Center-Fed Half-Wave Dipole (3-30MHz)
by Martin E. Meserve
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A Center-Fed Half-Wave Dipole is
probably the simplest of antennas to construct and use. It is usually
suspended between two supports, from it's end insulators, and has the
feedline hanging from the center. The drawing below shows the esential
parts of a dipole. A good wire to use is a #14 or #16 stranded copper
wire, for flexability and to minimize weight. Needless to
say, the end-insulators, center-insulator, and the wire need to be fairly strong,
especially when you are dealing with low frequency antennas due to their
The recommended height for a dipole is 1/2
wavelength above ground. Finding two supports at the recomended height may
be fairly easy for higher frequency antennas (15 and 10 Meters), but may
present a problem at lower frequencies (80 and 40 Meters). At low frequencies,
like 40 Meters, you would need two vertical supports that are 65 Feet
(20 Meter) high to meet the recomended height. But, don't let this bother
you too much. Lower heights will reduce the feed impedance and change the
radiation angle, diminishing the overall effectiveness of the antenna, but
the antenna will still be very usable.
The 1:1 Balun, pictured in the drawing, is a
device that transforms a balanced transmission line to an un-balanced
transmission line. The feed point of the antenna is balanced but the
coax is un-balanced. Connecting the coax directly to the antenna feed point
will work but you may experience feedline ratiation and have trouble
obtaining a good match at your transmitter.
The simplest of Baluns is known as a
Choke Balun. It doesn't really do a good job of transforming from a
balanced to an un-balanced condition but it does provide a good degree
of isolation to keep the feedline from radiating. This type of balun
is constructed from 8 to 10 loops of coax with a diameter of about
8 to 10 inches. Bind the turns together with electrical tape or UV
stabilized tie wraps.
There are also commercially available
Baluns. These are usually very well made and provide for strain relief
of the radiator elements. Some provide an attachment at the top that can
be used for hanging. Internally, these are usually made from several turns of wire
wrapped around a ferrite core. The usable bandwidth is very large making
it useful from 3 to 30 MHz. The picture on the left is only and example of
one type of commercial balun. They come in a variety of shapes and sizes.
Depending on your height above ground, you may
not be able to obtain a 1:1 SWR, however, properly adjusted it should be
some where between 1:1 and 1.5:1. Don't worry about a couple of tenths
in your SWR, just get it as low as possible, and use it. The SWR will change
anyway as you move across the band. The real point here is to just get
the dipole up as high as you can.
The dipole antenna is really only good for use on one
frequency band. Sometimes you can use it on it's
third harmonic, in a pinch. For example, a 40 Meter Dipole is usable
on 15 Meters, but the SWR may not be as good. For multi-band
operation see the sections on Trap Dipoles or Fan Dipoles.
The formulas on the right can be used as a starting
point for a center-fed half-wave dipole. It's a good idea to cut your wire
a few inches longer than the calculated values to allow for securing to
the insulators and final adjustments. The length can then be
changed incrementally for the best match. Enter your required center frequency
in the box on the right.
L (feet) = ------- L (meters) = -------
f (Mhz) f (Mhz)
Enter the required Center Frequency,
or the Frequency of Minimum SWR....
A Half-Wave Dipole for this frequency should be
x long, which would make each leg of the
dipole x in length.
Antenna Length Scaling
So, you say you cut your antenna to the
dimensions specified, but when you put it up the frequency of minimum
SWR is higher, or lower, then you intended. The equations assume that
your erecting your antenna at the recomended height of 1/2 wavelength
and there is nothing close by to interfere with it, like house wiring
or trees or just about anything. That's usually not the case and there
is the remotest possibility that you made a slight error in measuring.
But none of that really matters. The real point is that you should always
be ready to make some final adjustments.
However, if you planned well, you included some
extra wire for pruning purposes. If, at each end, you had a 1 foot loop
of wire you would probably be able to easily adjust the antenna for
any frequency offset. When you finally get the attenna to the length you
need you can cut off any excess. But a better idea is to just leave it
there. It won't bother the performance and will give you room to adjust
the antenna later, if necessary.
In the text boxes on the right, enter the
frequency where the SWR is currently at a minimum. Then enter the frequency
you would like the SWR to be a minimum. Finally enter the current dipole
dimension for one side of the dipole. Note that it is assumed that both
sides are equal and pruning will apply to both sides.
A synopsys of the scaling data is then presented
below. As we are really just dealing with a ratio, the dimension (Ft or M)
is not really needed but I included it for clarity.
Current Center Freq. (MHz)
Required Center Freq. (MHz)
Current Dimension (One Leg Only)
The current frequency of minimum SWR is
x and the required frequency of
minimum SWR is x. The
dimension of one leg, as specified above, is
x. The specified dimension would
need to be changed to x, which is
a x of