Although some of the basics of antenna construction and materials are covered here, this is not meant to be a
definitive course on building your own antennas. Many books have been written about antenna construction and you
could easily fill a few volumes on the subject. Instead, this section is mostly just intended to help you to choose the
type of antenna that would best work in your situation, and how that antenna should be constructed. For further
information on basic receiving antenna construction, see Build Your Own Shortwave Antennas (2nd Edition) by
Andrew Yoder from TAB/McGraw-Hill. Note, however, that this reference is not intended for transmitting antennas.
Most typical HF antennas are large (or at least long) because of the very long wavelengths of these signals. For
example, a full wavelength at 30000 kHz is 30 feet 14 inches and a full wavelength at 1700 kHz is a whopping 550
feet 8 inches! Because of the lack of space for such large antennas, most are designed to be either a half or a
quarter wavelength. Because of the large size and the low power levels involved (usually under 1000 watts for most
HF installations), most HF antennas (except for the mobile whips and the rotatable antennas) are constructed out
of wire.
Different types of wire can be used for HF antennas, but generally stranded copper wire is the best. Copper is
the best low-cost conductor (don't even think about using stranded silver, platinum, or gold!), so it allows more of
your signal to pass from the conductor to the air. Also, it is strong, won't easily stretch (unlike aluminum wire), and
won't quickly corrode away (unlike steel or iron). Stranded, rather than solid, wire is the best because the wires will
probably be bending and swaying in the breeze for most of their time in the air. As a result, an antenna made from
solid copper wire will break much quicker than one made from stranded copper wire.
The gauge of the wire is a factor that is important when constructing a transmitting antenna, but not a receiving
antenna. The importance of the wire gauge relates to a principle known as skin effect. Skin
effect is an interesting principle, whereby all of the signal from the transmitter is
concentrated on the outside ("skin") of the wire. Beyond a certain breakdown surface area-
to-signal ratio, the wire will begin to heat up. In one instance, a broadcast engineer was
using over 1000 watts into a small-gauge dipole antenna. The wire gauge was too small to
support the output power, and the wire heated up. After a few minutes of transmitting, the antenna wire burned up
and the remains of the dipole fell to the ground! For best results with powers under 1000 watts, you should use any
of the following wire gauges: #6, #8, #10, or #12.
The gauge of the wire also has an impact on the usable bandwidth of the antenna. The bandwidth is the
frequency range within which the antenna can be used for transmitting (without any matching devices) or most
effectively used for receiving. The bandwidth depends on the type of antenna that is used, the gauge of the wire, and
several other factors. A typical antenna for base operation can be used for approximately 200 to 300 kHz above and
below the frequency that it is cut for. The problems of having your antenna out of resonance are that it causes high
SWR, which is covered further in one of the following sections.
53
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The Bandwidth is the
frequency range
within which the
antenna can be used
for transmitting.
ANTENNA CONSTRUCTION BASICS
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