Wavelength can be calculated as
c = n λ = f λ
So, Wavelength, λ = V/ f
Where,
λ = Carrier wavelength
f = Carrier frequency
c = Velocity of light ( 3 × 108 m/s)
In case of wavelength, when wavelength decreases then the speed of electromagnetic wave propagation decreases and loss increases. Wavelength inversely proportional to rainy season .
The signal-to-noise ratio which is the ratio of the power in a signal to the power contained in the noise that is present at a particular point in the transmission. Typically, this ratio is measured at a receiver, because it is at this point that an attempt is made to process the signal and recover the data. For convenience, this ratio is often reported in decibels:
SNR dB= 10 log 10signal powernoise power
This expression the amount, in decibels, that the intended signal exceeds the noise level. A high SNR will mean a high-quality signal and low number of required intermediate repeaters .
The signal-to-noise ratio is important in the transmission of digital data because it sets the upper bound on the achievable data rate. Shannon’s result is that the maximum channel capacity, in bits per second, obeys the equation
C= B log2(1+SNR)
Where C is the capacity of the channel in bits per second and B is the bandwidth of the channel in hertz .
Bit Error Rate. The rate at which errors in transmission occur, normally related closely to the Signal to Noise Ratio (SNR). BER of 10-9, or one bit error for every billion bits, is a typical minimum system requirement .
Labels: Microwave antennaA waveguide is a hollow metal tube, either a cylinder or a rectangle in cross section that acts as a pipe for transmission of electromagnetic waves of the microwave range over short distance (1 to 10 cms) . A typical waveguide of rectangular cross section. Since the waveguide is of solid metal, any electromagnetic wave put into its cavity can not escape and travels through the guide to the other end. As a result, waveguide offers a very large bandwidth (2 GHz to 110 GHz) .
The RF power for acceleration of protons inside the accelerating structure, supplied from high-power klystrons, is taken up to the desired ports by means of wave-guide line. The factors of primary importance for a wave-guide system are: power handling capacity, insertion loss, impedance uniformity, band width, physical dimensions/tolerances, economic considerations and self strength. The wave-guides are made from aluminium alloy 6061 plates which are heliarc welded at four corners .
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Microwave antenna
The RF power for acceleration of protons inside the accelerating structure, supplied from high-power klystrons, is taken up to the desired ports by means of wave-guide line. The factors of primary importance for a wave-guide system are: power handling capacity, insertion loss, impedance uniformity, band width, physical dimensions/tolerances, economic considerations and self strength. The wave-guides are made from aluminium alloy 6061 plates which are heliarc welded at four corners .
There will always be some loss of signal strength through the cables and connectors used to connect to the antenna.
This loss is directly proportional to the length of the cable and generally inversely proportional to the diameter of the cable .Additional loss occurs for each connector used and must be considered in planning.Cable vendor can provide a chart indicating the loss for various types and lengths of cable. Table A-1 on page A-4 is an example of this kind of chart.
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Microwave antenna
Consider a dish antenna pointed at an isotropic antenna transmitting some distance away. We know that the isotropic antenna radiates uniformly in all directions, so it is a simple matter of spherical geometry to calculate how much of that power should be arriving at the dish over its whole aperture . Now to measure how much power is being received from the dish (at the electrical connection to the feed) – never greater than is arriving at the aperture. The ratio of power received to power arriving is the aperture efficiency.
The total antenna efficiency accounts for the following losses:
(1) Reflection because of mismatch between the feeding transmission line and the antenna and
(2) Antenna conductor and dielectric losses.
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Microwave antenna
The total antenna efficiency accounts for the following losses:
(1) Reflection because of mismatch between the feeding transmission line and the antenna and
(2) Antenna conductor and dielectric losses.
Antenna gain is essential for microwave communication---since it helps both transmitting and receiving, it is doubly valuable.
Antenna gain is an indicator of how well an antenna focuses RF energy in a preferred direction. Antenna gain is expressed in dBi (the ratio of the power radiated by the antenna in a specific direction to the power radiated in that direction by an isotropic antenna fed by the same transmitter). Antenna manufacturers normally specify the antenna gain for each antenna they manufacture .
The relationship between antenna gain and effective area is
G = 4πAe / λ 2 = 4π f 2Ae / c2
Where
G = Antenna Gain
Ae = Effective area
λ = Carrier wavelength
f = Carrier frequency
c = Speed of light ( 3 × 108 m/s)
The hypothetical isotropic antenna is a point source that radiates equally in all directions. Any real antenna will radiate more energy in some directions than in others. Since it cannot create energy, the total power radiated is the same as an isotropic antenna driven from the same transmitter: in some direction it radiates more energy than an isotropic antenna, so in others it must radiate less energy. The gain of an antenna in a given direction is the amount of energy radiated in that direction compared to the energy an isotropic antenna would radiate in the same direction when driven with the same input power. Usually we are only interested in the maximum gain-----the direction in which the antenna is radiating most of the power .
An antenna with a large aperture has more gain than smaller one; just as the captures more energy from a passing radio wave, it also radiates more energy in that direction. Gain may be calculated as
GdBi = 10 log10( η 4π/ λ 2 A)
With reference to an isotropic radiator; η is the efficiency of the antenna.
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Microwave antenna
Antenna gain is an indicator of how well an antenna focuses RF energy in a preferred direction. Antenna gain is expressed in dBi (the ratio of the power radiated by the antenna in a specific direction to the power radiated in that direction by an isotropic antenna fed by the same transmitter). Antenna manufacturers normally specify the antenna gain for each antenna they manufacture .
The relationship between antenna gain and effective area is
G = 4πAe / λ 2 = 4π f 2Ae / c2
Where
G = Antenna Gain
Ae = Effective area
λ = Carrier wavelength
f = Carrier frequency
c = Speed of light ( 3 × 108 m/s)
The hypothetical isotropic antenna is a point source that radiates equally in all directions. Any real antenna will radiate more energy in some directions than in others. Since it cannot create energy, the total power radiated is the same as an isotropic antenna driven from the same transmitter: in some direction it radiates more energy than an isotropic antenna, so in others it must radiate less energy. The gain of an antenna in a given direction is the amount of energy radiated in that direction compared to the energy an isotropic antenna would radiate in the same direction when driven with the same input power. Usually we are only interested in the maximum gain-----the direction in which the antenna is radiating most of the power .
An antenna with a large aperture has more gain than smaller one; just as the captures more energy from a passing radio wave, it also radiates more energy in that direction. Gain may be calculated as
GdBi = 10 log10( η 4π/ λ 2 A)
With reference to an isotropic radiator; η is the efficiency of the antenna.
A signal degrades as it moves through space. The longer the path, the more loss it experiences. This free-space path loss is a factor in calculating the link viability . Free-space path loss is easily calculated for miles or kilometers using one of the following formulas:
Lp = (96.6 + 20 log10 F) + (20 log10 D)
where
Lp = free-space path loss between antennas (in dB)
F = frequency in GHz
D = path length in miles
or
Lp = (92.4 + 20 log10 F) + (20 log10 D)
where
Lp = free-space path loss between antennas (in dB)
F = frequency in GHz
D = path length in kilometers
The sensitivity of a receiver is its ability to receive quick signal. This sensitivity may be defined in several ways.
First, it may be started in terms of the signal field strength of a signal that will produce a desired demodulated output level under a certain modulation level. The sensitivity is usually started in terms of the voltage developed by the antenna across the receiver antenna terminals in microvolts. This level ranges from a few microvolts to a few hundred microvolts for typical receiver .
Another way of stating the sensitivity is to state the antenna terminal signal voltage required to produce a specified signal- to- noise ratio. In the case of receiver for digital signals, the sensitivity is usually stated as the input signal level required to produce a desired bit- error rate which is related to signal- to- noise ratio.
Receiver sensitivity is typically specified in units of microvolts for a 12 dB SINAD (signal to noise and distortion). The amount of RF power required by the receiver to faithfully represent the actual signal transmitted is arbitrarily set at the 12 dB SINAD, which translates into -114 dBm for a typical receiver.
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Microwave antenna
The major issues with microwave link engineering are rain fade, multi path and interference. An RF engineer begins a design by doing a link budget analysis. A given radio system has a system gain that depends on the design on the radio and the modulation used. The gains from the antenna at each end are added to this gain. Larger antennas provide higher gain. The free space loss of the radio signal as it travels over the air is then subtracted from the system: the longer the link the higher the loss. These calculations result in a “fade margin” for the link. Anything that affects the radio signal within this margin will be overcome by the radio; if the margin is exceeded, then the link could go down. The next step, then, is to analyze impediments that could potentially affect the radio signal . With good understanding of the potential affects on the signal, the RF engineer can design links with availability and performance equal to or better than a wire line link. A link budget is a rough calculation of all known elements of the link to determine if the signal will have the proper strength when it reaches the other end of the link. To make this calculation, the following information should be available:
- Frequency of the link
- Free space path loss
- Power of the transmitter
- Antenna gain
- Total length of transmission cable and loss per unit length at the specified frequency
- Number of connectors used
- Loss of each connector at the specified frequency
- Path length
The amount of extra RF power radiated to overcome this phenomenon is referred to as fade margin. The exact amount of fade margin required depends on the desired reliability of the link, but a good rule-of-thumb is 20dB to 30dB.
Fade Margin = SG + AG - LC – LP
Where
SG = System gain (depend on modem)
AG = Antenna gain
LC = Cable loss
LP = Path loss
These parameters are also responsible for gain loss.
System Gain = Radiated Power of radio Equipment - (Receiver Sensitivity)
When planning for paths longer than seven miles, the curvature of the earth might become a factor in path planning and require that the antenna be located higher off the ground . The additional antenna height needed can be calculated using the following formula:
H = D2/8
Where,
H = Height of earth bulge (in feet)
D = Distance between antennas (in miles)
The minimum antenna height at each end of the link for paths longer than seven miles (for smooth terrain without obstructions) is the height of the First Fresnel Zone plus the additional height required to clear the earth bulge . The formula would be:
H = 43.3 √ (D/4F ) + D2/8
Where,
H = Height of the antenna (in feet)
D = Distance between antennas (in miles)
F = Frequency in GHz
The characteristics of a radio signal cause it to occupy a broad cross-section of space, called the Fresnel Zone, between the antennas. From figure shows the area occupied by the strongest radio signal, called the First Fresnel Zone, which surrounds the direct line between the antennas
Because of the shape of the First Fresnel Zone, what appears to be a clear line-of-sight path may not be. As long as 80 percent of the First Fresnel Zone is clear of obstructions, the link behaves essentially the same as a clear free-space path .
H = Height of the First Fresnel Zone (in feet)
D = Distance between the antennas (in miles)
F = Frequency in GHz
In the first Fresnel zone and all the odd numbered Fresnel zones, deflected signals are generally in phase with or the center lobe signal. In the second Fresnel zone, and all even-numbered Fresnel zones, deflected signals are up to 180 out of phase with the center lobe signal . Signals deflected from the second Fresnel zone can cause Inter Symbol Interference (ISI) which can result in great losses of the center lobe signal. To avoid this problem, must place the antenna at a height that is out of range from F2 deflections. (An antenna can be set too high as well as too low.) Where deflection and diffraction from ground-based objects cause interference, even a small relocation of the antenna often produces a substantial improvement.
The minimum antenna height at each end of the link for paths longer than seven miles (for smooth terrain without obstructions) is the height of the First Fresnel Zone plus the additional height required to clear the earth bulge . The formula would be:
H = 43.3 √ (D/4F ) + D2/8
Where,
H = Height of the antenna (in feet)
D = Distance between antennas (in miles)
F = Frequency in GHz
When planning antenna placement, it might be necessary to build a free-standing tower for the antenna. Regulations and limitations define the height and location of these towers with respect to airports, runways, and airplane approach paths. These regulations are controlled by the FAA. In some circumstances, the tower installations must be approved by the FAA, registered with the FCC, or both. To ensure compliance, review the current FCC regulations regarding antenna structure .
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Microwave antenna
In general, the best location for antenna placement is always the closest to the transmitter. The type of antenna selected also has an effect on placement as discussed earlier. Since the transmitted signal is typically vertically polarized, it is important to orient the receiver antennas in a vertical position. The best place is usually on an adjacent wall, near the vicinity of the transmitter, using the frequency length formula to determine the proper distance between the antennas. When a number of multiple systems are used in a given location it is a common practice to use antenna combiners to eliminate the antenna Farm. Antenna amplifier/combiners are a convenience item that allows a single pair of antennas to feed multiple receivers
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Microwave antenna
The orientation of the antenna will change the orientation of the signal. The transmitting and receiving antennas should be both polarized either horizontally or vertically. Adjacent antennas on different frequencies can be cross-polarized to help reduce interference between the two, if operating license permits this.
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Microwave antenna
Dipole antenna
Other antenna types include the “dipole”, where a section of wire, one-half the wavelength, is positioned either horizontally or vertically in the air to transmit signals. Dipoles emit their signals in more of a two dimensional semi-circular or “doughnut” pattern, the key being both the transmitter and receiver’s antennas must be aligned the same (horizontally or vertically). Dipoles do not require a ground-plane are considered “bi-directional,” in that their signals travel in two opposite directions, depending on how the antenna is oriented .
Yagi antenna
The more focused (uni-directional) type of antenna is called a “Yagi.” A Yagi antenna is basically a standard one-half wavelength antenna, but with additional “elements” placed in front of it to focus the energy for transmission in one direction. The “reflector” and “director” elements are just similar-sized resonators spaced appropriately to increase the strength and narrow the direction of the signal prior to transmission. Again, the key to successfully using Yagi antennas is the correct orientation and alignment of the transmitting/receiving antennas.
Sectoral antenna
The requirement to serve a number of small areas from a single base station has resulted in the development of the multiple beam technique, for which the sectoral antenna is ideal .
Parabolic Reflector antenna
This antenna consists of a parabolic metal surface (dish) with a feed antenna in front. The feed antenna consists of a directive antenna such as a dipole and reflector, log-periodic dipole array or horn antenna. This antenna is capable of producing extremely high gains, usually in the 20 - 30 dBi range
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Microwave antenna
Other antenna types include the “dipole”, where a section of wire, one-half the wavelength, is positioned either horizontally or vertically in the air to transmit signals. Dipoles emit their signals in more of a two dimensional semi-circular or “doughnut” pattern, the key being both the transmitter and receiver’s antennas must be aligned the same (horizontally or vertically). Dipoles do not require a ground-plane are considered “bi-directional,” in that their signals travel in two opposite directions, depending on how the antenna is oriented .
Yagi antenna
The more focused (uni-directional) type of antenna is called a “Yagi.” A Yagi antenna is basically a standard one-half wavelength antenna, but with additional “elements” placed in front of it to focus the energy for transmission in one direction. The “reflector” and “director” elements are just similar-sized resonators spaced appropriately to increase the strength and narrow the direction of the signal prior to transmission. Again, the key to successfully using Yagi antennas is the correct orientation and alignment of the transmitting/receiving antennas.
Sectoral antenna
The requirement to serve a number of small areas from a single base station has resulted in the development of the multiple beam technique, for which the sectoral antenna is ideal .
Parabolic Reflector antenna
This antenna consists of a parabolic metal surface (dish) with a feed antenna in front. The feed antenna consists of a directive antenna such as a dipole and reflector, log-periodic dipole array or horn antenna. This antenna is capable of producing extremely high gains, usually in the 20 - 30 dBi range
Antennas are mainly two types---
- Omni directional antenna
- Directional antenna
Omni directional Antenna
The omni directional antenna radiates or receives equally well in all directions. It is also called the "non-directional" antenna because it does not favor any particular direction.
Omni antennas usually resemble vertical rods but can come in other shapes as well. Some have horizontal rods of the same length placed at their base to increase their performance/distance. These are called “ground planes”.
The key factor to note is that for receivers all four signals (or signals from any direction, for that matter) are received equally well. For transmitters, the radiated signal has the same strength in all directions. This pattern is useful for broadcasting a signal to all points of the compass (as when calling "CQ"), or when listening for signals from all points.
The omni directional antenna radiates or receives equally well in all directions. It is also called the "non-directional" antenna because it does not favor any particular direction.
Omni antennas usually resemble vertical rods but can come in other shapes as well. Some have horizontal rods of the same length placed at their base to increase their performance/distance. These are called “ground planes”.
The key factor to note is that for receivers all four signals (or signals from any direction, for that matter) are received equally well. For transmitters, the radiated signal has the same strength in all directions. This pattern is useful for broadcasting a signal to all points of the compass (as when calling "CQ"), or when listening for signals from all points.
Directional Antennas
Gain and directivity are intimately related in antennas. The directivity of an antenna is a statement of how the RF energy is focused in one or two directions. Because the amount of RF energy remains the same, but is distributed over less area, the apparent signal strength is higher. This apparent increase in signal strength is the antenna gain. The gain is measured in decibels over either a dipole (dBd) or a theoretical construct called an isotropic radiator (dBi) . The isotropic radiator is a spherical signal source that radiates equally well in all directions. One way to view the omni directional pattern is that it is a slice taken horizontally through the three dimensional sphere
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Microwave antenna
"That part of a transmitting or receiving system which is designed to radiate or to receive electromagnetic waves". An antenna can also be viewed as a transitional structure (transducer) between free-space and a transmission line (such as a coaxial line). An important property of an antenna is the ability to focus and free shape the radiated power in space e.g.: it enhances the power in some wanted directions and suppresses the power in other directions .
Antennas focus the radio signal in a specific direction and in a narrow beam. The increase in the signal power (compared to an omni directional antenna) when it is focused in the desired direction is called gain.
Antennas are tuned to operate on a specific group of frequencies. Other specific attributes such as beam-width and gain are also fixed by the manufacturer. Antennas should be selected and placed according to the site and the application .
In general, the larger the antenna, the higher the gain and the larger the mast required. It is best to use the smallest antenna that will provide sufficient protection from interference and enough signal at the far end of the link to provide good reception even with fading.
Other considerations include antenna beam-width, front-to-side ratios, front-to-back ratios, and cross-polarization rejection. Where interference from other licensees on the same channel or adjacent channels is an issue, narrow beam-widths, high front-to-back and front-to-side ratios, and high cross-polarization rejection are likely to be required. Even when other licensees are not an issue, if using a network deployment using the “cell” approach, all these considerations is still important to reduce interference between own adjacent installations .
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