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Antenna Basics
Gain
For a particular antenna the stated gain figure applies only along the antenna bore-sight and often only in one
plane; if the remote point is off-axis relative to the antenna electrical centre-line or bore-sight then the gain will
depend on the degree of offset according to the polar diagram for that antenna.
For example, an antenna is advertised as having a gain of 12dBi and a beam-width of 60 degrees; this is normally
taken to refer to the half power or -3dB beam-width. In other words the antenna gain at ±30degrees off the bore-
sight will be 3db down on the peak gain or 9dBi. Beyond ±30 degrees off-axis the gain will be very much less.
For short vertical antennas of the "rubber duck" type the gain is fairly constant in the horizontal plane but the
vertical beam-width may be as little as 12 degrees for a 9dB antenna making it necessary to mount the antenna
slightly off vertical for best signal strength in a specific area; this of course means that the pattern on the
opposite side might now be pointing into the ground or up in the air.
Polarisation
Antenna polar diagrams show coverage in terms of variations in gain over vertical and horizontal planes relative
to the antenna. Depending on its method of construction the antenna will radiate the electric component of the
Electro-Magnetic (EM) wave in one plane and the magnetic component in the other (Polar diagrams identify these
two components as E for electric and H for magnetic). If the electric component of the EM wave is vertical then
the antenna is said to be vertically polarised and vice-versa.
There are some exceptions to this (such as crossed and circular polarisation) but for the sake of simplicity they
are not considered here.
For maximum power transfer all antennas on the same radio net must be mounted so that their signals have the
same polarisation; it is sometimes possible to take advantage of this to allow operation of adjacent but otherwise
unconnected radio nets on the same channel by having the antennas on one net vertically polarised and the other
horizontally polarised.
Mounting Requirements
Objects or structures within the operating region of the antenna will distort the horizontal and vertical space
pattern so that the antenna polar diagram no longer represents the actual coverage. For this reason, antennas
should be mounted to maximise separation from buildings or structures and away from areas where large objects
may be temporarily placed. This is usually achieved by fixing the antenna to a mast or tower by means of a
bracket that allows adjustment of antenna orientation. The mounting method should also allow for adjustment of
height as, very often, a small change in antenna elevation will improve signal strength.
Receiver Input Power and RSSI Relationship
During development of the Rad24 radio it was necessary to understand the relationship between signal power at
the receiver input and the indicated RSSI figure. A calibrated signal source was used to set RSSI readings at
intervals of 10 and the corresponding input levels recorded at each step. Figures in italics were not measured but
extrapolated from the measured values.
Input signal power
Indicated RSSI
(dBm)
Value
-7
-10
-17
-20
-28.5
-30
-36.5
-40
-44.5
-50
-54
-60
-64
-70
-74
-80
-79
-85
-84
-90
85
Mantracourt Electronics Limited T24 Technical Manual
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