Bedienungsanleitung Yaesu FT-857D (Seite 14 von 132) (Englisch)

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12 FT-857D Operating Manual

ANTENNA CONSIDERATIONS

The antenna systems connected to your FT-857D transceiver are, of course, critically im-

portant in ensuring successful communications. The FT-857D 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 power

amplifier’s protection circuitry will begin to reduce the power output of there is more than

a 50% divergence from the specified impedance (less than 33 Ω or greater than 75 Ω,

corresponding to a Standing Wave Ratio (SWR) of 1.5:1).

Two antenna jacks are provided on the rear panel of the FT-857D. 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 Ω 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 Ω 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-857D. 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

polarizations, 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 “miss-

ing 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

INSTALLATION

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