Radio (Rf) Measurements; Antenna System Swr And Transmitter Power Output; Introduction - GE MDS iNET Series Reference Manual

• Use a two-conductor cable to power to the radio. Then the currents in the positive and negative wires
are equal and opposite, causing their magnetic fields to cancel. The result is no net inductance in the
connection to cause voltage overshoot.
• Do not connect a radio to a power supply that is already powered up, unless necessary (that is, when
connecting a radio to a battery bank and charger). When power is applied by switching on a power
supply, the rise time of the supply is too slow to cause overshoot.
• Typically, there are multiple return paths for the negative side of the power supply, through the coax-
ial cable shield and the chassis, for example. Any imbalance in the currents in the power cable results
in voltage overshoot, so this should be minimized during initial power-up if the supply cannot be
turned off.
• Add a 1 to 2 , 2 Watt resistor in series with the positive lead. This greatly limits voltage overshoot.
Since these radios draw very little current in receive mode, and transmit only briefly, there is little
loss in power efficiency. In transmit, the voltage drop is minimal and has no effect.
• Minimize the length of the power cabling, within reason.
• When power is applied from a power source having a relatively high (1 or 2 ) source impedance,
or from a power source without a large amount of output capacitance, no overshoot occurs. There-
fore, use a power supply that is rated appropriately for the radio if possible—avoid using power sup-
plies that far exceed the radio's current requirements.
Direct any questions you have about interfacing to GE MDS radios to the Technical Services Department,
using the information provided at the back of this guide.

4.2 Radio (RF) Measurements

There are several measurements that are a good practice to perform during the initial installation. The will
confirm proper operation of the unit and if they are recorded, serve as a benchmark in troubleshooting
should difficulties appear in the future. These measurements are:
• Transmitter Power Output
• Antenna System SWR (Standing-Wave Ratio)
• Antenna Direction Optimization
These procedures may interrupt traffic through an established network and should only be performed by a
skilled radio-technician in cooperation with the network manager.

4.2.1 Antenna System SWR and Transmitter Power Output

Introduction

A proper impedance match between the transceiver and the antenna system is important. It ensures the max-
imum signal transfer between the radio and antenna. The impedance match can be checked indirectly by
measuring the SWR (standing-wave ratio) of the antenna system. If the results are normal, record them for
comparison for use during future routine preventative maintenance. Abnormal readings indicate a possible
trouble with the antenna or the transmission line that will need to be corrected.
The SWR of the antenna system should be checked before the radio is put into regular service. For accurate
readings, a wattmeter suited to 1000 MHz measurements is required. One unit meeting this criteria is the
™
Bird Model 43 directional wattmeter with a 801-1 or other appropriate element installed.
The reflected power should be less than 10% of the forward power (2:1 SWR). Higher readings usually
indicate problems with the antenna, feedline or coaxial connectors.
If the reflected power is more than 10%, check the feedline, antenna and its connectors for damage.
Record the current transmitter power output level, and then set it to 30 dBm for the duration of the test to
provide an adequate signal level for the directional wattmeter.
114 MDS iNET Series Reference Manual 05-2806A01, Rev. L