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industrial communications handbook 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
.
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.
Figure 2.3: Simple Alpine Horn explanation of radia-
tion.
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
.
λ
/ 2 @ 5.8 GHz
λ
/ 2 @ 2.45 GHz