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Gain a better understanding of RF power units, including watts and milliwatts, and learn how to measure power using decibels (dB) and dBm. Explore the concepts of directional gain and signal-to-noise ratio (SNR) in RF technology.
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Chapter 02Radio Frequency& Antenna Fundamentals(part 2) Center for Information Technology
Units of Power Watt (W) is a basic unit of power. One watt is equal to one ampere of current flowing at one volt. Milliwatt (mW) 1W = 1000 mW The reason to be concerned with milliwatts is because most of the 802.11 equipment that transmits at power levels between 1 and 100 mW. For indoor use, it is recommended that the output power is less than 100 mW. For outdoor WLANs may use more power if they are providing site-to-site links or are providing coverage to a large outdoor area. The FCC limits the total output power from the antenna to 4 W for point-to-multipoint applications in the 2.4 GHz ISM band.
Units of Power Decibel (dB) The decibel is a comparative measurement value. It is a measurement of the difference between two power levels. 1 bel is a ratio of 10:1 between two power levels. Therefore, a power ratio of 200:20 is 1 bel (10:1) and 200:40 is .5 bels (5:1) and 200:10 is 2 bels (20:1). bels = log(P1/P2) decibels = 10×log(P1/P2) = 10xlog(Pout/Pin) The decibel is relative where the milliwatt is absolute. The decibel is logarithmic where the milliwatt is linear.
dB The differences between values can become extremely large or small and more difficult to deal with. It is easier to say that a 100 mW signal decreased by 70 decibels than to say that it decreased to .00001 milliwatts. 10log(100/0.00001) = 70dB Comparison of Milliwatts and Decibel Change (relative to 1 mW)
10’s and 3’s Rules of RF Math 1. A gain of 3 dB magnifies the output power by two. 2. A loss of 3 dB equals one half of the output power. 3. A gain of 10 dB magnifies the output power by 10. 4. A loss of 10 dB equals one-tenth of the output power. 5. dB gains and losses are cumulative.
Quiz 1 • An RF signal of 2 Watts is applied to a 100-foot antenna cable, however, only 1 Watt of transmit power is actually developed at the input of the transmitting antenna. What is the resulting cable loss, measured in dB? • A. 0.5 dB • B. 1 dB • C. 3 dB • D. 5 dB Ans = C
Quiz 2 • A loss of -10dB yields a power ratio of? • A. 1:3 • B. 1:10 • C. 2:1 • D. 10:1 Ans = B -10dB = 10log(P1/P2) log(P1/P2) = -1 P1/P2 = 10-1 = 1:10
Quiz 3 • A gain of +3dB yields a power ratio of? • A. 2:1 • B. 3:1 • C. 10:1 • D. 1:10 Ans = A +3dB = 10log(P1/P2) log(P1/P2) = 3/10 P1/P2 = 103/10 = 1.995262 2:1
Quiz 4 You have an access point that is transmitting at 50 mW. The signal loss between the access point and the antenna is –1 dB, and the access point is using a 5 dBi antenna. Calculate mW output by antenna. A. 100 mW B. 125 mW C. 150 mW D. 200 mW Ans = B dB mW 50 +10 500 -3 250 -3 125
dBm The dBm represents an absolute measurement of power where the m stands for milliwatts. dBm references decibels relative to 1 milliwatt such that 0 dBm equals 1 milliwatt. The formula to get dBm from milliwatts is dBm = 10xlog(PowermW) The benefits of working with dBm values instead of milliwatts is the ability to easily add and subtract simple decibels instead of multiplying and dividing often huge and tiny numbers.
dBm mW 0 1 10 10 20 100 30 1000 dBm = 10xlog(PowermW) Quiz In terms of RF power, 1 Watt = ______ dBm. A. 3 B. 10 C. 20 D. 30 Ans = D
dBi The dBi (the i stands for isotropic) represents a measurement of power gain used for RF antennas. It is a comparison of the gain of the antenna and the output of a theoretical isotropic radiator. An isotropic radiator is an ideal antenna that we cannot create with any known technology. This is an antenna that radiates power equally in all directions. The dBi value must be calculated against the input power provided to the antenna to determine the actual output power in the direction in which the antenna propagates RF signals.
dBd dBi is a calculation of directional gain compared to an isotropic radiator, dBd is a calculation of directional gain compared to a dipole antenna. dBd is a value calculated against the input power to determine the directional output power of the antenna. The difference is that a dBd value is compared with a dipole antenna, which itself has a gain of 2.14 over an isotropic radiator. An antenna with a gain of 7 dBd has a gain of 9.14 dBi. To convert from dBd to dBi, just add 2.14. To convert from dBi to dBd, just subtract 2.14. 0 dBd = 2.14 dBi.
SNR Background RF noise, which can be caused by all the various systems and natural phenomena that generate energy in the electromagnetic spectrum, is known as the noise floor. The power level of the RF signal relative to the power level of the noise floor is known as the signal-to-noise ratio or SNR. The higher the ratio, the less obtrusive the background noise is.
RSSI The received signal strength indicator is an arbitrary measurement of received signal strength defined in the IEEE 802.11 standards. Cisco uses a range of 0–100 in their devices, and most Atheros-based chipsets use a range of 0–60. The RSSI rating is also arbitrarily used to determine when to reassociate and when to transmit. In other words, vendors will decide what the lowest RSSI rating should be before attempting to reassociate to a basic service set with a stronger beacon signal
Link Budget Link budget is an accounting of all components of power, gain, loss, receiver sensitivity, and fade margin. This includes the cables and connectors leading up the antenna, as well as the antennas themselves. It also includes free space path loss. A link budget is used to predict performance before the link is established. - Show in advance if it will be acceptable - Show if one option is better than another - Provide a criterion to evaluate actual performance
Receive Sensitivity The minimum signal strength needed at the receiver. The receive sensitivity is not a single dBm rating; it is a series of dBm ratings required to communicate at varying data rates. Ex: The lowest number in dBm, which is −94dBm is the weakest signal the radio can tolerate.
System Operating Margin The SOM is the amount of received signal strength relative to the client device’s receive sensitivity. The SOM = link budget, is the calculation of the amount of RF signal that is received minus the amount of signal required by the receiver. Ex: If we have a client device with a receive sensitivity of −94dBm and the card is picking up the wireless signal at −65dBm, the SOM is the difference between −94 and −65. Therefore, the link budget is: SOM = RS − S where S is the signal strength at the wireless client device and RS is the receive sensitivity of the client device. The resulting SOM is 29dBm. This means that the signal strength can be weakened by 29dBm and the link can be maintained.
Link budget calculation The receive sensitivity of both bridges is −94dBm. The calculations are as follows: Link budget calculation 1: 100mW = 20dBm Link budget calculation 2: 20dBm−3dB+7dBi−83dB = −59dBm Link budget calculation 3: (−94dBm) − (−59dBm) = 35 dBm SOM = 35 dBm
Fade Margin By including a few extra dB of strength in the required link budget, we can provide a link that will endure longer. This extra signal strength is fade margin. We do not add to the link budget/SOM dBm value, but instead we take away from the receive sensitivity. For example, we may decide to work off of an absolute receive sensitivity of −80dBm instead of the −94dBm supported by the Cisco Aironet card mentioned earlier. This would provide a fade margin of 14dBm. SOM in this case is 21 dBm.
Intentional Radiator The intentional radiator is the point at which the antenna is connected. The signal originates at a transmitter and may pass through connectors, amplifiers, attenuators, and cables before reaching the antenna. These components amplify or attenuate the signal, resulting in the output power at the intentional radiator before entering the antenna. The FCC sets the rules regarding the power that can be delivered to the antenna and radiated by the antenna. The FCC allows 1 watt of output power from the intentional radiator and 4 watts of antenna output power in a point-to-multipoint link in the 2.4 GHz ISM band.
EIRP The equivalent isotropically radiated power (EIRP) is the output power from the intentional radiator plus the directional gain provided by the antenna. Power radiated out of antenna of a wireless system
Example 1 We have a wireless bridge that generates a 100 mW signal. The bridge is connected to an antenna using cable that creates 3 dB of signal loss. The antenna provides 10 dBi of signal gain. In this example, calculate the IR and EIRP values.
Solution 10xlog100mW = 20 dBm IR = 20 dBm - 3 dB = 17 dBm EIRP = 17 dBm + 10 dB = 27 dBm
dBm = 10xlog(PowermW) dBm mW 0 1 10 10 20 100 20dBm – 3dB + 9dBi = 26 dBm Quiz 1 • An access point is emitting a 100 mW signal that is connected to a length of cable with a 3dB loss. If the cable is then connected to a +9dBi antenna, what is the EIRP from the antenna in dBm? • A. 20 dBm • B. 23 dBm • C. 26 dBm • D. 29 dBm Ans = C
Quiz 2 • An access point that emits a 1000 mW signal is connected to a cable and its connectors with 10dB loss. The cable is then connected to a 3dB gain antenna. What is the resulting in mW from the antenna? • A. 100 mW • B. 200 mW • C. 300 mW • D. 500 mW Ans = B dB mW 1000 -10 100 +3 200
Visual LOS - RF LOS If we can physically see something, it is said to be in our visual line of sight. This LOS is actually the transmission path of the light waves from the object we are viewing (transmitter) to our eyes (receiver). RF LOS is more sensitive than visible LOS to interference near the path between the transmitter and the receiver. More space is needed for the RF waves to be seen by each end of the connection. This extra space is called the Fresnel zone.
The Fresnel Zone The Fresnel zones are a theoretically infinite number of ellipsoidal areas around the LOS in an RF link.
Beamwidths Beamwidth is the measurement of how broad or narrow the focus of the RF energy is as it propagates from the antenna along the main lobe. The main lobe is the primary RF energy coming from the antenna. Beamwidth is measured both vertically and horizontally The beamwidth is a measurement taken from the center of the RF signal to the points on the vertical and horizontal axes where the signal decreases by 3 dB or half power.
Beamwidths Beamwidth measurements give the propagation pattern of an antenna, they are less than perfect in illustrating the actual areas that are covered by the antenna. For more useful visual representations: reference Azimuth and Elevation charts.
Azimuth and Elevation Azimuth and Elevation charts provide a visualization of the antenna’s propagation patterns. The Azimuth chart shows a top-down view of the propagation path The Elevation chart shows a side view of the propagation path
Isotropic Radiator The isotropic radiator is a fictional device or concept that cannot be developed using today’s technology. We cannot currently create an antenna that propagates RF energy equally in all directions. dBi is a measurement of the gain of an antenna in a particular direction over the power level that would exist in that direction if the RF energy were propagated by an isotropic radiator. In other words, dBi is a measurement of the difference between the power levels at a point in space generated by a real antenna versus the theoretical isotropic radiator. The Sun is often used as an analogy of an isotropic radiator.
Polarization Antenna polarization refers to the physical orientation of the antenna in a horizontal or vertical position. The electric field forms what is known as the E-plane. The magnetic field forms what is known as the H-plane. The E-plane determines the polarization of the antenna, since it is parallel to the antenna. Therefore, if the antenna is in a vertical position, it is said to be vertically polarized. If the antenna is in a horizontal position, it is said to be horizontally polarized. Vertical polarization means that most of the signal is being propagated horizontally, and horizontal polarization means that most of the signal is being propagated vertically.
Antenna Diversity Antenna diversity is a feature that allows the device to receive signals using two antennas and one receiver. The reason to use antenna diversity is that a device transmit a signal and it may arrive at a receiving device from multiple angles with multiple signal strengths. The device supporting antenna diversity will look at the signal that comes into each antenna during the frame preamble of a single frame and choose the signal that is best on a frame-by-frame basis. The best frame preamble will determine which antenna is used to receive the rest of the current frame.
Omnidirectional/Dipole Antennas Omnidirectional antennas, the most popular type being the dipole antenna, are antennas with a 360-degree horizontal propagation pattern.
Omnidirectional Antenna Usage Omnidirectional antennas provide coverage on a horizontal plane with some coverage vertically and outward from the antenna. This means they may provide some coverage to floors above and below. To reach people farther away horizontally: use higher gain To reach people farther up or down vertically: use lower gain
Semidirectional Antennas Semidirectional antennas are antennas that focus most of their energy in a particular direction. Patch, Panel, and Yagi are semidirectional antennas. Patch and panel antennas usually focus their energy in a horizontal arc of 180 degrees or less, whereas Yagi antennas usually have a coverage pattern of 90 degrees or less.
Highly Directional Antennas Highly directional antennas are antennas that transmit with a very narrow beam. These types of antennas often look like the satellite dish. They are generally called parabolic dish or grid antennas. They are mostly used for PtP or PtMP links.
Sectorized and Phased-Array Antennas A sectorized antenna is a high-gain antenna that works back-to-back with other sectorized antennas. A phased-array antenna is a special antenna system that is actually composed of multiple antennas connected to a single processor. The antennas are used to transmit different phases that result in a directed beam of RF energy aimed at client devices.
MIMO Antenna Systems Multiple-Input Multiple-Output (MIMO) can be described as any RF communications system that has multiple antennas at both ends of the communications link being used concurrently. The proposed 802.11n standard will include MIMO technology.
