Table 10 - Swr / Rev Pwr Interpretation - CalAmp INTEGRA-TR User Manual

Wireless modem for licensed spectrum

User manual (64 pages)

Table of Contents

Interpreting RSSI Readings

Typical values of RSSI will be in the range of -110 to -60 dBm (depending on network speed), with higher values (i.e. less

negative values) indicating a stronger signal.

Reliability of data reception depends largely on signal strength. Good design practice calls for a minimum 30 dB "fade

margin", based on a threshold reception level of -107 dBm (1 uV) at speed of 9600 b/s. Experience indicates that this will

give about 99.5% reliability.

Some representative performance values for 9600 b/s operation are given below. These values assume that the units are

correctly aligned and installed in a quiet location. Environments with high electrical or RF noise levels will require an

increase (less negative) in the numbers shown to achieve a given level of reliability.

-100 dBm Approximately 50% reliability. Fading may cause frequent data loss.

-90 dBm Approximately 90% reliability. Fading will cause occasional data loss.

-80 dBm Approximately 99% reliability. Reasonable tolerance to most fading.

-70 dBm Approximately 99.9% reliability with high tolerance to fading.

If RSSI values drops seasonally the most likely cause is tree foliage which can interfere with radio transmissions during the

spring and summer.

Interpreting Power Readings

The values returned are approximate and should not be regarded as an absolute indicator of performance. For example, a

unit that shows a forward power of 4.5 watts may actually measure at 5.0 watts on a lab quality wattmeter. For this reason,

these values should not be used to indicate that a unit is out of spec or to compare one unit to another.

However, the values returned should be consistent for any given unit. If statistics are kept on a unit per unit basis, changes

in forward or reflected power are significant. The following conditions are worthy of investigation.

1. Forward power output (in watts) drops or rises by more than 10% from its established value. Reflected power

remains low. This indicates that the transmitter may need alignment or that a component may be in need of

replacement.

2. Forward power output drops by more than 10% from its established value or reflected power shows an increase.

This indicates a possible antenna or feedline problem that affects SWR (Standing Wave Ratio).

Reverse Power and SWR

A reverse power reading above zero is an indication that the antenna, feedline or connectors are damaged, corroded or

improperly tuned. This creates standing waves that are reported as a Standing Wave Ratio (SWR). Table 10 is based on a

forward power of 5 watts (it may be scaled for lower power settings) and gives guidelines to interpreting these figures: