by 1885. Edison patented his system in US pateint466,972 . Antennas were also used in 1888 by Heinrich Hertz (1857-1894) to prove the existence of elecromagnetic waves predicted by the theory of James Maxwell. Hertz placed the emitter dipole in the focal point of a parabolic reflector. He published his work and installation drawings in Annalen der physik and Cheime (vol. 36, 1889).

Antennas

2008-02-23T23:11:00.000-08:00

As the name wireless already indicates ,this communication mode involves gettin rid of wires and transmitting signals through spaces without guidance . We do not need any medium for transport of electromagnetic waves .Somehow ,we have to couple the energy from transmitters to the outsider and in reverse from outside world to the receiver .This is exactly that antenna do.Antennas couple electromagnetic energy to and from space and a wire or co-axial cable(or any other appropriate conductor)
A theoritical referance antenna is the istotropic radiator , a point in space radiating equal power in all directions ,i.e all the points with equal poer are located on a sphere with antenna as its center .The radiation pattern is symmetric in all directions (a three dimesnional in real sense)
Howeaver ,such antenna doesnt exist in reality .Real antennas all inhibit directive effects , the intensity of radiation is not the same in all directions from antenna .The simplest real antenna is a thin ,center fed dipole, also called Hertzian dipole.The dipole consist of two non-collinear conductors of length ,seperated by a small feeding gap.The lenght of dipole is not arbitary ,but for example half the wave-length of the signal to transmit results in an efficent way ,radiation of energy. If mounted in the roof of car, the length of lanbda /4 .This is known as macroni- antenna.