12
FT-857 Operating Manual
A
NTENNA
C
ONSIDERATIONS
The antenna systems connected to your
FT-857
transceiver are, of course, critically impor-
tant in ensuring successful communications. The
FT-857
is designed for use with any an-
tenna system providing a 50
W
resistive impedance at the desired operating frequency. While
minor excursions from the 50
W
specification are of no consequence, the power amplifier’s
protection circuitry will begin to reduce the power output of there is more than a 50% diver-
gence from the specified impedance (less than 33
W
or greater than 75
W
, corresponding to
a Standing Wave Ratio (SWR) of 1.5:1).
Two antenna jacks are provided on the rear panel of the
FT-857
. The “
HF/50 MHz ANT
”
jack is used for HF and 50 MHz, while the “
144/430 MHz ANT
” jack is used for 144 MHz
and 430 MHz.
Guidelines for successful base and mobile station installations are shown below.
Mobile Antenna Installations
Mobile antennas for the HF bands, with the possible exception of those designed for 28
MHz, display very high “Q” due to the fact that they must be physically shortened, then
resonated using a loading coil. Additional system bandwidth may be realized using the Yaesu
FC-30
Automatic Antenna Tuner, which will present a 50
W
impedance to your transceiver
on the 1.8 ~ 50 MHz bands so long as the SWR on the coaxial line connected to the
FC-30
is below 3:1.
On the VHF and UHF bands, coaxial line losses increase so rapidly in the presence of SWR
that we recommend that all impedance matching to 50
W
be performed at the antenna
feedpoint.
Yaesu’s Active-Tuned Antenna System (
ATAS-100/-120
) is a unique HF/VHF/UHF mo-
bile antenna system, which provides automatic tuning when used with the
FT-857
. See page
68 for full details on the
ATAS-100/-120
.
For VHF/UHF weak-signal (CW/SSB) operation, remember that the antenna polarization
standard for these modes is horizontal, not vertical, so you must use a loop or otherwise
horizontally-polarized antenna so as to avoid cross-polarization loss of signal strength (which
can be 20 dB or more!). On HF, signals propagated via the ionosphere develop mixed polar-
izations, so antenna selection may be made strictly on mechanical considerations; vertical
antennas are almost always utilized on HF for this reason.
In mobile (and portable) installations, when vertical antennas are used, remember that the
grounding of the base area of the antenna is critically important to proper operation. Since
most HF vertical antennas emulate a quarter-wavelength “monopole” antenna, the “missing
half” of the dipole antenna consists of a counterpoise of radial ground system. In a vehicle,
if mounting the antenna to a door or hatch, it is recommended that you bond the door to the
I
NSTALLATION
