Industrial Communications Handbook August 2016

Outdoor, or at least a relatively large distance, with not much inbetween, the Friis link equation, given above in Equation 3.4 works best. But it depends on line-of-sight. Now this is not as simple as it may at first seem. At least 60% of the first Fresnel zone must be clear of any obstruction, otherwise the link will be intermit- tent at best. Remember that the Fresnel zone is a three dimen- sional ellipsoid between the transmitter and the receiv- er. The earth gets in the way, trees, hills, buildings … Assuming a symmetrical link, with the worst obstruc- tion in the middle of the link (e.g. earth), we can get the First Fresnel zone radius in metres from Equation 3.5 .

height at the ends of the link may need adjustment. A fully redundant system has several antennas, positioned vertically to account for this. An example is the classic Microwave towers seen dotted around the country for telephony in the days before fibre. Finally, remember that 2,45 GHz is the frequency used in your microwave oven to cook your potato, de- signed to make water molecules within it vibrate and cause heating. Since WiFi is at the same frequency, and a tree has leaves containing water, a highly directional link through a tree works in winter in the absence of leaves, but is useless in summer with luxuriant growth. It gets worse in the rain when all the leaves are nice and wet. Rain, in itself, at these frequencies is a problem, hence the clamoring for TV frequencies that will be freed up after digital migration, as VHF does not suffer from such efficient absorption. When planning an outdoor link, a link margin is cru- cial to the success of the link, and strongly influences cost. Hence, how much do you need to spend to get a reliable link? How long is a piece a of string? A margin of ‘only’ 10 dB means 10 times the power. 3.3 Indoor and diversity The majority of industrial communications will occur in an environment that would be classified as indoor. This is defined by lots of clutter, both metallic and nonmetal- lic. Metallic clutter introduces a fully reflective surface, and strongly reflects the electromagnetic wave, interact- ing with the strong forward signal, leading to interfer- ence: Constructive and Destructive! Remember from before that a quarter wavelength, 90 ° , exists between a point of absolute destruction and beautiful addition. Thus, at 2,45 GHz WiFi, that is 30 mm. On this basis, it can be seen that communication at this frequency in a busy environment is simply impos- sible. The only way WiFi actually works is by having Di- versity . Diversity ensures that when one antenna is in a de- structive interference zone, there is another antenna that can still receive. This of course requires two differ- ent radios, and the ability to be able to switch between the signals very rapidly indeed: requiring a computing platform to decide which signal is stronger. Even the

D f

km ( )

(3.5)

8.656 = ×

F

( )

1 m

(

)

GHz

Or, since we are only really interested in 2,45 and 5,8 GHz, this reduces to:

and

The first Fresnel zone under these conditions is shown in Table 3.1 .

Table 3.1: Fresnel zone radii in metres at the centre of a symmetric ellipsoid. Link distance [km] F1 [m] 2,45 GHz F1 [m] 5,8 GHz 1 5,5 3,2 2 7,8 4,5 3 9,6 5,5 4 11,1 6,3 5 12,4 7,0 Table 3.1 thus also shows why the higher frequency is most often used for longer distance links, as no one wants a 13 m mast at both ends of a 5 km link! (There are other considerations, but this is a positive!) A point-to-point link that works perfectly in Winter may not work when Summer comes around. Over a ki- lometre or so link, the refractive index of the air chang- es, especially with temperature, changing the so-called ‘K-factor’ which accounts for earth curvature. So the

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industrial communications handbook 2016