Chapter 2 RF Fundamentals
What unit of measurement is used to quantify the power gain or loss of an RF signal? Choose all that apply. A. dBi B. dBm C. Watts D. dB
A, D. Power gain or loss is a change in power, not an absolute amount of power. For this reason, dB and dBi are correct because they are measurements of a change in power.
A year ago, while working for your current organization, you installed a wireless link between two buildings. Recently you have received reports that the throughput of the link has decreased. After investigating the connection problems, you discover there is a tree within the Fresnel Zone of the link that is causing 25% blockage of the connection. Which of the following statements are true? Choose all that apply. A. The tree cannot be the problem, because only 25% of the connection is blocked B. The tree might be the problem, because up to 40% of the Fresnel Zone can be blocked without causing problems C. If the tree is the problem, raising the heights of both antennas will fix the problem D. If the tree is the problem, increasing the power at the transmitters at each end of the link will fix the problem
B, C. Raising the antennas on each end will fix the problem, but what happens in another year when the tree has grown again? This is only a short-term fix. Turning up the power will not always solve the problem since the problem may be retransmissions due to packet errors instead of decreased signal amplitude. Trimming the tree as a test, and then cutting it down if it is the problem is the solution to this problem. The tree could be the problem because any encroachment into the Fresnel Zone can cause signal degradation. Typically, 20-40% blockage of the Fresnel Zone does not present a problem, but as previously stated any blockage of the Fresnel Zone should be avoided if at all possible. In this case, it's quite possible that only 25% blockage could be a serious problem as there may be other factors to consider that we are unaware of.
Given a wireless bridge with 200 mW of output power connected through a 100 foot cable with 6 dB of loss to an antenna with 9 dBi of gain, what is the EIRP at the antenna in dBm? A. 20 dBm B. 26 dBm C. 30 dBm D. 36 dBm
B. Converting 200 mW to dBm, we see that we start out with 23 dBm. From this point, it's a simple addition/subtraction problem. 23 dBm - 6 dB + 9 dBi = 26 dBm.
1Given an access point with 100 mW of output power connected through a 50-foot cable with 3 dB of loss to an antenna with 10 dBi of gain, what is the EIRP at the antenna in mW? A. 100 mW B. 250 mW C. 500 mW D. 1 W
C. 100 mW with a 3 dB loss results in 50 mW of output power remaining because a 3 dB loss cuts the power in half. A gain of 10 dBi will multiply the remaining power of 50 mW by a factor of 10 yielding 500 mW of output power at the antenna element (referred to as EIRP).
When RF LOS is present, visual LOS will always be present. A. This statement is true B. This statement is false C. It depends on the specific factors
C. Due to weather, smog, or distance, the person configuring the RF link may not be able to see the other end, yet because there is nothing in the Fresnel Zone interfering with the RF signal, the link works fine.
Which one of the following RF behaviors is defined as "the bending of a wave as it passes through a medium of different density"? A. Diffraction B. Reflection C. Refraction D. Distraction
C. Refraction of RF waves works similarly to visible light bending as it passes from air into water. Be careful not to confuse refraction and diffraction.
dBi is a relative measurement of decibels to which one of the following? A. Internet B. Intentional radiator C. Isotropic radiator D. Isotropic radio
C. There's no such thing as the perfect antenna, yet that's exactly what an isotropic radiator is. An isotropic radiator radiates power evenly in all directions (a spherical pattern). Omni-directional antennas have a doughnut-shaped coverage pattern that has gain relative to an isotropic radiator due to the squeezing of the RF field. All antennas are referenced against this imaginary antenna for quantifiable measurement.
What is the net gain or loss of a circuit if it is using two cables with 3 dB loss each, one amplifier with a 12 dB gain, 1 antenna with 9 dBi gain, and an attenuator with a loss of 5 dB? A. 5 dB B. 10 dB C. 15 dB D. 20 dB
B. Instead of the usual question that begins and ends with an absolute amount of power (measured in mW or dBm), this question is testing your understanding of the additive nature of power change. - 3 dB -3 dB +12 dB +9 dBi - 5dB yields a combined power change of +10 dB.
Given an access point with an output power of 20 dBm connected through a cable with a loss of 6 dB to an amplifier with a 10 dB gain, then through a cable with 3 dB of loss to an antenna with 6 dBi of gain, what is the EIRP in dBm? A. 18 dBm B. 23 dBm C. 25 dBm D. 27 dBm
D. This question is a simple addition/subtraction problem from start to finish. 20 dBm - 6 dB + 10 dB - 3 dB + 6 dBi = 27 dBm. It starts and ends with absolute amounts of power, but adds and subtracts changes in power along the way.
Using which of the following will reduce VSWR? Choose all that apply. A. Cables and connectors that all have an impedance of 50 Ohms B. Cables with a 50 Ohm impedance and connectors with 75 Ohm impedance C. Cables and connectors that all have an impedance of 75 Ohms D. Cables with 75 Ohm impedance and connectors with 50 Ohm impedance
A, C. It's important in keeping VSWR to a minimum that all devices in an RF system be impedance-matched. This means that all devices in the system must be 50 ohms, 75 ohms, or whatever impedance is being used, just so that all devices match.
In regards to gain and loss measurements in wireless LANs, the statement that gains and losses are additive is: A. Always true B. Always false C. Sometimes true D. Sometimes false E. It depends on the equipment manufacturer
A. Gains and losses in a wireless LAN system using like units (dB & dBi) are always additive. Sometimes this means adding in negative numbers (losses) and sometimes the numbers are positive (gain). As you've seen in some of the practice questions in this section, gain and loss can be calculated by adding all gains and losses in a system.
Which of the following is a cause of VSWR? A. Mismatched impedances between wireless LAN connectors B. Too much power being radiated from the antenna element C. The incorrect type of antenna used to transmit a signal D. Use of the incorrect RF frequency band
A. Having mismatched impedances between any two devices in a system can cause VSWR at that point of connection. This concept applies to cables, connectors, the output circuit of the transmitter, the input circuit of the receiver, the antenna, and any other device in the system.
Radio waves propagate (move) away from the source (the antenna) in what manner? A. In a straight line in all directions at once within the vertical and horizontal beamwidths B. In circles spiraling away from the antenna C. In spherical, concentric circles within the horizontal beamwidth D. Up and down across the area of coverage
A. The angles of the horizontal and vertical beamwidth determine exactly what direction the RF waves will propagate from the antenna. Waves propagate within these angles in all directions simultaneously.
When visual line of sight (LOS) is present, RF LOS will always be present. A. This statement is true B. This statement is false C. It depends on the configuration of the antennas
B. In order to have RF LOS, the Fresnel Zone must be clear. Having a clear visual line of sight between two points does not necessarily mean that your Fresnel Zone is clear. The radius of the Fresnel Zone can be calculated with a simple formula.
In an RF circuit, what is the intentional radiator defined as? A. The output of the transmitting device B. The output of the last connector before the signal enters the antenna C. The output as measured after the antenna D. The output after the first length of cable attached to the transmitting device
B. The FCC defines the Intentional Radiator as the input power to the antenna. This definition means that the power output of the transmitter, plus all cabling, connectors, splitters, amplifiers, and attenuators before the antenna is included as part of Intentional Radiator.
Which one of the following is considered impedance in an RF circuit? A. The inability to transmit RF signals B. The pressure that causes current flow C. Resistance to current flow, measured in Ohms D. The frequency on which an RF transmitter sends signals
C. Impedance (measured in Ohms) is a resistance to current flow in any electrical circuit. RF signals are high-frequency alternating current (AC) and experience loss due to resistance while in a copper conductor such as cabling.
Given an access point with an output power of 100 mW connected through a cable with a loss of 2 dB to antenna with a gain of 11 dBi, what is the EIRP in mW? A. 200 mW B. 400 mW C. 800 mW D. 1 W
C. Starting with 100 mW of output power, and not being able to use the 10's and 3's, we have a more complex math problem. A trick here is to first realize that gains and losses are additive so a 2 dB loss and an 11 dBi gain equals 9 dB of change. Luckily, 9 dB is divisible by 3, so we have 100 x 2 x 2 x 2 = 800 mW of output power at the antenna (EIRP).
Why is the Fresnel Zone important to the integrity of the RF link? A. The Fresnel Zone defines the area of coverage in a typical RF coverage cell B. The Fresnel Zone must always be 100% clear of any and all blockage for a wireless LAN to operate properly C. The Fresnel Zone defines an area around the LOS that can introduce RF signal interference if blocked D. The Fresnel Zone does not change with the length of the RF link
C. The Fresnel Zone is an area around the direct path between the transmitter and the receiver that must be mostly clear (60-80%) in order to avoid RF signal interference. Trees, buildings, and many other objects tend to get into the Fresnel Zone of long-distance RF links. It's important to try to keep the Fresnel Zone as clear as possible. There is a simple formula for calculating the radius of the Fresnel Zone.
The FCC allows how many watts of power to be radiated from an antenna in a point-to-multipoint wireless LAN connection using unlicensed 2.4 GHz spread spectrum equipment? A. 1 watt B. 2 watts C. 3 watts D. 4 watts
D. The FCC defines EIRP as the output from the antenna. EIRP in a point-to-multipoint configuration (which includes any configuration using an omni-directional antenna) is limited to 4 watts by the FCC.
In RF mathematics, 1 watt equals what measurement of dBm? A. 1 B. 3 C. 10 D. 30 E. 100
D. The mathematical reference point most commonly used with wireless LANs is 0 dBm = 1 mW. Using the simple calculation of adding 30 dB, we can quickly see that 1 watt = 30 dBm.