A quick way to operate the HF amateur radio
bands is to construct your antenna from wire. The dipole is a wire antenna
that can be easily constructed, but you must be sure that you have enough
space to put one up. On the lower frequencies the dipole lengths become
quite large. A dipole on the 40 meter band is nearly 66 feet long!
There are a couple of tricks though that you can use if your space is
limited. For example, you could just extend the center part of the antenna
(out from the feed point) as far as you have room, and then let
the ends hang down vertically for the remainder of the antenna. The ends
of a dipole are not responsible for much of the radiated signal so this
method will work well.
Another method is to zig-zag the legs of the antenna in a sort of Z
shape. This will reduce the area required to mount the antenna.
The main thing to remember is to try and not fold the antenna
back on itself. This could cause cancellation of some of your signal!
There are just two easy steps to design a dipole. First,
decide the frequency of operation, and second calculate the length.
The formula for the calculation is
468 / Frequency (in MHz). Let's try an example: Design a dipole for the 10 meter band.
The frequency that is chosen might be 28.5 MHz. Plugging this in to the
formula (468 / 28.5) gives a dipole with a length of 16.42 feet.
A good rule of thumb is to cut the wire a little longer than you need, then you can shorten the wire to adjust your match.
The feed point impedance is around 75 ohms so the SWR should be about a
1.5 : 1 . (Note that the actual impedance and the SWR will vary do to location,
mounting height, near by structures, etc.)
We've provided you with computer modeled dipole lengths which were designed for the best SWR over ground at each given band. The lengths were rounded off
to the nearest 1/16 of an inch.
VHF and UHF
Here is an illustration of the free space figure 8 pattern of a dipole.
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