Yaesu FT-920, Operating Manual

Page: 11 / 98

FT-920 Operating Manual
9
Antenna Considerations
The FT-920 is designed for use with any antenna
system providing a 50
Ω
resistive impedance at the
desired operating frequency. While minor excursions
from the 50
Ω
specification are of no consequence,
the transceiver’s Automatic Antenna Tuner may not
be able to reduce the impedance mismatch to an
acceptable value if the Standing Wave Ratio (SWR)
present at the Antenna jack is greater than 3:1.
Among the undesirable consequences that high SWR
may produce are:
r
The transceiver’s power amplifier protection
circuitry will reduce power if the Automatic
Antenna Tuner is unsuccessful in reducing the
SWR.
r
Even if the Automatic Antenna Tuner success-
fully normalizes the impedance presented to
the radio, feedline losses will escalate rapidly
with high SWR at the higher operating frequen-
cies, especially 28 MHz and 50 MHz.
r
Although high SWR itself does not cause
feedline radiation, the sudden onset of high
SWR may well indicate a mechanical failure in
a matching device, leading to an electrical con-
dition which may cause excessive feedline ra-
diation, which can cause interference to nearby
home-entertainment devices.
Every effort should, therefore, be made to ensure that
the impedance of the antenna system utilized with
the FT-920 be as close as possible to the specified
50
Ω
value.
Any antenna to be used with the FT-920 must, ulti-
mately, be fed with 50
Ω
coaxial cable. Therefore,
when using a “balanced” antenna such as a dipole,
remember that a balun or other matching/balancing
device must be used so as to ensure proper antenna
performance.
Use high-quality 50
Ω
coaxial cable for the lead-in to
your FT-920 transceiver. All efforts at providing an
efficient antenna system will be wasted if poor qual-
ity, lossy coaxial cable is used. Losses in coaxial lines
increase as the frequency increases, so a coaxial
line with only 0.5 dB of loss at 7 MHz may have 2 dB
of loss at 28 MHz. For reference, the chart below
shows approximate loss figures for typically-available
coaxial cables frequently used in amateur radio in-
stallations.
Memory Backup
The lithium memory B
ACKUP
switch on the rear panel
is turned on at the factory, allowing VFO, memory,
and menu data to be retained while power is off.
Backup current is minuscule, so it is not necessary
to turn the B
ACKUP
switch off unless the transceiver is
to be stored for an extended period.
After five or more years of operation, the transceiver
may fail to retain memories. At this point, the lithium
battery should be replaced. Contact your dealer for
procurement of the battery, and follow the instruc-
tions on page 91 for replacement of the battery.
Before completing installation of your FT-920 trans-
ceiver, please take the time to review the following
safety guidelines.
Cable Ty pe Loss: 2 MH z Los s: 1 5 MHz Los s: 2 8 MHz
RG-5 8A a 0.55 1.75 2.60
RG- 58 Foa m a
0.54 1.50 2.00
RG-8 X a
0.39 1.07 1.85
RG-8 A , R G -213 a
0.27 0.85 1.25
RG -8 Foa m a
0.22 0.65 0.88
Belden
®
991 3 a
0.18 0.50 0.69
RG-1 7A a
0.08 0.30 0.46
Loss in dB per 30m (100 feet) for
Selected 50
Ω Ω
Ω Ω
Ω
Coaxial Cables
(Assumes 50
Ω
Input/Output Terminations)
Loss figures are approximate; consult cable manufacturers’ cata-
logs for complete specifications.
Loss figures can increase significantly if high SWR is present on
the transmission line.