|How To Build a 1/2 Wave Dipole:
Determine your center operating frequency, and using the following
formula, calculate your wire length in feet:
For example, if I wanted to build a 20 meter dipole
for a center frequency of 14.300 MHz, I would divide 468 by 14.300 and get
the result of 32.72 feet. Multiply that by the velocity factor of the
wire used, for the overall total length. If I am using 12 AWG stranded
copper wire with the jacket on (I never use bare) then I would multiply my
length by .975 for an overall total length of 31.9 feet. If you
multiply the .9 by 12, that will give you your length in inches. So 30 feet,
10 and 3/4" is very close.
If I take 31.9 and divide that by 2, I get 15.95
feet, which is one leg of my 20 meter dipole. So cut two wires 17 feet
long and wrap the wires thru the center insulator and the end insulators.
Leave a little stub hanging at the ends so we can tune the antenna once it's built.
There are countless ways to make the feed point of
your dipole. Some people use plexi-glass, some use PVC T-fittings,
some use a piece of thin PVC Pipe, there are feed points for sale at most
ham shops. We called them "Cobra Heads" in the military.
Alpha-Delta makes a great dipole "kit" called the Delta-C that includes two
end insulators and the feed point head with a replaceable lightning arrestor
Whatever you decide to use is of little consequence
as long as you provide a means to keep water from getting into the end of
your feed line.
The left picture above shows the feed-line soldered
to the wires on an insulator. This design is not the correct method,
the coax should be looped over the insulator so as to remove the strain from
the solder joints.
The right picture above shows a good example of an
end-insulator. Note, you can leave some wire hang down from the
insulator, creating "tuning stubs" to help match your antenna.
The above formulas and the lengths given in the charts below
can also be hung as an "Inverted Vee" configuration.
Dipoles are not omni-directional, in other words, looking
down on a dipole, the signal radiates perpendicular to the antenna itself,
not in all directions like a vertical antenna does. Therefore, if you
hang a dipole with the end insulators oriented to the East and West, the
antenna will primarily transmit and receive to the North and South.
The picture below is looking directly down at a transmitting dipole.