AvMesh
®
RF Communications Systems
Installation & Service Manual
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9
Figure 4:
Creation of intermodulation products
Furthermore, the third order intermodulation products can potentially pass through the
receiver since they are close in frequency to the fundamental frequencies. Consider the
situation in which 3 signals are present at the input of a non-linear amplifier or mixer; one
is the signal of interest at frequency f3 (e.g. 140 MHz) and the other two are competing
signals at frequencies f1 (e.g. 100MHz) and f2 (e.g. 120MHz). There are 15 combinations
of third order intermodulation products (including harmonics) from the interactions
between the signal of interest (f3) and the competing signals (f1 and f2). One of these
intermodulation products (i.e. 2f2-f1=2x120 – 100=140MHz) is equal to the frequency of the
signal of interest (f3) and as a result, this intermodulation product will enter the receiver
and interfere with the signal of interest. As a consequence, the receiver is at risk of
extracting and processing the incorrect information.
It is therefore essential to calculate the 3rd order harmonics and intermodulation products
for all combinations of competing frequencies in a site to determine if there is a possibility
of interference.
Antennas are a key component of any radio frequency communications system as they
enable the transfer of information from the transmitter to the receiver. For this reason, poor
antenna performance limits the performance of the overall system.
Antennas provide a radio communication system with three main properties; gain,
direction and polarization. A description of each is given in the following sections.
Typically, antennas do not transmit or receive radio frequency energy uniformly in all
directions and instead, transmit or receive more in some directions compared to others.
The antenna gain can be defined as a ratio of the highest signal strength transmitted or
received in a particular direction to that of a reference antenna. The two main types of
reference antenna are either the isotropic antenna which radiates the radio frequency
energy uniformly in all directions or the dipole which does not.
f1
-f2
f1 f2
Frequency
A
m
p
lit
ud
e
2f1
-f2
2f2
-f1
2f1
f1
+f2
2f2
3f
1
2f1
+f2
f1
+2
f2
3f
2
f1
-f2
f1 f2
Frequency
A
m
p
lit
ud
e
2f1
-f2
2f2
-f1
2f1
f1
+f2
2f2
3f
1
2f1
+f2
f1
+2
f2
3f
2
3.4 Antennas
3.4.1 Gain
