|The dipole antenna is the
most widely used type of antenna for high frequencies and in physics
theory, is the simplest form of antenna. A one-half
wavelength dipole antenna commonly consists of two quarter-wavelength
pieces of copper wire, called "legs", placed end to end, then
trimmed to the resonant center
frequency of the desired band. The feedline is then connected to
each leg. In a dipole antenna design, the highest voltage is
found at the center end of each dipole leg whilst the lowest voltage is
found at each furthest end of the dipole leg. Thus, the
highest resistance is found at each furthest end of each dipole leg
whilst the lowest resistance is found at the center of the dipole leg.
The transmitter cycles alternating current through each leg of
the dipole, and each leg is always the inverse polarity of it's
counterpart. Any type of conductive metal wire can be used to
make the legs of a dipole; copper being the best conductor whilst steel
being the most durable conductor. Copper is a soft metal and
tends to stretch, causing the dipole leg to snap in high winds, yet
steel is a poorer conductor and is susceptible to corrosion. So,
oftentimes, copper-coated steel wire is preferred as a trade off
between superior conductivity VS strength.
Heinrich Hertz first demonstrated the concept of the dipole antenna in
1887, and Gugliemo Marconi perfected the design. Thru his
design, Marconi was able to design dipole antennae in the early
20th century to achieve
long-distance radio communications from ships at sea.
At that time, telegraph was the main means of long-distance
communication, but it required stations to be connected by wires. Many lighthouses and
certainly ships at sea couldn't be connected together by a cable, so
Marconni set out in 1897 to invent a wireless communication system.
By 1910, Marconi had equipped most passenger ships operating
in the Atlantic with his wireless radio, and provided Morse operators
to passenger ships operating between Europe and North
America for a fee. Under Marconi's license agreement,
wireless operators were employees of the Marconi Company, operated under Marconi Company rules, and were considered
officers aboard their ships. Their duties included sending important
operational and safety messages to other vessels in the area, and
messages to coastal stations for a fee. The new technology was something of a
fashionable novelty, and first-class passengers enjoyed being able to
send messages to friends and family hundreds of miles away on shore.
wireless transmitted about 300 miles during the day and about
800 - 1,000 miles at night due to the refraction of the radio waves in
ionosphere. Titanic was outfitted with the best wireless
equipment Marconi had to offer, and her multi-wire "T" dipole antenna
was over 150 feet in height, ran nearly 400 feet in length between
Titanics fore and aft smokestacks, and was made of #18 B&S
uninsulated silicon bronze wire.
I built my 20-meter dipole HF antenna from 14 gauge stranded copper
wire. I made friction end insulators from 3/4" schedule 40
PVC pipe and the center yoke from a "T" electrical fitting with a weatherproof access
cover. I trimmed my 20 meter dipole to 14.280 MHz and
achieved a 1.5 SWR.
I have talked to other hams in Salerno Italy, Germany, Chicago, Florida, Germany, Japan, and
Boston with my home made 20 meter dipole HF antenna.
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