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Formulas To Design Your Own Dipoles And Inverted Vees
INTRODUCTION-The longwire antenna is a very effective antenna for the listener who wants
to cover all of the shortwave bands from 530 KHZ to 30 MHZ. However if you have some
favorite frequencies that you listen to on a regular basis you may wish to consider a dipole
antenna. This antenna is a fairly easy to construct antenna and will give you better reception on
the frequency it is cut for. Think of a dipole as a longwire that has a insulator in the middle.
FREQUENCY-A dipole antenna will not only work well on the frequency it is cut for, but also
for the multiples of that frequency. For example if you cut a dipole for 7.0 Mhz will also
work well on 14 Mhz, 21 Mhz and 28 Mhz. This way if you can pick and choose your frequency
you can make one antenna work on two or three bands.
LENGTH- To find out how long the antenna should be all you have to do is fill in a simple
468 divided by FREQUENCY IN MHZ = LENGTH IN FEET
300 divided by FREQUENCY
IN MHZ = LENGTH IN METERS (wavelength)
168 divided by FREQUENCY IN MHZ = DIPOLE LENGTH
(with end effect
That is the only formula you need ever know to build a dipole antenna.
Calculator purposes: Each dipole leg length is a
half wave leg
CONSTRUCTION- Once you have selected a frequency and calculated the length of wire you
need add two feet to this length. This is done so you will have six (6) inches of wire at each end to
wrap around the insulators. Once you have this extended length of wire cut it in half. This will
give you both sides of the dipole.
Attach an end insulator to one end of each piece of wire. You can use the egg shaped
insulators sold by many radio supply stores or make your own out of a piece of plastic. This can
be done by cutting a piece of heavy plastic or plexi-glass to a size of about six (6) inches in
length and about 2-3 inches wide.
Drill a small hole one to one and a half inches from each end of the plastic to wrap the wire
around. It is best to solder these connections and wrap them in a sealant tape to keep the effects
of the weather from harming them.
The other free ends are attached to a center connector which you can buy with a built in coaxial
cable connecter, or make your own. This will look similar to the end connectors but you will
have to find a way to secure the coax lead wire to the insulator. If you build your own when you
attach the coax to the ends of the wires, insure that you solder and wrap the connections. One
wire will go to the center of the coax, while the other wire will go to the shielded braid of the
coax. This will give you a perfect half wave dipole. You should also wrap the coax fitting of the
commercially available center insulator to keep water and other moisture out. Moisture will ruin
the connections on any type of insulator and make the antenna less effective or at worst
MULTI-BAND DIPOLES- As was stated above you can use the dipole on the harmonics or
multiples of the frequency it is cut for. However if you are short on space you can build a
multi-band dipole. This way you will get an antenna that will operate on several frequencies.
Instead of using a single strand of wire you can use wire that has several insulated wires in it.
These MUST be insulated wires to insure that they do not touch each other. You then cut the top
wire to be the longest, the second wire to be the second longest, the third wire to be the third
longest etc.. Only the longest wire is attached to the end insulators and all wires are fed to the
center insulator to attach to the coax feed line.
INSTALLATION- Once you have the antenna cut all you have to do in put it between two
masts. Make sure that you use the free side of the end insulators to attach some rope. Tie this
rope from the end insulators to the masts. Leave some slack on the antenna. If you pull too tight
the antenna will break in the wind or if snow and or ice should coat the antenna. KEEP AWAY
FROM OVERHEAD WIRES!! Keep away from these as should the antenna ever come into
contact with an overhead wire you will do permanent damage to your radio if not yourself. All
you have to do is feed the coax to your radio and listen to the stations come in. It would be best
to install a lightening arrester in the coax feed line to help protect your receiver. These are
available from many radio supply stores. Follow the instructions carefully! In areas where thunder
storms or snow storms are common a lightening arrester is a must for safety.
You can install them flat or at an angle. If at an angle they will be more directional the direction
that they are sloped. You can also install them as an inverted V shape. This dipole has a higher
center with lower ends to save on space in smaller back yards. All three versions work well.