User's Manual

14221-1510-4440, Rev. B
63
range, replace the antenna, make sure all connections are seated firmly, and repeat the testing
procedure. If problems persist, contact the Technical Assistance Center (see page 12).
If the radio installation employs a ¼-wave unity-gain antenna (part number AN-
225006-001), the driven element (i.e., whip) of the antenna must be trimmed to the
proper length in order to minimize antenna system VSWR. Refer to the instructions
included with the antenna for trimming instructions.
10. Position the wattmeter’s element to measure reverse (reflected) RF power from the antenna. The
arrow on the face of the element must point away from the antenna and to the radio to measure
reverse power.
11. Key the radio’s transmitter via the microphone’s PTT button.
12. For a good mobile radio installation, an antenna system VSWR of between 1.2:1 and 1.5:1 is
reasonable. If the utilized wattmeter/power-meter cannot directly measure and display VSWR, use the
following formula to calculate the reverse power at a 1.5:1 VSWR level:
P
reflected (Watts)
= P
forward (Watts)
* 0.04, where P
forward (Watts)
is the measured forward power.
For example: For a P
forward
of 30 Watts, P
reflected
= 30 * 0.04 = 1.2 Watts. See Table 15-3 for
more examples.
Table 15-3: Example Measured Forward Power Levels to
Reflected Power Levels for a 1.5:1 VSWR
RADIO
MAXIMUM
TRANSMIT
POWER
APPLICABLE
RF BANDS
MEASURED
TRANSMIT FORWARD
POWER
P
forward
REFLECTED POWER
AT A VSWR = 1.5:1
P
reflected
Watts dBm Watts dBm
35 Watts
700 MHz
23.8 43.8 0.95 29.8
30.0 44.8 1.20 30.8
37.7 45.8 1.51 31.8
800 MHz
27.8 44.4 1.11 30.5
35.0 45.4 1.40 31.5
44.1
46.4
1.76 32.5
50 Watts
VHF
and
UHF
39.8 46.0 1.59 32.0
50 47.0 2.00 33.0
63 48.0 2.52 34.0
Transmit only for as long as needed to take the measurement, then immediately disable
the transmission.
The formula to convert from watts to dBm is: (Power in dBm) = 10 * Log(Power in milliwatts),
where Log is the base 10 logarithm.
NOTE
NOTE
NOTE