Industrial Communications Handbook August 2016

Remembering that λ /4 represents the case where an open-circuit transforms to a short circuit, at 50 Hz (0,000050 MHz), this is 1 500 km, roughly the distance between Cape Town and Ogies, the centre of our gener- ating capacity. So when some nice chap switches Cape Town off the grid, Ogies is in trouble. Grid stabilisation is a challenge on long distance transmission, hence the need for HVDC. At WiFi frequencies, this calamitous situation occurs at a mere 30mm in free-space. Figure 2.2 shows comparative sizes of Sleeve Di- poles at 2,45GHz, and 5,8GHz. These are usually termi- nated with an SMA connector, and this shows the clear dependence of size on the frequency.

ed, or NOT INTENDED! ). This is illustrated in the sim- ple alpine horn antenna in Figure 2.3 .

λ / 2 @ 5.8 GHz

Figure 2.3: Simple Alpine Horn explanation of radia- tion.

λ / 2 @ 2.45 GHz

Note that the radiation is launched in a particular po- larisation, vertically in the direction of propagation. Ad- ditionally, Maxwell tells us that Electric fields get lonely without an accompanying Magnetic field in the plane 90° away from both propagation and the electric field. Thus, sufficiently far away from the antenna, both the electric and magnetic fields are transverse to the propa- gation, as shown in Figure 2.4 .

Figure 2.2: Half-wave (sleeve) dipoles.

From Equation 2.1 , your TV1,2,3 antenna at 200 MHz has elements 3/4 m long, your MNET antenna has 250mm elements, your WiFi at 2,45GHz is at 61mm, and at 5,8GHz, it's at 26mm. Everything in Electromagnetics scales exactly as a function of frequency. For the vast majority of Industrial Communications, we deal with the unlicenced ISM (Industrial, Scientific and Medical) bands of 2,45 and 5,8GHz. So the choice of antenna depends very strongly on the frequency of operation. 2.3 Radiation Quite what causes radiation, we don’t really know, but we do put forth some theories, almost always associ- ated with accelerating charged particles. What we do know is how to get it radiating: Time, Length, Phase. (You may have heard that one before …) Essentially, if we take a transmission line, and split it apart, so the conductors are more than a tenth of a wavelength ( λ /10) apart, radiation will happen (intend-

Figure 2.4: A Transverse ElectroMagnetic (TEM) Wave.

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industrial communications handbook 2016