SMALL LOOP ANTENNA BASIC INFORMATION AND TUTORIALS

What is small loop antenna?
Small loop antenna defined.



Large loop antennas are those with overall wire lengths of 0.5λ to more than 2λ. Small loop antennas, on the other hand, have an overall wire length that is much less than one wavelength (1λ).

According to a Second World War US Navy training manual such antennas are those with an overall length of ≤0.22λ. Jasik’s classic 1961 text on radio antennas uses the figure ≤0.17λ, while John Kraus (1950) used the figure ≤0.10λ.

An amateur radio source, The ARRL Antenna Book, recommends ≤0.085λ for small loop antennas. For the purposes of our discussion we will use Kraus’s figure of ≤0.10λ.

A defining characteristic of small loops versus large loops is seen in the current distribution. In the small loop antenna the current flowing in the loop is uniform in all portions of the loop. In the large loop, however, the current varies along the length of the conductor, i.e. there are current nodes and antinodes.

The small loop antenna also differs from the large loop in the manner of its response to the radio signal. A radio signal is a transverse electromagnetic (TEM) wave, in which magnetic and electrical fields alternate with each other along the direction of travel.

The large loop, like most large wire antennas, respond primarily to the electrical field component of the TEM, while small loops respond mostly to the magnetic field component. The importance of this fact is that it means the small loop antenna is less sensitive to local electromagnetic interference
sources such as power lines and appliances.

Local EMI consists largely of electrical fields, while radio signals have both magnetic and electrical fields. With proper shielding, the electrical response can be reduced even further.

TRANSMISSION LINE NOISE BASIC INFORMATION AND TUTORIALS

What is transmission line noise?
Transmission Line Noise Defined.



Transmission lines are capable of generating noise and spurious voltages that are seen by the system as valid signals. Several such sources exist.

One source is coupling between noise currents flowing in the outer and inner conductors. Such currents are induced by nearby electromagnetic interference and other sources (e.g. connection to a noisy ground plane).

Although coaxial design reduces noise pick-up compared with parallel line, the potential for EMI exists. Selection of high-grade line, with a high degree of shielding, reduces the problem.

Another source of noise is thermal noise in the resistances and conductances of the line. This type of noise is proportional to resistance and temperature.

There is also noise created by mechanical movement of the cable. One species results from movement of the dielectric against the two conductors.

This form of noise is caused by electrostatic discharges in much the same manner as the spark created by rubbing a piece of plastic against woollen cloth.

A second species of mechanically generated noise is piezoelectricity in the dielectric. Although more\ common in cheap cables, one should be aware of it.

Mechanical deformation of the dielectric causes electrical potentials to be generated.

Both species of mechanically generated noise can be reduced or eliminated by proper mounting of the cable. Although rarely a problem at lower frequencies, such noise can be significant at microwave
frequencies when signals are low.

VOLTAGE STANDING WAVE RATION BASIC INFORMATION AND TUTORIALS

What is Voltage standing wave ratio (VSWR)?
Voltage standing wave ratio (VSWR) Defined.



When an RF cable is mismatched, i.e. connected to a load of a different impedance to that of the cable, not all the power supplied to the cable is absorbed by the load. That which does not enter the load is reflected back down the cable.

This reflected power adds to the incident voltage when they are in phase with each other and subtracts from the incident voltage when the two are out of phase. The result is a series of voltage – and current – maxima and minima at halfwavelength intervals along the length of the line.

The maxima are referred to as antinodes and the minima as nodes. The voltage standing wave ratio is the numerical ratio of the maximum voltage on the line to the minimum voltage: VSWR = Vmax/Vmin.

It is also given by: VSWR = RL/Z0 or Z0/RL (depending on which is the larger so that the ratio is always greater than unity) where RL = the load resistance.

The return loss is the power ratio, in dB, between the incident (forward) power and the reflected (reverse) power. The reflection coefficient is the numerical ratio of the reflected voltage to the incident voltage.

The VSWR is 1, and there is no reflected power, whenever the load is purely resistive and its value equals the characteristic impedance of the line. When the load resistance does not equal the line impedance, or the load is reactive, the VSWR rises above unity.

A low VSWR is vital to avoid loss of radiated power, heating of the line due to high power loss, breakdown of the line caused by high voltage stress, and excessive radiation from the line. In practice, a VSWR of 1.5:1 is considered acceptable for an antenna system, higher ratios indicating a possible defect.

THE INERTIAL NAVIGATION SYSTEM (INS) BASIC INFORMATION

The inertial navigation system provides information similar to that supplied by a doppler system, and it also gives readings of latitude and longitude when desired. The inertial system utilizes extremely sensitive gyros and accelerometers to develop signals from which navigation information is computed.

The heart of the Collins INS-61 inertial system is the inertial sensor unit (ISU) which contains the

inertial measurement unit (IMU), the system power supply, and the battery charger. The IMU is manufactured by the Kearfott Systems Division of Singer- General Precision, Incorporated.

This unit contains a stabilized platform on which are mounted two 2 degrees-of-freedom gyros, a two axis accelerometer, and 401 a single-axis accelerometer. The stable platform is isolated from angular motions in the aircraft by a shock-mounted gimbal system providing isolation from aircraft vibration.

The IMU is contained within a fixed outer frame which serves to mount the platform, the IMU connectors, and the electronics for instrument compensation and electrical adjustments.

Aircraft pitch, roll, and heading data is provided by synchros mounted on the platform-gimbal axis. The gyros in the IMU sense pitch, roll, and yaw and produce signals which are converted to usable information by the computer.

Acceleration in any direction is sensed by the accelerometers, and this information is also employed by the computer to provide navigation information. The information developed by the computer is displayed on the control/ display panel.

The following information is available: track angle and ground speed, true heading and drift angle, crosstrack distance and trackangle error, present position in latitude and longitude, latitude and longitude of any of the nine way points stored in the system, wind speed and direction, distance and time to the next way point, desired track, from or to a way point, way-point alert, system-failure warning, battery mode of operation annunciation, and attitude reference-mode annunciation.

Desired information is displayed when the selector switch on the control/display panel is rotated to the appropriate position.

THE SUPERHETERODYNE BASIC INFORMATION

The superheterodyne uses one or more mixers to convert an incoming signal, regardless of its frequency, to an identically modulated signal at some other, constant frequency. The signal frequency can be heterodyned once, twice, or even three times. Thus, you might hear of a single-conversion, double-conversion, or triple-conversion superheterodyne receiver.

A single-conversion superhet

The incoming signal first passes through a sensitive, low-noise, tunable front-end amplifier. The tuning range of this amplifier must be sufficient to cover all the desired reception frequencies fIN.

The second stage is a mixer/LO combination. The LO has a variable frequency that tunes over the received-signal range plus 9.000 MHz. The LO frequency control is the main tuning control for the entire receiver.

The LO tuning might track along with the tuning of the front end, or the front end might tune independently by means of a separate preselector control. The mixer output is always at 9.000 MHz, no matter what the incoming signal frequency.

The intermediate frequency

The 9.000-MHz mixer output signal is called the intermediate frequency (IF) of the superhet. This signal has the same modulation waveform, and the same bandwidth, as the incoming signal.

The only difference is that it might be “upside down”; LSB would be changed to USB, or the sense of FSK would be reversed. But this is an inconsequential difference insofar as it has no effect on the quality of the received signal.

The IF is easy to process because its frequency never changes. Several IF amplifier stages, along with filtering, provide the best possible sensitivity and selectivity. This part of the receiver is the IF amplifier chain or IF chain.

MICROCOM NETWORKING PROTOCOL (MNP) CLASSES BASIC INFORMATION

Class1 The lowest performance level. Uses an asynchronous byte-oriented half-duplex method of exchanging data. The protocol efficiency of a Class 1 implementation is about 70% (a 2400 bps modem using MNP Class 1 will have a 1690 bps throughput).

Class 2 Uses asynchronous byte-oriented full-duplex data exchange. The protocol efficiency of a Class 2 modem is about 84% (a 2400 bps modem will realize a 2000 bps throughput).

Class 3 Uses synchronous bit-oriented full-duplex data exchange. This approach is more efficient than the asynchronous byte-oriented approach, which takes 10 bits to represent 8 data bits because of the ‘start’ and ‘stop’ framing bits. The synchronous data format eliminates the need for start and stop bits. Users still send data asynchronously to a Class 3 modem but the modems communicate with each other synchronously. The protocol efficiency of a Class 3 implementation is about 108% (a 2400 bps modem will actually run at a 2600 bps throughput).

Class 4 Adds two techniques: Adaptive Packet Assembly and Data Phase Optimization. In the former technique, if the data channel is relatively error-free, MNP assembles larger data packets to increase throughput. If the data channel is introducing many errors, then MNP assembles smaller data packets for transmission. Although smaller data packets increase protocol overhead, they concurrently decrease the throughput penalty of data retransmissions, so more data are successfully transmitted on the first try.

Data Phase Optimization eliminates some of the administrative information in the data packets, which further reduces protocol overhead. The protocol efficiency of a Class 4 implementation is about 120% (a 2400 bps modem will effectively yield a throughput of 2900 bps).

Class 5 This class adds data compression, which uses a real-time adaptive algorithm to compress data. The real-time capabilities of the algorithm allow the data compression to operate on interactive terminal data as well as on file transfer data. The adaptive nature of the algorithm allows it to analyze user data continuously and adjust the compression parameters to maximize data throughput.

The effectiveness of the data compression algorithm depends on the data pattern being processed. Most data patterns will benefit from data compression, with performance advantages typically ranging from 1.3 to 1.0 and 2.0 to 1.0,although some files may be compressed at an even higher ratio. Based on a 1.6 to 1 compression ratio, Microcom gives Class 5 MNP a 200% protocol efficiency, or 4800 bps throughput in a 2400 bps modem installation.

Class 6 This class adds 9600 bps V.29 modulation, universal line negotiation, and statistical duplexing to MNP Class 5 features. Universal link negotiation allows two unlike MNP Class 6 modems to find the highest operating speed (between 300 and 9600 bps) at which both can operate. The modems begin to talk at a common lower speed and automatically negotiate the use of progressively higher speeds.

Statistical duplexing is a technique for simulating full-duplex service over half-duplex, high-speed carriers. Once the modem link has been established using full-duplex V.22 modulation, user data streams move via the carrier’s faster half-duplex mode. However, the modems monitor the data streams and allocate each modem’s use of the line to best approximate a full-duplex exchange. Microcom claims that a 9600 bps V.29 modem using MNP Class 6 (and Class 5 data compression) can achive 19.2 kbps throughput over dial circuits.

Class 7 Uses an advanced form of Huffman encoding called Enhanced Data Compression. Enhanced Data Compression has all the characteristics of Class 5 compression, but in addition predicts the probability of repetitive characters in the data stream. Class 7 compression, on the average, reduces data by 42%.

Class 8 Adds CCITT V.29 Fast-Train modem technology to Class 7 Enhanced Data Compression, enabling half-duplex devices to emulate full-duplex transmission.

Class 9Combines CCITT V.32 modem modulation technology with Class 7 Enhanced Data Compression, resulting in a full-duplex throughput that can exceed that obtainable with a V.32 modem by 300%. Class 9 also employs selective retransmission, in which errors packets are retransmitted, and piggybacking, in which acknowledgment information is added to the data.

Class 10Adds Adverse Channel Enhancement (ACE),which optimizes modem performance in environments with poor or varying line conditions, such as cellular communications, rural telephone service,and some international connections.

Adverse Channel Enhancements fall into five categories:

Negotiated Speed Upshift: modem handshake begins at the lowest possible modulation speed, and when line conditions permit, the modem upshifts to the highest possible speed.

Robust Auto-Reliable Mode: enables MNP10 modems to establish a reliable connection during noisy call set-ups by making multiple attempts to overcome circuit interference. In comparison,other MNP classes make only one call set-up attempt.

Dynamic Speed Shift: causes an MNP10 modem to adjust its operating rate continuously throughout a session in response to current line conditions.

Aggressive Adaptive Packet Assembly: results in packet sizes varying from 8 to 256 bytes in length. Small data packets are used during the establishment of a link, and there is an aggressive increase in the size of packets as conditions permit.

Dynamic Transmit Level Adjustment (DTLA): designed for cellular operations, DTLA results in the sampling of the modem’s ransmit level and its automatic adjustment to optimize data throughput.

SCEINTIFIC ATLANTA CABLE MODEM BASIC INFORMATION

In this examination of cable modems, we will focus upon the asymmetric architecture of a Scientific Atlanta cable modem. The Scientific Atlanta cable modem we will examine is based upon an asymmetric design, using QAM in a 6MHz downstream channel to obtain an operating rate of 27 MHz.

In the opposite direction the modem uses QPSK modulation to provide an operating rate of 1.5 Mbps upstream. The modem supports downstream frequencies in the 54 to 750MHz spectrum and frequencies in the 14MHz to 26.5MHz range for upstream communications.

The Scientific Atlanta cable modem’s modulation method was proposed to the IEEE 802.14 Working Group and became the basis for use in both the IEEE standard and the DOCSI specification. Scientific Atlanta noted that QAM is non-proprietary and was previously selected as the European Telecommunications Standard.

In the firm’s proposal, two levels of modulation based upon 64 QAM and 256 QAM were defined to permit implementation flexibility. The standardization of QAM for downstream transmission results in a signaling rate of 5MHz using a carrier frequency between 151MHz and 749MHz spaced 6MHz apart to correspond to TV channel assignments.

The use of a 5MHz signaling rate and 64 QAM which enables six bits to be encoded in one signal change permits a transmission rate of 6 bits/symbol#5 MHz,or 30 Mbps. In comparison, the use of 256 QAM results in the packing of eight bits per signal change, resulting in a transmission rate of 8 bits/signal change#5 MHz,or 40 Mbps.

Through the use of forward error coding, the data rate throughput is slightly reduced from the modem’s operating rate to 35.504 Mbps for 256 QAM and 27.37 Mbps for 64 QAM. This reduction results from extra parity bits becoming injected into the data stream to provide the forward error detection and correction capability.

COMPUTER PRINTER AND DISPLAY TERMINALS BASIC INFORMATION

A serial or character printer, whose name resulted from the fact that they print one character at a time, was the display mechanism first used with terminals. The first serial printers were ‘fully formed’ impact printers in which different types of mechanisms, including a daisy wheel, type-ball or rotating cylinder which formed characters from a single piece of type, were used to strike a ribbon to produce a printed image.

The editing capability provided by the terminal was minimal, typically permitting the operator to delete a previously entered character or the current line, since the terminal transmitted and received data on a line by line basis. A second type of impact printer which grew in popularity during the 1970s and 1980s to where it has virtually replaced fully formed printers is the dot matrix printer.

The dot matrix printer employs a matrix of pins in its print head. The first dot matrix printers used a rectangular matrix of dots, typically 7 dots high by 5 dots wide or 9 dots high by 7 dots wide. The pins in the matrix are selectively ‘fired’ to form each character. Printing of characters results from the movement of the print head containing a column of 7 or 9 pins across the paper, with the printer selectively firing the pins at 5 or 7 successive intervals to form each character.

Until the mid-1980s, the matrix of pins used to form characters resulted in a considerable amount of white space between dots. This space made the dot matrix pattern easily discernible to the eye and limited the use of printed output produced by this type of printer to draft correspondence.

By the mid 1980s, advances in print head technology resulted in the inclusion of more pins on some print heads. The additional pins were used to considerably reduce the space between pin impacts in forming a character.

Other dot matrix developments included two-pass printing in which the first pass of printing a line was followed by the printer feeding the paper upward by a slight amount, perhaps 1/256th of an inch, prior to the line being printed a second time.

One result of placing more pins on the print head and using a two-pass printing technique was. a higher quality print. Since this print resembled the letter quality print of a full impact printer, it became known as near letter quality (NLQ).

The firing of additional pins to form a better print image required additional time, resulting in NLQ printing being slower than conventional dot matrix printing. Thus, most modern dot matrix printers have two or more user selectable print modes – draft and NLQ – with the draft print mode providing

a considerably faster print rate than the NLQ print mode.

A second major category of printers employs non-impact technology to form characters. Non-impact printers include thermal matrix ink jet and laser devices. The thermal matrix printer forms characters by applying a voltage to pins in a matrix, causing the pins to be heated. The heated pins interact with heat-sensitive paper used in these printers, resulting in the formation of characters.

The ink jet printer has a nozzle consisting of a matrix of holes out of which ink is squirted to form characters. Thus, both thermal matrix and ink jet printers are based upon dot matrix technology. In comparison, the laser printer uses a rotating drum and a small amount of current to generate a magnetic field, which results in toner from a cartridge adhering to distinct locations on paper passing around the drum. Laser printers have resolutions between 300 and 1200 dots per inch (dpi) and through software can form characters in almost any shape.

The key limitation associated with the use of printers for both input and output is the elementary editing capability provided by this type of terminal device. Data entered from the keyboard are either printed and transmitted as each character is pressed or stored in a buffer area.

The buffer storage area contained in most ASR and KSR terminals is only capable of holding one line of data or 72 to 80 characters depending upon the type of terminal. By using the backspace key to eliminate a previously entered character, an operator can perform elementary editing.

Once the carriage return key has been pressed, however, the entire line is transmitted, resulting in an operator having to re-enter the line with any changes he or she desires to correct a previously entered line. As an alternative to the use of the backspace key, an operator can simultaneously press the control (Ctrl) key and an alphabetic key, canceling the present line and removing its contents from buffer storage. This action causes a carriage return and line feed to be automatically generated, permitting the operator to begin his or her data entry anew.

STEREO FM RADIO BROADCASTING BASIC INFORMATION AND TUTORIALS

An important advantage of FM over AM radio broadcasting is the availability of stereo sound. This requires the transmission and reception of the left and right audio signals, to produce a stereo image.

However, the introduction of stereo had to be achieved while still catering for large numbers of listeners who wanted to continue receiving in mono. The introduction of stereo FM radio had to go unnoticed by anyone using old mono radios. Without this condition, a simpler method could have been chosen.

The standard FM stereo system uses frequency division multiplexing (FDM) to combine the two signals of the left and right channels. The signals are filtered to limit the bandwidth to 15 kHz. The left (L) and right (R) signals are then added to produce a sum signal and subtracted one from the other to produce a difference signal.


The sum signal provides a monophonic signal, which provides a baseband signal for the frequency modulator. This was the technique used in mono FM and thus was the obvious choice for stereo FM, to allow backward compatibility.

An mono FM radio can receive this signal and recover the combined L and R channels, thereby satisfying the requirement for providing unchanged service to mono radios. The difference signal is used to amplitude modulate a 38 kHz sinewave. By utilizing a balanced mixer, double sideband suppressed carrier (DSBSC) is generated.

However, the modulation method must take into account the ease of demodulation. In particular, demodulating a DSBSC signal can be difficult. Both frequency and phase of the carrier are needed to perform faithful demodulation.


In the stereo system the DSBSC demodulation problem is dealt with by including a 19 kHz pilot tone in the broadcast. This tone is generated by a divide-by-two frequency converter circuit, which takes the 38 kHz carrier and produces the 19 kHz pilot tone.

The 19 kHz pilot tone falls midway in the spectral region between the mono sum signal (up to 15 kHz) and below the DSBSC difference signal information  The DSBSC signal extends from 23 kHz to 53 kHz, since the input modulating signals are band limited to 15 kHz. The DSBSC output is added to the baseband (L and R sum) signal and the 19 kHz pilot tone before being sent to the FM modulator.

A mono FM receiver ignores the stereo information by using a filter after its FM demodulator to block everything above 15 kHz. It passes the combined L and R channel signal, which is monophonic. A stereo receiver has an additional circuit after the FM demodulator to detect and demodulate the DSBSC signal. The stereo receiver detects a 19 kHz pilot tone and uses this to generate a 38 kHz signal.

This is then used to demodulate the DSBSC signal that carries the L and R channel difference information. The stereo receiver then has both the sum and difference signals, which is all that is needed to recreate the separate left and right signals.

Separation is achieved by adding and subtracting sum and difference signals. The noise power spectral density of a demodulated FM signal tends to increase with the square of the modulation frequency. This is why pre-emphasis is used to boost the high frequency baseband signals for maintaining the signal-to-noise ratio of the transmitted signal.

However, this means that there will be more noise in the 23 kHz to 53 kHz band used for the difference signal than for the 0–15 kHz band used for the sum signal. Consequently a significantly higher input signal level is required to receive a stereo transmission compared with a mono signal for the same output signal-to noise ratio.

Thus stereo reception requires far higher radio signal levels than for mono reception and is more susceptible to interference from other radio sources.

QUARTZ CRYSTAL SPECIFICATION BASIC INFORMATION AND TUTORIALS

The details which must be specified when ordering crystals are:

1. Frequency. Normally specified in kHz up to 9999.999 kHz and in MHz from 10.0MHz upwards except for integer values which are all specified in MHz. The frequency must be described to seven significant figures, otherwise any figure that might follow those given will be taken as zero.

2. Mounting or holder style.

3. Frequency tolerance. This is the cutting or calibration tolerance acceptable at 25◦C. It should be borne in mind that cost rises with increased manufacturing accuracy and a slight adjustment (pullability) is possible in the circuit.


4. Frequency stability. Normally specified as a plus or minus value measured over a defined temperature range. A crystal designed for a restricted range has a better performance over that range than one designed for a wider range so it is important not to overspecify.

5. Temperature range. The range over which the crystal is required to operate and meet the performance specified in 4. Standard temperature ranges are:

0 to 5◦C
−10 to 60◦C
−20 to 70◦C
−30 to 80◦C
−40 to 90◦C
−55 to 105◦C
−55 to 125◦C

It is sufficient when ordering from some manufacturers to quote only the lower temperature limit. For ovened operation the quoted figure, say 80◦C, would denote the oven temperature.

6. Circuit condition. This specifies the shunt capacitance that the circuit will place across the crystal in parallel mode operation.

7. Drive level. The maximum power that the crystal can safely dissipate. 1mW is a typical value for crystals used in radio transmitters and receivers.

A typical specification therefore reads: 16.66667MHz HC49 20 30 10 30
1 2 3 4 5 6 referring to the items listed above

When the crystal is for operation in series mode, it is usually sufficient to replace the last figure with ‘S’. The drive level is not normally specified in the ordering details.

TYPES OF BROADBAND INTERNET ACCESS BASIC INFORMATION

Broadband Access—The Fat Internet Pipe
What started as a healthy communication vehicle among students, researchers, and university professors is today the most pervasive communications technology available. The Internet has positively affected millions of people around the world in areas such as communication, finance, engineering, operations, business, and daily living.

Perhaps the Internet’s best-known feature is the World Wide Web (the Web), which provides unlimited resources for text, graphics, video, audio, and animation applications.

Unfortunately, many people who access the Web experience the frustration of endless waiting for information to download to their computers. The need for fast access to the Internet is pushing the demand for broadband access.

We are moving from phone line dial-up to new and improved connectivity platforms. Demand for high-speed Internet access solutions has fueled the proliferation of a number of rival technologies that are competing with each other to deliver high-speed connections to the home.

The bulk of these connections are delivered by technologies based on delivery platforms such as:

■ Cable
■ DSL
■ ISDN
■ Wireless local loop
■ Ethernet
■ Fiber
■ Satellite or Powerline

Telecommunication, satellite, and cable companies are looking for ways to enhance their revenue sources by providing high-speed Internet access. While the average phone call lasts three minutes, an Internet transaction using an analog modem averages over three hours and keeps the circuits busy.

This increased traffic requires more circuits, but comes with no incremental revenue gain for the service provider.

Phone companies had to look for new techniques for Internet access. One such technology is DSL, which offers several advantages.

While phone lines are limited to speeds up to 56 Kb/s (kilobits per seconds) that many find inadequate, broadband communications offer much higher transfer rates. And broadband connections are always on, which means users don’t have to go through a slow log-on process each time they access the Internet.

TYPES OF MOBILE PHONE NETWORKS BASIC INFORMATION

Networks Available
Most of the phones available today are digital phones, but if they operate only on a single digital frequency and not on the more ubiquitous analog network, you may end up with a phone that works only in very limited areas.
Phones on the market today fall into three categories:

� Single Mode
� Dual Mode
� Tri-Mode

Single Mode
Some of the newest digital phones being introduced support only a single digital network because the manufacturer wanted to get it to market quickly to make sure they got the newest, coolest phone on the shelf first.

All manufacturers produce single mode phones. But the problem may be that it’s cool when it works downtown, but if you try to use it when driving to your house in the suburbs you may be out of luck.

Before purchasing a phone, remember to check the phone’s specifications, or ask a customer service representative to explain the phone’s mode to make sure it will work everywhere you need it to work.

Dual Mode
Many digital phones are dual mode; that is, they work on two networks— digital and analog. Some of the phones that are available as dual mode actually work on three networks—for example, the 800 MHz analog network, the 800 MHz digital network, and the 1900 MHz digital network.These phones will allow you the most use, because they are most likely to find an available network wherever you are.

Tri-Mode
A small number of phones available today are called tri-mode.This means that they work on three frequencies: 800 MHz analog, 1900 MHz U.S. digital, and another international digital frequency. Tri-mode phones are the ones that will work overseas in many countries around the world.

VOLTAGE STANDING WAVE RATIO BASIC INFORMATION AND TUTORIALS

When an RF cable is mismatched, i.e. connected to a load of a different impedance to that of the cable, not all the power supplied to the cable is absorbed by the load. That which does not enter the load is reflected back down the cable.

This reflected power adds to the incident voltage when they are in phase with each other and subtracts from the incident voltage when the two are out of phase. The result is a series of voltage – and current – maxima and minima at halfwavelength intervals along the length of the line. The maxima are referred to as antinodes and the minima as nodes.


The voltage standing wave ratio is the numerical ratio of the maximum voltage on the line to the minimum voltage: VSWR = Vmax/Vmin. It is also given by: VSWR = RL/Z0 or Z0/RL (depending on which is the larger so that the ratio is always greater than unity) where RL = the load resistance.

The return loss is the power ratio, in dB, between the incident (forward) power and the reflected (reverse) power. The reflection coefficient is the numerical ratio of the reflected voltage to the incident voltage.

The VSWR is 1, and there is no reflected power, whenever the load is purely resistive and its value equals the characteristic impedance of the line. When the load resistance does not equal the line impedance, or the load is reactive, the VSWR rises above unity.

A low VSWR is vital to avoid loss of radiated power, heating of the line due to high power loss, breakdown of the line caused by high voltage stress, and excessive radiation from the line. In practice, a VSWR of 1.5:1 is considered acceptable for an antenna system, higher ratios indicating a possible defect.

SCATTER IN COMMUNICATION (SPORADIC E) BASIC INFORMATION AND TUTORIALS

There are a number of scatter modes of propagation. These modes can extend the radio horizon a considerable amount. Where the radio horizon might be a few tens of kilometres, underscatter modes permit very much longer propagation.

For example, a local FM broadcaster at 100MHz might have a service area of about 40 miles, and might be heard 180 miles away during the summer months when Sporadic- E propagation occurs. One summer, a television station in Halifax, Nova Scotia, Canada, was routinely viewable in Washington, DC in the United States during the early morning hours for nearly a week.

Sporadic-E is believed to occur when a small region of the atmosphere becomes differentially ionized, and thereby becomes a species of ‘radio mirror’. Ionospheric scatter propagation occurs when clouds of ions exist in the atmosphere.

These clouds can exist in both the ionosphere and the troposphere, although the tropospheric model is more reliable for communications. A signal impinging this region may be scattered towards other terrestrial sites which may be a great distance away. The specific distance depends on the geometry of the scenario.

There are at least three different modes of scatter from ionized clouds: back scatter, side scatter, and forward scatter. The back scatter mode is a bit like radar, in that signal is returned back to the transmitter site, or in regions close to the transmitter.

Forward scatter occurs when the reflected signal continues in the same azimuthal direction (with respect to the transmitter), but is redirected toward the Earth’s surface. Side scatter is similar to forward scatter, but the azimuthal direction might change.

Unfortunately, there are often multiple reflections from the ionized cloud. When these reflections are able to reach the receiving site, the result is a rapid, fluttery fading that can be of quite profound depths.

Meteor scatter is used for communication in high latitude regions. When a meteor enters the Earth’s atmosphere it leaves an ionized trail of air behind it. This trail might be thousands of kilometres long, but is very short lived.

Radio signals impinging the tubular metre ion trail are reflected back towards Earth. If the density of meteors in the critical region is high, then more or less continuous communications can be achieved.

This phenomenon is noted in the low VHF between 50 and about 150 MHz. It can easily be observed on the FM broadcast band if the receiver is tuned to distant stations that are barely audible. If the geometry of the scenario is right, abrupt but short-lived peaks in the signal strength will be noted.

MULTI PATH RADIO PROPAGATION BASIC INFORMATION AND TUTORIALS

Reflection, refraction and diffraction may provide signals in what would otherwise be areas of no signal, but they also produce interference.

Reflected – or diffracted – signals may arrive at the receiver in any phase relationship with the direct ray and with each other. The relative phasing of the signals depends on the differing lengths of their paths and the nature of the reflection.

When the direct and reflected rays have followed paths differing by an odd number of half-wavelengths they could be expected to arrive at the receiver in anti-phase with a cancelling effect. However, in the reflection process a further phase change normally takes place.

If the reflecting surface had infinite conductivity, no losses would occur in the reflection, and the reflected wave would have exactly the same or opposite phase as the incident wave depending on the polarization in relation to the reflecting surface.

In practice, the reflected wave is of smaller amplitude than the incident, and the phase relationships are also changed. The factors affecting the phasing are complex but most frequently, in practical situations, approximately 180◦ phase change occurs on reflection, so that reflected waves travelling an odd number
of half-wavelengths arrive in phase with the direct wave while those travelling an even number arrive anti-phase.

As conditions in the path between transmitter and receiver change so does the strength and path length of reflected signals. This means that a receiver may be subjected to signal variations of almost twice the mean level and practically zero, giving rise to severe fading.

This type of fading is frequency selective and occurs on troposcatter systems and in the mobile environment where it is more severe at higher frequencies. A mobile receiver travelling through an urban area can receive rapid signal fluctuations caused by additions and cancellations of the direct and reflected signals at half-wavelength intervals.

Fading due to the multi-path environment is often referred to as Rayleigh fading. Rayleigh fading, which can cause short signal dropouts, imposes severe restraints on mobile data transmission.

ENHANCED SPECIALIZED MOBILE RADIO (ESMR) BASIC DEFINITION AND INFORMATION

What Is ESMR?


Enhanced Specialized Mobile Radio (ESMR) systems use digital radio transmissions similar to other digital technologies. Spread-spectrum modes, such as frequency hopping, are common.

One major difference from other networks is that in an ESMR system, connections between users is almost instantaneous, compared to the typical delays required to dial and set up a call in a public cellular network.This capability allows the ESMR carrier to offer walkie-talkie–like services on its network, as well as cellular calling.

This is a great advantage for large work groups who need to be in constant contact with just a touch of a button, for example a construction crew. ESMR services also allow customers to contact many people at the same time, much like a CB radio, thus creating a multiple person “call.”

Examples of ESMR networks include Ericsson’s Enhanced Digital Access Communications System (EDACS), Motorola’s Integrated Dispatch Enhanced Network (iDEN), and the Nextel System. Delivery and courier services, which depend on mobility and speed, also typically employ ESMR for voice communications between the delivery vehicles and the office.

The technology consists of a dispatcher in an office plotting out the day’s events for the driver.When the driver arrives at his location, he radios the dispatcher and lets him know his location.The benefit of ESMR is its ability to act like a CB radio, allowing all users on one channel to listen, while still allowing two users to communicate personally.This arrangement allows the dispatcher to coordinate schedules for both pick-ups and deliveries, and to track the driver’s progress.

Drivers with empty loads can be routed to assist backlogged drivers. Drivers that are on the road can be radioed if a customer cancels a delivery.This type of communication benefits delivery services in two major areas, saving time and increasing efficiency.

Benefits of ESMR
ESMR is a unique digital service that allows the user a couple ways to communicate, including:

Push-to-talk features that operate like walkie-talkies allow users to talk directly to another person without delay in setting up the call

Group calling allows users to talk to many people at one time

Digital transmissions assure privacy

Two-way data features are available

International systems allow the use of one handset anywhere the service is available

TIME DIVISION MULTIPLE ACCESS (TDMA) BASIC DEFINITION AND INFORMATION

What Is TDMA? 


Time Division Multiple Access (TDMA) divides wireless conversations by frequency and time to increase the capacity of the network. TDMA uses a single voice channel for multiple calls by taking each call, breaking it into timed sections of a digital transmission to the tower, and reassembling the call based on the timeslot.

For example, call A has all its parts put into timeslot A, call B has its parts put into timeslot B, and each are sent in one transmission to the tower, where the calls are put back together for transmission to the other party. TDMA was one of the original digital systems used and has gone through many revisions to make sure it utilizes the network to the best of its abilities.

Based on a limited number of time slots for each call in a channel, TDMA has no accommodations for silence in a telephone conversation. In other words, once a call is initiated, the channel/timeslot pair belongs to the phone for the duration of the call.

Each channel using TDMA technology for wireless calls is further divided into time slots. Each timeslot is assigned to a different user.The transmitter transmits information for all time slots at the same frequency and the receivers receive all the timeslots but listen only to the time slot they have been allocated.

The net effect is that the efficiency of the channel is increased by a multiple of the number of timeslots that are being used. Most common second-generation TDMA phone systems use three timeslots.


Benefits of TDMA
TDMA was one of the first technologies to expand analog’s voice capabilities beyond one call per channel.TDMA has many other benefits:

Provides advanced features like caller ID, text messaging

Economizes bandwidth

Provides voice clarity and overall call quality, even over long distances

Is difficult to decode, therefore only minimal eavesdropping is possible

Uses less battery power (when compared to analog)

Offers voice privacy

GLOBAL SYSTEM FOR MOBILE COMMUNICATION (GSM) BASIC DEFINITION AND INFORMATION

What Is GSM?


Global Systems for Mobile Communication (GSM) is the main technology used by the international digital wireless systems; however, GMS is used only by a small percentage of wireless carriers in the United States. GSM is interesting in that it uses a modified and far more efficient version of TDMA.

GSM keeps the idea of time slots on frequency channels, but corrects several major shortcomings. Since the GSM timeslots are smaller than TDMA, they hold less data but allow for data rates starting at 300 bits per second. Thus, a call can use as many timeslots as necessary up to a limit of 13 kilobits per second.

When a call is inactive (silence) or can be compressed more, fewer timeslots are used.To facilitate filling in gaps left by unused timeslots, calls do frequency hopping in GSM.This means that calls will jump between channels and timeslots to maximize the system’s usage.A control channel is used to communicate the frequency hopping and other information between the antenna tower and the phone.

The architecture used by GSM consists of three main components: a mobile station, a base station subsystem, and a network subsystem. These components work in tandem to allow a user to travel seamlessly without interruption of service, while offering the flexibility of having any device used permanently or temporarily by any user.

Utilizing the three separate components of the GSM network, this type of communication is truly portable.A user can place an identification card called a Subscriber Identity Module (SIM) in the wireless device, and the device will take on the personal configurations and information of that user.

This includes telephone numbers, home system, and billing information. Although the United States has migrated towards CDMA and TDMA as the premier mode of wireless communications, a large part of the world uses GSM.


Benefits of GSM
GSM networks cover the most wireless users around the world and the technology is gaining favor in the United States because of the following benefits:


Provides integrated voice, high-speed two-way data, fax, and short message services capabilities

Offers advanced features such as caller ID, text messaging

Offers superior voice clarity and overall call quality

Provides personal identification tied to a Subscriber

 Information Module (SIM) card that can be used in multiple phones, not tying the user to one phone

 Offers voice privacy

Uses less battery power (when compared to analog)

Enables a single technology handset to work around the world where GSM is available (as long as the frequencies are accessible by the handset)

CODE DIVISION MULTIPLE ACCESS (CDMA) DEFINITION BASIC INFORMATION AND TUTORIALS

What Is CDMA?


Code Division Multiple Access (CDMA) is often referred to as the most interesting, but hardest to implement method of carrying wireless services. CDMA systems have no channels, but instead encode each call as a coded sequence across the entire frequency spectrum. Each conversation is modulated, in the digital domain, with a unique code that makes it distinguishable from the other calls in the frequency spectrum.

CDMA is the newest of the multiple access technologies; it is not yet as widely used but is showing great promise. CDMA does not divide the allocated block of frequencies into individual channels. It assigns a unique code to each signal and then combines all the signals into a single large channel.

The receiver receives the integrated signal and uses the same code just to process the desired signal. CDMA is gaining popularity as a third-generation (3G) wireless phone technology because it is very efficient at utilizing bandwidth, plus it is natively very secure because all conversations are uniquely encoded.

The fact that CDMA shares frequencies with neighboring wireless towers allows for easier installation of extra capacity, since extra capacity can be achieved by simply adding extra cell sites and shrinking power levels of nearby sites.The downside to CDMA is the complexity of deciphering and extracting the received signals, especially if there are multiple signal paths (reflections) between the phone and the wireless tower (called multipath interference).

As a result, CDMA phones are sometimes more expensive than other digital phones and CDMA antenna site equipment is three to four times the price of the other digital network equivalents.


Benefits of CDMA
CDMA networks cover more wireless users in the United States than any other digital standard and include benefits such as:

Advanced features like caller ID, text messaging

Voice clarity and overall call quality

The ability to filter out background noise and interference

Fewer dropped calls (as compared to analog)

Improved security and privacy—the digitally encoded, spread spectrum transmissions minimizes eavesdropping

Α large number of customers who can share the same radio frequencies

The greatest customer capacity of network equipment for low cost

Less battery power (when compared to analog)

TYPES OF COAXIAL CABLE BASIC INFORMATION AND TUTORIALS

Coaxial cable consists of two cylindrical conductors sharing the same axis (hence ‘co-axial’) and separated by a dielectric. For low frequencies (in flexible cables) the dielectric may be polyethylene or polyethylene foam, but at higher frequencies Teflon and other materials are used.

Also used in some applications, notably high powered broadcasting transmitters, are dry air and dry nitrogen.

Several forms of coaxial line are available. Flexible coaxial cable discussed earlier in this chapter is perhaps the most common form.

The outer conductor in such cable is made of either braided wire or foil. Again, television broadcast receiver antennas provide an example of such cable from common experience.

Another form of flexible or semi-flexible coaxial line is helical line in which the outer conductor is spiral wound. This type of coaxial cable is usually 2.5 or more centimetres in diameter.

Hardline is coaxial cable that uses a thin-walled pipe as the outer conductor. Some hardline coax used at microwave frequencies has a rigid outer conductor and a solid dielectric.

Gas-filled line is a special case of hardline that is hollow, the centre conductor being supported by a series of thin ceramic or Teflon insulators. The dielectric is either anhydrous (i.e. dry) nitrogen or some other inert gas.

Some flexible microwave coaxial cable uses a solid ‘air-articulated’ dielectric, in which the inner insulator is not continuous around the centre conductor, but rather is ridged. Reduced dielectric losses increase the usefulness of the cable at higher frequencies.

Double shielded coaxial cable provides an extra measure of protection against radiation from the line, and EMI from outside sources from getting into the system.