Archive for the ‘RF Engineering’ Category

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Basic Antenna Concept

Thursday, November 12th, 2009
Basic Antenna ConceptsAntenna
An antenna is a device that transmits and/or receives electromagnetic waves. Electromagnetic waves are often referred to as radio waves. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned to the same frequency band that the radio system to which it is connected operates in, otherwise reception and/or transmission will be impaired.

Wavelength
We often refer to antenna size relative to wavelength. For example: a half-wave dipole, which is approximately a half-wavelength long. Wavelength is the distance a radio wave will travel during one cycle. The formula for wavelength is:

 

Note: The length of a half-wave dipole is slightly less than a half-wavelength due to end effect. The speed of propagation in coaxial cable is slower than in air, so the wavelength in the cable is shorter. The velocity of propagation of electromagnetic waves in coax is usually given as a percentage of free space velocity, and is different for different types of coax.

Impedance_Matching
For efficient transfer of energy, the impedance of the radio, the antenna, and the transmission line connecting the radio to the antenna must be the same. Radios typically are designed for 50 ohms impedance and the coaxial cables (transmission lines) used with them also have a 50 ohm impedance. Efficient antenna configurations often have an impedance other than 50 ohms, some sort of impedance matching circuit is then required to transform the antenna impedance to 50 ohms. Radiall/Larsen antennas come with the necessary impedance matching circuitry as part of the antenna. We use low loss components in our matching circuits to provide the maximum transfer of energy between the transmission line and the antenna.

VSWR_and_Reflected_Power
The Voltage Standing Wave Ratio (VSWR) is an indication of how good the impedance match is. VSWR is often abbreviated as SWR. A high VSWR is an indication that the signal is reflected prior to being radiated by the antenna. VSWR and reflected power are different ways of measuring and expressing the same thing.

A VSWR of 2.0:1 or less is considered good. Most commercial antennas, however, are specified to be 1.5:1 or less over some bandwidth. Based on a 100 watt radio, a 1.5:1 VSWR equates to a forward power of 96 watts and a reflected power of 4 watts, or the reflected power is 4.2% of the forward power.

Bandwidth
Bandwidth can be defined in terms of radiation patterns or VSWR/reflected power. Bandwidth is often expressed in terms of percent bandwidth, because the percent bandwidth is constant relative to frequency. If bandwidth is expressed in absolute units of frequency, for example MHz, the bandwidth is then different depending upon whether the frequencies in question are near 150, 450, or 825 MHz.

 

Directivity_and_Gain
Directivity is the ability of an antenna to focus energy in a particular direction when transmitting or to receive energy better from a particular direction when receiving. The relationship between gain and directivity: Gain = efficiency/Directivity. We see the phenomena of increased directivity when comparing a light bulb to a spotlight. A 100 watt spotlight will provide more light in a particular direction than a 100 watt light bulb, and less light in other directions. We could say the spotlight has more “directivity” than the light bulb. The spotlight is comparable to an antenna with increased directivity. An antenna with increased directivity is hopefully implemented efficiently, is low loss, and therefore exhibits both increased directivity and gain.

Gain is given in reference to a standard antenna. The two most common reference antennas are the isotropic antenna and the resonant half-wave dipole antenna. The isotropic antenna radiates equally well in “all” directions. Real isotropic antennas do not exist, but they provide useful and simple theoretical antenna patterns with which to compare real antennas. An antenna gain of 2 (3 dB) compared to an isotropic antenna would be written as 3 dBi. The resonant half-wave dipole can be a useful standard for comparing to other antennas at one frequency or over a very narrow band of frequencies. To compare the dipole to an antenna over a range of frequencies requires an adjustable dipole or a number of dipoles of different lengths. An antenna gain of 1 (0 dB) compared to a dipole antenna would be written as 0 dBd.

Gain_Measurement
One method of measuring gain is by comparing the antenna under test against a known standard antenna. This is technically known as a gain transfer technique. At lower frequencies, it is convenient to use a 1/2-wave dipole as the standard. At higher frequencies, it is common to use a calibrated gain horn as a gain standard, with gain typically expressed in dBi.

Another method for measuring gain is the 3 antenna method. Transmitted and received power at the antenna terminals is measured between three arbitrary antennas at a known fixed distance. The Friis transmission formula is used to develop three equations and three unknowns. The equations are solved to find the gain expressed in dBi of all three antennas.

Use the following conversion factor to convert between dBd and dBi: 0 dBd = 2.15 dBi.

Antenna_Placement
Correct antenna placement is critical to the performance of an antenna. An antenna mounted on the roof will function better than the same antenna installed on the hood or trunk of a car. Knowledge of the vehicle may also be an important factor in determining what type of antenna to use. You do not want to install a glass mount antenna on the rear window of a vehicle in which metal has been used to tint the glass. The metal tinting will work as a shield and not allow signals to pass through the glass.

Radiation_Patterns
The radiation or antenna pattern describes the relative strength of the radiated field in various directions from the antenna, at a fixed or constant distance. The radiation pattern is a “reception pattern” as well, since it also describes the receiving properties of the antenna. The radiation pattern is three-dimensional, but it is difficult to display the three-dimensional radiation pattern in a meaningful manner, it is also time consuming to measure a three-dimensional radiation pattern. Often radiation patterns are measured that are a slice of the three-dimensional pattern, which is of course a two-dimensional radiation pattern which can be displayed easily on a screen or piece of paper. These pattern measurements are presented in either a rectangular or a polar format.

Absolute_and_Relative_Patterns
Absolute radiation patterns are presented in absolute units of field strength or power. Relative radiation patterns are referenced in relative units of field strength or power. Most radiation pattern measurements are relative pattern measurements, and then the gain transfer method is then used to establish the absolute gain of the antenna.

Near-Field_and_Far-Field_Patterns
The radiation pattern in the region close to the antenna is not exactly the same as the pattern at large distances. The term near-field refers to the field pattern that exists close to the antenna; the term far-field refers to the field pattern at large distances. The far-field is also called the radiation field, and is what is most commonly of interest. The near-field is called the induction field (although it also has a radiation component).

Ordinarily, it is the radiated power that is of interest, and so antenna patterns are usually measured in the far-field region. For pattern measurement it is important to choose a distance sufficiently large to be in the far-field, well out of the near-field. The minimum permissible distance depends on the dimensions of the antenna in relation to the wavelength. The accepted formula for this distance is:

 

When extremely high power is being radiated (as from some modern radar antennas), the near-field pattern is needed to determine what regions near the antenna, if any, are hazardous to human beings.

Beamwidth
Depending on the radio system in which an antenna is being employed there can be many definitions of beamwidth. A common definition is the half power beamwidth. The peak radiation intensity is found and then the points on either side of the peak represent half the power of the peak intensity are located. The angular distance between the half power points traveling through the peak is the beamwidth. Half the power is —3dB, so the half power beamwidth is sometimes referred to as the 3dB beamwidth.

Antenna Pattern Types

  1. a.     Omnidirectional_Antennas
    For mobile, portable, and some base station applications the type of antenna needed has an omnidirectional radiation pattern. The omnidirectional antenna radiates and receives equally well in all horizontal directions. The gain of an omnidirectional antenna can be increased by narrowing the beamwidth in the vertical or elevation plane. The net effect is to focus the antenna’s energy toward the horizon.

Selecting the right antenna gain for the application is the subject of much analysis and investigation. Gain is achieved at the expense of beamwidth: higher-gain antennas feature narrow beamwidths while the opposite is also true.

Omnidirectional antennas with different gains are used to improve reception and transmission in certain types of terrain. A 0 dBd gain antenna radiates more energy higher in the vertical plane to reach radio communication sites that are located in higher places. Therefore they are more useful in mountainous and metropolitan areas with tall buildings. A 3 dBd gain antenna is the compromise in suburban and general settings. A 5 dBd gain antenna radiates more energy toward the horizon compared to the 0 and 3 dBd antennas to reach radio communication sites that are further apart and less obstructed. Therefore they are best used in deserts, plains, flatlands, and open farm areas.

  1. b.     Directional_Antennas
    Directional antennas focus energy in a particular direction. Directional antennas are used in some base station applications where coverage over a sector by separate antennas is desired. Point to point links also benefit from directional antennas. Yagi and panel antennas are directional antennas.

 

Antenna_Polarization
Polarization is defined as the orientation of the electric field of an electromagnetic wave. Polarization is in general described by an ellipse. Two often used special cases of elliptical polarization are linear polarization and circular polarization. The initial polarization of a radio wave is determined by the antenna that launches the waves into space. The environment through which the radio wave passes on its way from the transmit antenna to the receive antenna may cause a change in polarization.

With linear polarization the electric field vector stays in the same plane. In circular polarization the electric field vector appears to be rotating with circular motion about the direction of propagation, making one full turn for each RF cycle. The rotation may be right-hand or left-hand.

Choice of polarization is one of the design choices available to the RF system designer. For example, low frequency (< 1 MHz) vertically polarized radio waves propagate much more successfully near the earth than horizontally polarized radio waves, because horizontally polarized waves will be cancelled out by reflections from the earth. Mobile radio systems waves generally are vertically polarized. TV broadcasting has adopted horizontal polarization as a standard. This choice was made to maximize signal-to-noise ratios. At frequencies above 1 GHz, there is little basis for a choice of horizontal or vertical polarization, although in specific applications, there may be some possible advantage in one or the other. Circular polarization has also been found to be of advantage in some microwave radar applications to minimize the “clutter” echoes received from raindrops, in relation to the echoes from larger targets such as aircraft. Circular polarization can also be used to reduce multipath. 

Antenna Selection Guideline

Thursday, November 12th, 2009

                         ANTENNA SELECTION GUIDELINE

 

 

1.0  Objective

 

The objective of this document is to frame a general guideline for selection of antenna at proposed CDMA BTS sites in various circles. Selection of correct antenna type is of utmost importance for the optimum performance of RF network.

 

2.0  Selection of Antenna Type

 

Two major categories of antennas have been finalized for deployment at proposed BTS sites viz. i) antennas with 65° horizontal beamwidth and ii) 90° horizontal beamwidth.

 

3.0  Deployment of 65° Antennas

 

The broad category of 65° antennas has been further classified in three subgroups mentioned below.

 

a)     65° antenna with 6° electrical downtilt

b)     65° antenna with 4° electrical downtilt

c)      65° antenna with 2° electrical downtilt

 

 

3.1 Deployment of 65°-6° Antenna

 

In case of this antenna, the vertical beamwidth of antenna is provided with an electrical downtilt of 6° below horizon. These antennas are proposed to be deployed in the following cases:

 

a)     Cities having 15 or more BTSs.

b)     Sectors oriented towards dense urban or urban morphology.

c)      Site to site distance is upto 1.5 km

 

3.2 Deployment of 65°-4° Antenna       

 

65°-4° antennas are proposed to be deployed in the following cases:

 

a)     Cities having more than 7 BTS sites.

b)     Sectors oriented towards the city area with urban, low urban clutter.

c)      Site to site distance is between 1.5 km and 2.0 km

 

Due to site-specific requirement like hilly terrain, lesser site-to-site distance, deviations from above guideline were made in the following cities.

 

 

Amritsar (7 Sites)                        :                       3 Sites

Jodhpur (7 Sites)                        :                       2 Sites

Mysore (7 Sites)                          :                       1 Site

Udaipur (6 Sites)             :                       4 Sites

 

3.3 Deployment of 65°-2° Antenna

 

For this type of antenna, vertical beam is provided with an electrical downtilt of 2° below horizon. These antennas are proposed to be deployed in the following cases:

 

a)     Cities/towns having more than 2 BTS sites and upto 15 BTS sites.

b)     Sectors oriented towards the city areas having urban, low-urban or suburban morphology.

c)      Site to site distance should be more than 2 km.

 

4.0  Deployment of 90° Antenna

 

This category has been subdivided into two groups.

 

a)     90° antenna with no downtilt

b)     90° antenna with 2° electrical downtilt

 

4.1  Deployment of 90°-0° Antenna

 

These antennas are proposed to be deployed in

 

a)     Every sector of single/two BTS city/towns

b)     Sectors directed towards rural areas and highways of medium sized cities having upto 8 sites.

 

4.2  Deployment of 90°-2° Antennas

 

These antennas are proposed to be deployed in following cases:

 

a)     Sectors of BTSs directed towards rural areas or highways of cities having more than 8 BTS sites.

 

In big cities, sites located at good clutter type but at one extreme corner of the city limits have been proposed to have a combination of 65°-4° and 90°-2° antennas. In these cases, 90°-2° antenna type was preferred to 90°-0° as sidelobes/backlobes of 90°-0° may result in coverage overlap with 65°-4° antennas deployed in other sectors.

 

 

5.0 City Specific Antenna Selection

 

For all city/towns, barring a few specific requirements, antennas have been selected on the basis of the following guidelines. Combining all technical requirements together, cities can be classified in the following groups:

 

a)     A-Category: Top cities having combination of 65°-6°, 65°-4° and 90°-2°. For big cities, since number of BTS sites are more, use of 65°-2° may result in coverage overlap leading to pilot pollution. Also, this combination will make material handling much easier without deteriorating the performance of RF network. Among all the big cities, the combination has not been followed in Calcutta due to Radio Network requirement.

 

b)     B-Category: Other major cities having combination of 65°-4°, 65°-2° and 90°-2°. This combination is used in case of medium sized cities where site-to-site distance is more than 1.5km. There are a few exceptions in this case also as mentioned earlier.

 

 

c)      C-Category: Other cities having combination of 65°-2° and 90°-0° or 65°-2° and 90°-2°. Cities/towns in this group are having only a few BTSs and main aim in these cases is to use the antenna type which can extend coverage span to the maximum extent.

 

d)     D-Category Towns having upto 2 BTS sites. In these cases, only 90°-0° antennas have been proposed to maximize the SDCC coverage boundary.

BroadCast Engineering

Wednesday, October 21st, 2009

 

Broadcast engineering is the field of electrical engineering, and now to some extent computer engineering and information technology, which deals with radio and television broadcasting. Audio engineering and RF engineering are also essential parts of broadcast engineering, being their own subsets of electrical engineering.

Broadcast engineering involves both the studio end and the transmitter end (the entire airchain), as well as remote broadcasts. Every station has a broadcast engineer, though one may now serve an entire station group in a city, or be a contract engineer who essentially freelances his services to several stations (often in small media markets) as needed.

 Duties

Modern duties of a broadcast engineer include maintaining broadcast automation systems for the studio and automatic transmission systems for the transmitter plant. There are also important duties regarding radio towers, which must be maintained with proper lighting and painting. Occasionally a station’s engineer must deal with complaints of RF interference, particularly after a station has made changes to its transmission facilities.

Titles

Broadcast engineers may have varying titles depending on their level of expertise and field specialty. Some widely used titles include:

  • Broadcast design engineer
  • Broadcast systems engineer
  • Broadcast IT engineer
  • Broadcast network engineer
  • Broadcast maintenance engineer
  • Video broadcast engineer
  • TV studio broadcast engineer
  • Outside broadcast engineer
  • Remote broadcast engineer

Qualifications

Broadcast engineers may need to possess some or all of the following degrees, depending on the broadcast technical environment. If one of the formal qualifications is not present, a related degree or equivalent professional experience is desirable.

  • Degree in electrical engineering
  • Degree in electronic engineering
  • Degree in telecommunications engineering
  • Degree in computer engineering
  • Degree in management information system
  • Degree in broadcast technology

Knowledge

Broadcast engineers are generally required to have knowledge in the following areas, from conventional video broadcast systems to modern Information Technology:

  • Conventional broadcast
    • Audio/Video instrumentation measurment
    • Baseband video – standard / high-definition
    • Broadcast studio acoustics
    • Television studios – broadcast video cameras and camera lenses
    • Production switchers (video mixer)
    • Audio mixer
  • Broadcast IT
    • Video compression – DV25, MPEG, DVB or ATSC (or ISDB)
    • Digital server playout technologies. – VDCP, Louth, Harris, control protocols
    • Broadcast automation
    • Disk storage – RAID / NAS / SAN technologies.
    • Archives – Tape archives or grid storage technologies.
    • Computer networking
    • Operating systems – Microsoft Windows / Mac OS / Linux / RTOS
    • Post production – video capture and non-linear editing systems (NLEs).
  • RF
    • RF satellite uplinking – High powered amplifiers (HPA)
    • RF communications satellite downlinking – Band detection, carrier detection and IRD tuning, etc.
    • RF transmitter maintenance – IOT UHF transmitters, Solid State VHF transmitters, antennas, transmission line, high power filters, digital modulators.
  • Health & safety
    • Occupational safety and health
    • Fire suppression systems like FM 200.
    • Basic structural engineering
    • RF hazard mitigation

Above mentioned requirements vary from station to station.

Skills

Broadcast engineers must also have skillset and methodology to problem solving and soft skills, that helps in making effective use of their knowledge base.

  • Self-motivated
  • Enthusiasm to learn about emerging technologies, hardware/software and applications.
  • Logical approach to problem solving and troubleshooting
  • Detail oriented.
  • Quick thinking
  • Calm under high-pressure situations
  • Good oral and written business communications, negotiation and time management skills
  • Leadership skills – Organizing and motivating a group of engineers
  • Drawing skills – To draw graphical Visio workflow diagrams or CAD schematic drawings
  • Training and mentoring skills – To train and mentor junior or fellow engineers or operational staff.

 

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RF…!!!

Wednesday, October 21st, 2009

RF Engineering, also known as Radio Frequency Engineering, is a subset of electrical engineering that deals with devices which are designed to operate in the Radio Frequency spectrum. These devices operate within the range of about 3 Hz up to 300 GHz.

RF Engineering is incorporated into almost everything that transmits or receives a radio wave which includes, but not limited to, Mobile Phones, Radios, WiFi and walkie talkies.

RF Engineering is a highly specialized field. To produce quality results, an in-depth knowledge of Mathematics, Physics and general electronics theory is required. Even with this, the initial design of an RF Circuit usually bears very little resemblance to the final physical circuit produced, as revisions to the design are often required to achieve intended results.

Duties

RF Engineers are specialists in their respective field and can take on many different roles, such as design, and maintenance. An RF Engineer at a broadcast facility is responsible for maintenance of the stations high power broadcast transmitters, and associated systems. This includes transmitter site emergency power, remote control, main transmission line and antenna adjustments, microwave radio relay STL/TSL links and more. Typically, transmission equipment is past its expected lifetime, and there is little support available from the manufacturer. Often, creative and collaborative solutions are required. The range of technologies used is vast due to the wide array of frequencies allocated for different radio services, and due to the range in age of equipment. In general, older equipment is easier to service.

 

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