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Chapter 6: Installation
Frequency
The 900-MHz frequency ranges are not significantly affected by rain and can penetrate through foliage and “around obstacles”
to a certain degree. Path calculations provide results that specify “required” antenna heights. For cost savings and in taking
advantage of the characteristics of the frequency range, sometimes the height requirements are not adhered to—this may result
in unreliable communications.
Power Requirements
The MDR100A-R5 or MDR112A-R5 may be integrated into a system that accepts a range of DC input voltages (supply current
requirements must also be met). In some deployments, power consumption is critical. A number of features related to minimize
power consumption are available with the MDR100A-R5 or MDR112A-R5 Series: sleep/sniff modes, LED dimming, and the ability
to operate at less transmit power given the receive sensitivity of the distant modem.
Interference
The frequency hopping spread spectrum (FHSS) operation of the MDR100A-R5 or MDR112A-R5 most often allows it to work well
in an environment within which there may be sources of in-band interference. Frequency Restriction is a built-in feature that may
be used to avoid specific frequencies or ranges of frequencies; the Spectrum Analyzer function may be used to identify areas of
potential interference. Cavity filters are also available if required.
6.1 Path Calculation
Assuming adequate antenna heights, a basic formula to determine if an adequate radio signal path exists (i.e. there is a reason-
able fade margin to ensure reliability) is:
Fade Margin = System Gain - Path Loss
where all values are expressed in dB.
As discussed on the previous page, a desired fade margin is 20 dB.
System gain is calculated as follows:
System Gain = Transmitter Power + (Transmitter Antenna Gain - Transmitter Cable and Connector Losses) + (Receiver Antenna
Gain - Receiver Cable and Connector Losses) + | Receiver Sensitivity |.
where all values are expressed in dB, dBi, or dBm, as applicable.
Assuming a path loss of 113 dB for this example, the fade margin = 143-113 = 30 dB. 30 dB exceeds the desired fade margin of
20 dB, therefore this radio communications link would be very reliable and robust.
On the following page are examples of actual path loss measurements taken in an open rural environment; the path loss numbers
do not apply to urban or non-LOS environments.
CAUTION: FCC regulations allow for up to 36 dBi effective isotropic radiated power (EIRP). The sum (in dBm) of the transmitted
power, the cabling loss, and the antenna gain cannot exceed 36 dBi.
Example:
Tx power = 30 dBm
Tx antenna gain = 6 dBi
Tx cable/connector loss = 2 dB
Rx antenna gain = 3 dBi
Rx cable/connector loss = 2 dB
Rx sensitivity = -108 dBm
System Gain = [30+(6-2)+(3-2)+108]dB
= [30+4+1+108]dB
= 143dB.
