RF Behavior Calculations
In this Lab, students use an online calculator to compute power, cable loss, antenna gain, free space path loss, link budget, and Fresnel zone clearance.
Go to www.swisswireless.org/wlan_calc_en.html and locate the online calculator. If the web page has been moved, try searching for "swiss wireless", without the double quotation marks,on the Internet to locate the main page.
Task 1: Power calculations
The reference point that relates the logarithmic decibel (dB) scale to the linear milliwatt scale is known as the dBm. This reference point specifies that 1 mW = 0 dBm. Here, 1 mW is a measurement of absolute power, and 0 dBm is a measurement of relative power.
Notice that the calculator expects absolute power in watts. If you are given an absolute power level in milliwatts, which is fairly common on WLANs, you need to convert it to watts before using the calculator. Notice that 1 watt = 1000 milliwatts, and 1 milliwatt = 0.001 watts.
Simply put, dBm reflects the relationship between a given power level and 1 milliwatt. This comparison yields three outcomes:
a) If the given power level is more than 1 milliwatt, the resultant dBm is a positive value.
b) If the given power level is less than 1 milliwatt, the resultant dBm is a negative value. Note that a negative dBm value doesn't mean there is no power!
c) If the given power level is equal to 1 milliwatt, the resultant dBm is 0.
Use the calculator in the Power section to complete the following steps:
1. On the lower UNII-1 band (i.e., 5.150-5.250 GHz with 100 MHz channels), the maximum output power of theintentional radiator (IR) allowed by the FCC is 50 mW. The IR is also referred to as a wireless transmitter.
Click in the watts box and type 0.05 (50 mW = 0.05 watts). What is the dBm of 50 mW?
2. On the middle UNII-1 band (i.e., 5.250-5.350 GHz with 100 MHz channels), the maximum output power of the intentional radiator (IR) allowed by the FCC is 250 mW.
Click in the watts box and type 0.25 (250 mW = 0.25 watts). What is the dBm of 250 mW?
3. On the upper UNII-1 band (i.e., 5.725-5.825 GHz with 100 MHz channels), the maximum output power of the intentional radiator (IR) allowed by the FCC is 800 mW.
Click in the watts box and type the given power level in watts. What is the dBm of 800 mW?
Scroll down to the Receive Sensitivity section. Review the information regarding receive sensitivity.
4. The receive sensitivity of a LinksysWUSB600N wireless network adaptor at 54 Mbps is -70 dBm. Click in the dBm box and type -70. What are the watts of -70 dBm?
5. The receive sensitivity of a LinksysWUSB300N wireless network adaptor at 54 Mbps is -68 dBm. Click in the dBm box and type -68. What are the watts of -68 dBm?
6. If you have to choose between these adaptors based on their receive sensitivity at the bit rate of 54 Mbps, which adaptorwill potentially perform better in achieving the desired bit rate?
Task 2: Cable loss calculations
As an antenna accessary, RF cables introduce loss/attenuation to signals being transmitted on the communications link. Cable manufactures such as Times Microwave Systems (TMS) often provide an attenuation chart to list different types of cables with their signal loss (e.g., in dB per 100 feet) at a specific frequency.
When selecting a coaxial cable for RF communications, note that:
a) The impedance of the cable needs to match that of the antenna and wireless transceiver to avoid signal loss caused by the voltage standing wave ration (VSWR) effect.
b) Not all coaxial cables support the transmission of 2.4 GHz and 5 GHz signals.
c) Given an RF cable with a certain length, signal loss/attenuation in the cable increases with frequency.
d) Given a RF cable and a signal frequency, signal loss/attenuation in the cable increases with distance.
Use the calculator in the Loss in a Coaxial Cable at 2.45 GHz sectionto complete the following steps:
1. Next to Choose type of cable, select LMR 400. This is a TMS cable that supports both 2.4 GHz and 5 GHz RF signals. 100 feet of such cable used on the 2.5 GHz range decreases the signal strength by about 6.76 dB (that is, 6.76 dB signal loss per 100 feet).
2. Click in the Length (meter) box and type 30.48 (100 feet = 30.48 meters). Click m→dB. What is the loss at this length? _____
3. Click in the Length (meter) box and type 60.92 (200 feet = 30.48 meters). Click m→dB. What is the loss at this length? _____
4. When the cable length doubles, how does the loss change approximately? _____
Task 3: Antenna gain calculations
Antennas are often used to increase the power output of a transmitter. Antennas achieve this by focusing the existing power in a specific direction. Notice that the amount of power provided to the antenna from the transmitter does not change; the signal gain created by antennas is a passive gain.
Antenna gain in dBi or dBd is a parameter that describes the directionality characteristic of an antenna. Given a particular type of antenna, the higher the antenna gain, the more directional the antenna is, and the more focused the existing power is in a specific direction.
Parabolic or dish antennas are an example of highly directional antennas. Due to their relatively high antenna gain, dish antennas are typically used for point-to-point transmission links. The antenna gain of a parabolic antenna is directly related to the diameter of a dish antenna's reflector and the signal frequency.
Use the calculator in the Antenna section to complete the following steps:
1. Next to frequency band, select 2.41-2.48 GHz.
2. Next to antenna diameter in meters, type 0.1 (0.1 meters = 3.9 inches). This is an optional antenna that could be added to an access point (AP).
3. Click D→ dB. What is the maximum theoretical antenna gain? _____
4. Next to frequency band, select 5.15-5.85 GHz.
5. Next to antenna diameter in meters, type 0.1 (0.1 meters = 3.9 inches)
6. Click D→ dB. What is the maximum theoretical antenna gain? _____
7. Given the same sized reflector, which signals, high-frequency or low-frequency, can be more efficiently focused by parabolic antennas (i.e., result in a higher antenna gain)?
8. Next to frequency band, select 5.15-5.85 GHz.
9. Next to antenna diameter in meters, type 0.2 (0.1 meters = 7.8 inches)
10. Click D→ dB. What is the maximum theoretical antenna gain? _____
11. Given the same signal frequency, which dish antennas, large-sized or small-sized, are more efficient at focusing the signal (i.e., result in a higher antenna gain)?
Task 4: Free space loss calculations
Free space path loss (FSPL) is the amount of signal loss/attenuation caused by signal dispersion over a distance. As does the light emitted from a flash light, RF signals spread out and weaken when propagating from an antenna. Notice that FSPL occurs regardless of the obstacles that cause reflection, diffraction, etc.; this is indicated by the "free space" phrase in its name.
Use the calculator in the Free space loss section to complete the following steps:
1. Next to frequency band, select 2.41-2.48 GHz.
2. Next to kilometers, type 0.1 (100 meters = 0.1 kilometers).
3. Click dB← km. What is the free space path loss in dB? _____
4. Change the frequency band to 5.15-5.85 GHz.
5. Next to kilometers, type 0.1 (100 meters = 0.1 kilometers).
6. Click dB← km. What is the free space path loss in dB? _____
7. How does the free space path loss for 802.11a (operating on the 5 GHz band) compare with 802.11g (operating on the 2.4 GHz band)?
8. Next to frequency band, select 2.41-2.48 GHz.
9. Next to kilometers, type 0.02 (20 meters = 0.02 kilometers).
10. Click dB← km. What is the free space path loss in dB? _____
11. Next to kilometers, type 0.04 (40 meters = 0.04 kilometers).
12. Click dB← km. What is the free space path loss in dB? _____
13. Next to kilometers, type 0.08 (80 meters = 0.08 kilometers).
14. Click dB← km. What is the free space path loss in dB? _____
15. When the distance doubles, how does free space path loss in dB change?
16. Next to frequency band, select 17.1-17.3 GHz.
17. Next to kilometers, type 1.
18. Click dB← km. What is the free space path loss in dB? _____
19. Next to kilometers, type 2.
20. Click dB← km. What is the free space path loss in dB? _____
21. Next to kilometers, type 4.
22. Click dB← km. What is the free space path loss in dB? _____
23. When the distance doubles, how does free space path loss in dB change?
Without the calculator, we could use the 6 dB rule to estimate free space loss: doubling the distance results in a signal loss/attenuation of 6 dB.
Task 5: Link Budget Calculations
The ultimate goal of link budget calculations is to ensure that the received signal strength is above the receive sensitivity of the receiver. A link budget is computed by adding and subtracting gains and losses represented in dB forms from the original power level of the transmitter or IR.
Fade margin is the amount of desired signal (i.e., the received signal) above what is required (i.e., the receive sensitivity). If the receive sensitivity of a receiver is -75 dBm, and the received signal is measured as -75 dBm, a transmission link may or may not be successful. The fact is that the received signal strength cannot be maintained at -75 dBm, due to interference, obstacles, and weather conditions. A 10 dB to 25 dB margin is commonly planned to accommodate received signal strength fluctuations. This range may seem to be wide, but the longer a transmission link (especially outdoors), the higher the margin should be.
Use the calculator in the Link budget section to complete the following steps:
1. Enter the following values for an office WLAN:
Transmit-
Transmit output power: +15 dBm
Cable loss: -6 dB
Antenna gain: +2 dBi
NOTE: the previous values are used to computeeffective isotropically radiated power (EIRP): (+15 dBm) + (-6 dB) + (+2 dBi) = 11 dBm. By definition, EIRP is the amount of power an ideal isotropic radiator can generate. In reality, EIRP is the power radiated from an antenna; it is regulated by the FCC.
Propagation-
Free space loss: -81 dB
Reception-
Antenna gain: +2 dBi
Cable loss: -4 dB
Receiver sensitivity: -80 dBm
2. Click Compute. What's the total remaining margin in dB? _____
3. Is this margin sufficient to accommodate received signal fluctuations? Why?
4. Enter the following values for a 10-killometer outdoor transmission link:
Transmit-
Transmit output power: +10 dBm
Cable loss: -3 dB
Antenna gain: +25 dBi
NOTE: the previous values are used to computeeffective isotropically radiated power (EIRP): (+10 dBm) + (-3 dB) + (+25 dBi) = 32 dBm. EIRP is regulated by the FCC.
Propagation-
Free space loss: -120 dB
Reception-
Antenna gain: +25 dBi
Cable loss: -3 dB
Receiver sensitivity: -80 dBm
5. Click Compute. What's the total remaining margin in dB? _____
6. Is this margin sufficient to accommodate received signal fluctuations? Why?
Task 6: Link budget calculations
Between two point-to-point antennas, the area that surrounds the visual line of sight path (i.e., the straight line drawn between two antennas) is called the Fresnel zone. If trees, buildings, and other obstacles encroachon this football shaped area, RF signals could experience loss caused by reflection, scattering, and diffraction. This contributes to signal loss fluctuation, and could cause a transmission link to fail.
To determine if a tree or building is obstructing the Fresnel zone, the radius of the Fresnel zone at the location of the potential obstacle is calculated. Actions needed to maintainFresnel zone clearance include removing the obstacle or raising the antenna. Often, this Fresnel zone clearance is relaxed by 40%, that is, only 60% of the Fresnel zone is clear of obstacles.
Use the calculator in the Fresnel ellipsoid section to complete the following steps:
1. Next to distance "D" between transmitter and receiver [meters], type 114. This is close to the maximum distance of an office WLAN.
2. Next to distance "d" between transmitter and obstacle [meters], type 65. This assumes an obstacle is at the midpoint between two antennas.
3. Click Compute radius. What's the radius of the Fresnel zone at the middle point? _____
4. Next to distance "D" between transmitter and receiver [meters], type 16000. This refers to an approximately 10-mile outdoor point-to-point transmission link.
5. Next to distance "d" between transmitter and obstacle [meters], type 4800. The obstacle is about 3 miles from one antenna.
6. Click Compute radius. What's the radius of the Fresnel zone atthis specific location?