Monoband HF Dipoles - Tips and Hints for Construction and Use
Half-wave Dipoles and Inverted Vees are very easy
to construct, and are great for homebrew projects. You may purchase
commercially made dipoles or a kit with end insulators and center insulators
with coax connectors built in; or, make it homebrew from scratch and make your
own hardware and purchase the wire. If
you are going to use a balun at the feedpoint, depending on how much home brewing
you want to do, you can purchase a balun to use with the other materials you have
gathered or perhaps take a shorter route and purchase a center insulator with
the balun built in.
The antenna is called a half-wave antenna because
its length corresponds to an electrical half wave at the frequency for which it
is cut. The formula to calculate the
(approximate) overall physical length of a dipole is:
Shown below are typical
installation configurations, i.e. as a Flat Top supported by trees or
structures of some type at each end, or, as an Inverted VEE requiring a center
attach point and lower attach points at the ends.
Half-wave Flat Top Dipole
Half-wave Inverted Vee Dipole
The calculated length of
wire given by the formula above is cut in half to make both sides of the dipole
equal. Thus, this is called a balanced
antenna because it is fed at its exact center. The above formula gives an
approximate result because the length at the desired resonant frequency is affected by its operating environment
conditions. It is recommended to cut your dipole wire 2-3% longer than the
length given by the formula·
- Raise the dipole to its operating height
- Measure the SWR at several frequencies within the intended frequency
- Note the frequency (F_min) at which minimum SWR is obtained
- Trim both ends of the dipole equally down to the final length to
achieve lowest SWR at the desired frequency range.
You can feed your dipole
with coax. Acceptable feedline can be 50
O coax, such as RG-58, RG-8X, RG-8, RG-213, or 75 O types such as RG-11, RG-59. The coax should be sized to the power
handling capability you intend to operate.
A balun is not absolutely required, but a good
idea. Coaxial cable is unbalanced. When feeding a balanced load, such as a
dipole, with an unbalanced transmission line, the antenna will induce RF
currents on the outer shield of the coax.
There are a number of fine “current” type baluns commercially
available. For the advanced and
determined, builder one can also homebrew his own and info on baluns is readily
When feeding the single
band dipole with an unbalanced feedline (Coax), the use of a current balun will
force equal currents into the unbalanced legs of the dipole. Thus, typical properties and radiation
patterns are more predictable.
dimensional HF Dipole free space horizontal azimuth radiation pattern.
radiation is at right angles to the antenna wire. This
pattern in free space is with the dipole running North/South, thusmaximum
radiation is equally East and West.