MT-MR PLATINUM. POINT TO POINT AURAL RADIO RELAY SYSTEM
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G
ar
= Receiver antenna gain (in dBi)
A
ir+con
.= Transmission line & connectors losses at Receiver end.
(in dB).
S
rec.
= Received Signal (in dBm or dBw)
In our illustrative Example, we can replace these parameters by its numerical values in Expression ( 3) :
+36 - 2.94 + G
at
- 130.9 + G
ar
- 2.94 = S
rec
.
That is: G
at
+ G
ar
- 100.8 = S
rec
.
Lets choose our parabolic antennas having the same gain (and diameter) both in Transmitter & Receiver ends, so
G
at
= G
ar
= G
ant
.
We have: 2 G
ant
. - 100.8 = S
rec
.
Analysis of this expression leads to a direct relationship between antennas' gain and Received Signal strength. This
is a commonly used trick to calculate in a safe and easy way the required antenna's diameter.
Now, we proceed to calculate the required minimum signal strength at receiver input. For performing this, we go to
the concept of Fade Margin, defined as the difference in dB existing between the nominal received signal and
receiver noise threshold. This Fade Margin, under normal conditions, should be kept at least at 40 dB (the higher
the better). So, the minimum required signal level at Receiver input will be:
(4) S
rec
. = S
thr
. + FM
Assuming that our receiver has a Noise Threshold signal level of -80 dBm, we substitute the numerical values in
(4):
S
rec
. = -80 +40 = -40 dBm.
This value of -40 dBm will be the minimum required signal strength at Receiver's end in order to assure a reliable
value of signal-to-noise ratio in the received signal.
Replacing this value in the expression relating received signal strength with antennas' gain we have the following:
2 Gant. - 100.8 = -40
or : 2 Gant. = 60.8 dBi
Concluding: Gant. = 30.4 dBi
The next step is to consult any table of Parabolic Antennas in order to find the required diameter in our parabolic
reflectors to obtain this gain at the system working frequency. At 2 GHz band, mesh reflector antennas are widely
used, both for lighter weight and less wind load, taking into account the relatively large diameters required. An
example of these mesh antennas Table is as follows:
