In this Lab, you can do the calculations manually if you like to do so, or you can use an online calculator to calculate the power, cable loss, antenna gain, free space path loss, link budget, and Fresnel zone clearance.
The best way to learn is to use both! Do the manual calculations and then check them using the calculator!
One of many online calculators exists at www.swisswireless.org/wlan_calc_en.html (Links to an external site.) . If the web page has been moved (I checked the site today, July 27, 2016 and it is sound and good!), 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.
Here, the mathematical relation between the dB, dBm power, and the Watt power is:
Power in (dB) = 10 Log (Power in (watt)),
Power in (dBm) = 10 Log (Power in (milli-watt))
Important:
- The reference point is specified as 1 mW = 0 dBm.
- Log(1)=0, Log(2)=0.3, Log(10)=1,
- Log(A*B) = Log (A) + Log (B)
- Log(A/B) = Log (A) – Log (B)
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 thus, 1 milliwatt = 0.001 watt.
Simply put, dBm reflects the relationship between a given power level referenced to 1 milliwatt. This comparison yields three outcomes:
- If the given power level is more than 1 milliwatt, the resultant dBm is a positive value.
- 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!
- If the given power level is equal to 1 milliwatt, the resultant dBm is 0.
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:
- 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.
- Not all coaxial cables support the transmission of 2.4 GHz and 5 GHz signals.
- Given an RF cable with a certain length, signal loss/attenuation in the cable increases with frequency.
- Given a RF cable and a signal frequency, signal loss/attenuation in the cable increases with distance.
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.
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.
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–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.
Task 6: Link budget calculations.
Between two point-to-point antennas, the area that surrounds the visual Line Of Sight (LOS) path (i.e., the straight line drawn between two antennas) is called the Fresnel Zone. If trees, buildings, and other obstacles encroach on 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 maintain Fresnel 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.
