Fresnel Zone Interference –
The Fresnel Zone is a circular area perpendicular to and centered on the line of sight. In radio wave theory, if 80% of the first Fresnel Zone is clear of obstacles, the wave propagation loss is equivalent to that of free space.
The equation for calculating the first Fresnel Zone utilizes distances to a point in the line of sight with a possible obstruction in the path is:
where FZ = 72.1 x sq. root (D1 x D2) / (f x Rm )
f = frequency in GHz
Rm = distance between antennas in miles
D1 = first distance to obstruction in miles
D2 = second distance to obstruction in miles = Rm – D1 FZ = radius of Fresnel Zone in feet from direct line of sight
We will calculate a Fresnel Zone radius later in this discussion. In the home and office network in a building, the Fresnel Zone calculation is usually unnecessary because of all the
wall/ceiling/floor pass- through considerations for any RF signal path. But in outside RF signal paths (links), the Fresnel Zone calculations can be very important from quarter mile distances and longer.
My experience with tall loblolly pines on a project is a good case in point. A wireless link was designed and setup for two medical facilities (two-story structures) in Wilmington, NC which were located 0.5 and 0.75 miles from an eleven-story hospital. There was no direct line of sight between the two medical facilities, but there was from both buildings to the hospital roof. After securing proper approvals, an RF signal link was setup from each building antenna to hospital roof-mounted antennas. Even though there was a good visual path from one building to the hospital roof, some very tall, very scrawny loblolly pines were infringing into the Fresnel Zone radius that was calculated for the link. It was just a few branches with the wide-spaced loblolly needles, but we had to top the trees to obtain a satisfactory signal-to-noise ratio for dependable communication. It is amazing how much microwave (2.4 GHz) energy those long needles absorbed, reflected, deflected, and/or scattered.
In the earlier wireless link analysis example using the 3 Km distance between antennas and assuming a mid-path constriction (D1 = D2), the Fresnel Zone is calculated as follows using common conversion factors for US standard measurements.
Convert 3 Km to miles by dividing by a conversion factor of 1.6 kilometers per mile, which yields using f = 2.4 GHz:
Rm = 3Km / 1.6Km/mile = 1.88 miles
D1 = D2 = 0.94 mile
FZ = 31.9 feet
The 80% Fresnel Zone radius for Free Space Loss equivalence would be obtained by multiplying FZ by 0.8, which yields a radius of 25.5 feet. So the clear path concentric cylinder around your systems line of sight for the distances and frequency analyzed would be 51 feet in diameter at the middle of the RF link.
System Operating Margin (SOM)
SOM (System Operating Margin), also known as fade margin, is the difference of the receiver signal level in dBm minus the receiver sensitivity in dBm. It is a measure of the safety margin in a radio link. A higher SOM means a more reliable over the air connection. We recommend a minimum of 10 dB, but 20 dB or more is the best.
SOM is the difference between the signal a radio is actually receiving vs. what it needs for good data recovery (i.e. receiver sensitivity). By using the transmit and receive RF signal power, the cable losses, the antenna gains, and the free space losses as considered in this lesson, we can calculate the SOM. Thus we have a method for designing and analyzing RF signal links used in wireless networking.
Rx Signal Level = Tx Power - Tx Cable Loss + Tx Antenna Gain – Free Space Loss + Rx Antenna Gain - Rx Cable Loss
SOM = Rx Signal Level - Rx Sensitivity
We can modify the SOM expression to consider attenuation losses due to transmission through walls, etc., in an actual building wherein a home or office network would be installed. It is simply adding more loss terms to the SOM equation. But first we will have to consider the level of losses through various materials. The signal attenuation loss for 2.4 GHz transmission through the following structures can be included in the Rx Signal Level equation for each pass-through in the straight line signal path (line of sight). The dB loss values will be subtracted from the transmitted signal power to reflect the loss of passing through the material structures.
Which Building Materials Can Block Wi-Fi Signals?
Are you curious why a part of your home, or even an area just outside the house, has poor Wi-Fi reception? It could be due to the material used for the wall, or other physical barriers that block or weaken Wi-Fi signals.
|Structure||Loss dB in 2.4 GHz|
|Clear Glass Window||2|
|Brick Wall next to a Metal Door||3|
|Cinder Block Wall||4|
|Sheetrock/Wood Frame Wall||5|
|Sheetrock/Metal Framed Wall||6|
|Metal Frame Clear Glass Wall||6|
|Metal Screened Clear Glass Window||6|
|Metal Door in Office Wall||6|
|Metal Door in Brick Wall||12|
|Masonry Block (203 mm)||12|
|Brick faced concrete (190 mm)||14|
|Masonry Block (400 mm)||17|
|Concrete (200 mm)||23|
|Masonry Block (610 mm)||28|
|Concrete (300 mm)||35|