Antennas
If antennas are the most important part of an amateur station, that is
doubly true on HF where long-distance contacts place a premium on the
antenna's ability to transmit and receive efficiently. Antennas are
generally rated in terms of gainùhow well they concentrate signals in
a preferred direction. Gain is specified in dB and every 3 dB of gain
doubles your signal's strength.
The simplest antenna (and a very effective one) is a half-wave dipole
made of wire, one-half wavelength long, and installed horizontally.
You can build it yourself as described on the ARRL's Technical Information
Service (TIS) Web page or you can buy one or any of several common
variations; off-center-fed, multi-wire, and G5RV antennas are popular.
While a dipole's gain is low, it's efficient and hard to beat for the
price. You'll need one or two supports (trees work well) at least 20
feet tall. Large-scale versions of the Yagi beam antennas mentioned in
the VHF/UHF section can be placed atop steel towers, although this is
not required to get started.
Vertical antennas are also popular, particularly where a horizontal
antenna may be difficult to put up, for portable use, and where a "low profile"
antenna will be more suitable. The simplest is a quarter-wave vertical made of
metal tubing and radial wires fanning out from the
base to act as a ground plane. To use it on several of the HF bands
will require an antenna tuner described in the next section. Multi-band
verticals are constructed to operate on several bands without the antenna
tuner. Ground-independent verticals are available that operate without
the radial wires.
Mobile antennas for HF use come in two common styles; fixed-tuned
and tunable whips. A fixed-tuned whip is adjusted to present the proper
load to the transmitter on one band or over a portion of a band. You will
need one for each band you intend to use, but they are inexpensive. The
whips have a 3/8 inch to 24 threaded base that screws in to the antenna
mount on the vehicle, similarly to what is shown in the VHF/UHF section.
A tunable whip with an internal coil (called a "screwdriver" antenna) can
vary its length continuously to tune up on nearly any HF frequency. A
controller is mounted in the vehicle. Only one tunable whip is required,
although they are much more expensive than the fixed-tune whips.
Mobile antennas can be mounted on the vehicle temporarily or permanently.
A permanent mount generally results in a better electrical ground
connection to the vehicle, which is important for the HF antennas to
work well. Temporary mounts, such as larger versions of the VHF/UHF
magnet or mag-mounts, are usable for most purposes, but have lower
efficiency than mounts attached directly to the vehicle and can be knocked
loose. If you purchase the mount separately from the antenna, make sure
the mount and antenna have the same type of mechanical connection!
The most popular antenna feed line is coaxial cable or coax. There are
many types, but the most common are (from smallest to largest) are RG-58, RG-8X,
and RG-8 or RG-213.
Use RG-58 only for short (50 feet or less) distances due to its higher
losses and never at the output of an amplifier. RG-8X will carry the full
legal power, but is not a good choice for feed lines longer than 100 feet
or mistuned antennas. RG-213 is suitable for all amateur HF uses, except
for extremely long feed lines. At HF, the standard connectors are the
"UHF" connector family's PL-259 (cable plug) and SO-239 (equipment
receptacle) illustrated in the VHF section (see the PDF file). Cable is
available with connectors pre-installed or with a little "Elmering" you
can learn how to install them yourself as described on the ARRL TIS Web
site.
The other type of feed line is open-wire, ladder, or window line
consisting of two parallel wires coated with plastic insulation.
Open-wire line has very low losses, but is not as convenient to use as
coaxial cable and requires an antenna tuner or some other kind of
impedance transformer to work with most HF radios along with a
parallel-to-coaxial balun so that you can connect the feed line to the
radio.
--- SBBSecho 3.01-Win32
* Origin: The Thunderbolt BBS - wx1der.dyndns.org (57:57/10)
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