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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 40 feet over ground at each given band. The lengths
were rounded off to the nearest 1/16 of an inch and are for one half of the antenna.
You will require two equal lengths for each antenna.
| HF Dipole Antennas |
| Band |
Frequency |
Length of wire |
| 10 Meters |
28.450 MHz |
99-7/8" |
| 10 Meters |
28.100 MHz |
101-1/4" |
| 12 Meters |
24.950 MHz |
115-1/8" |
| 15 Meters |
21.300 MHz |
134-3/16" |
| 15 Meters |
21.100 MHz |
135-3/8" |
| 17 Meters |
18.120 MHz |
13'-3/8" |
| 20 Meters |
14.250 MHz |
16' 9- 7/16" |
| 20 Meters |
14.100 MHz |
16' 11-3/4" |
| 30 Meters |
10.125 MHz |
23' 9-7/8" |
| 40 Meters |
7.200 MHz |
32' 10-7/16" |
| 40 Meters |
7.050 MHz |
33' 6-7/16" |
| 80 Meters |
3.800 MHz |
61' 9-1/16" |
| 80 Meters |
3.600 MHz |
65' 3-3/16" |
| 160 Meters |
1.900 MHz |
125' 8-3/8" |
|
| VHF Dipole Antennas |
| Band |
Frequency |
Length of wire |
| 6 Meters |
50.500 MHz |
56-7/16" |
| 6 Meters |
52.000 MHz |
54-11/16" |
| 2 Meters |
145.000 MHz |
19-1/2" |
| 2 Meters |
147.000 MHz |
19-1/4" |
| 1.25 Meters |
223.000 MHz |
12-5/8" |
| 1.25 Meters |
224.000 MHz |
12-9/16" |
| 70 Centimeters |
430.000 MHz |
6- 1/2" |
| 70 Centimeters |
440.000 MHz |
6-3/8" |
| Please note that the 160 meter dipole SWR did not model below a 3.3 : 1 . |
|
Here is an illustration of the free space figure 8 pattern of a dipole.
Copyright © 1996-2006 Ed Oros and Bob Cocco All Rights Reserved.
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