favorite type of antenna.
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 shield and center conductors are then
connected to a 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.
Instantaneously, the dipole is charged negatively on one leg, beginning
at zero and rising to a maximum charge proportional to the power
supplied by the transmitter; then the charge decreases to zero.
That leg of the dipole becomes charged positively on the next
half-cycle of the existing waveform. This process creates a rising and
falling electric field from one leg of the dipole to the other, which
moves away from the antenna. Similarly, the current in the dipole
establishes a magnetic field encircling the dipole, which also moves
away from the antenna. The electric and magnetic fields together
form the radiated electromagnetic field, which is the "radio signal"
that is heard by the receiving antenna. In an ideal dipole
antenna design, identical current will exist on each leg of the
dipole. 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.
A German physicist named Heinrich Hertz first demonstrated the concept
dipole antenna in 1887, and an Italian inventor named Gugliemo Marconi
perfected the 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. Marconi's
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 the
Earth's 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 have talked to other hams in Italy, Germany, Germany, Japan, the
Caribbean, and Ireland with my home made 20 meter dipole HF antenna.