User Manual

33
that provides the proper VSWR and radiation pattern without the use of a
ground plane. Typical antenna gain without a ground plane is 0 dB.
The case/antenna mounting plate (188D6437P1) was specifically designed
to fit the hole mounting pattern for the 19B209568P5 mount.
CAUTION
Since the Allen Telecom, Inc. antenna (part number
19B209568P5) is a high impedance, voltage fed antenna,
the length of the coaxial cable from the antenna to the
J
AGUAR
725M RF port is critical. It cannot be just any
arbitrary length. To prevent possible extraneous radiation,
RFI feedback from the antenna to the radio, high VSWR,
and antenna ground currents, select an antenna coaxial cable
length that is close to a multiple of an odd quarter-
wavelength.
To properly use a thru-line wattmeter, one of the following two conditions
must be met:
•
If the wattmeter is installed right at the J
AGUAR
725M RF port and the
odd multiple of a quarter-wavelength of coax to the antenna is used,
correct readings will result. Any other length of coax will result in
improper readings.
•
If a length of coax equal to a multiple of a half-wave length is installed
between the J
AGUAR
725M RF port and the thru-line wattmeter and an
odd multiple of a quarter-wavelength of coax to the antenna is used,
correct readings will also result. Any other length of coax will result in
improper readings.
To calculate the proper cable length of the antenna coaxial cable, proceed as
follows:
1. Determine the center frequency of the majority of programmed transmit
channel frequencies.
2. Use the formula:
¼
λ
in = 2805
F(MHz)
Example 1: Assume the customer has an 800 MHz J
AGUAR
725M with
most of the transmit frequencies between 816-821 MHz. Therefore, center
frequency is 818.5 MHz. Applying the above formula results in: