Dipole antennas are one of the simplest antennas to make as well as being one of the most effective. They’re inexpensive, too, when compared to many other different types of antennas that you can buy, especially when you make them yourself.
The term “dipole antenna” is actually a bit of a misnomer. A more complete name for the antenna we’re going to build is “center-fed, half-wavelength, dipole antenna.” Dipole antennas that are a half-wavelength long are said to be “resonant” and have an input impedance of about 70 ohms.
Figure 1 shows a typical dipole antenna. It consists of:
- a center insulator,
- two end insulators,
- two quarter-wave elements, and
- a feedline, usually a length of coaxial cable.
The two quarter-wave elements can be made from almost any kind of wire, but I like to use antenna wire because it will stretch less than hookup wire. AmateurRadioSupplies sells many different kinds of antenna wire, including the traditional hard-drawn copper wire and FLEX-Weave antenna wire.
You’ll note that there are many different gauges available, from 26-ga. up to 12-ga. If you’re antenna must be stealthy, you could go with a smaller wire, but if stealth isn’t a problem in your installation, I’d suggest the 14-ga. wire. 14-ga. is stronger than 26-ga. wire and will easily handle up to 100 W.
In addition to the wire, you’ll need a center insulator and two end insulators. A good choice for the center insulator is the Budwig HQ-1. There are many other options–you can even make your own, but one good thing about using the HQ-1 is that it provides the connector to which you connect your coax. For the ends, you can use the Budwig HQ-2 end insulators or the generic dog bone insulator.
You’ll also need a feedline. The input impedance of a dipole antenna is 70 ohms, and you can use either 70-ohm coax or 50-ohm coax to feed the antenna. 50-ohm coax is somewhat of a mismatch, but not enough to make a big difference. And, if you used 70-ohm coax, you’ll still have a mismatch at the transmitter end as most modern transmitters have a 50-ohm output impedance.
If you’ll be running 100W or less, as most amateurs do, RG-8X is a good choice. If you buy a cable with the PL-259 connectors already installed, you’ll be able to get on the air quickly.
Let’s Build an Antenna!
Now, you’re ready to build the antenna. As you can see from Figure 1, the overall length of the dipole, in feet, is 468/f(MHz). For a dipole designed to operate on 28.3 MHz, the overall length would be:
L = 468/28.3 = 16-ft., 6-in.
The individual element lengths would then be half that, or 8-ft., 3-in. When you cut the lengths of wire, you might want to tack on a few inches. It’s much easier to make a piece of wire shorter than it is to add some wire to make it longer. If you make each element 8-ft., 9-in. long, you should be OK.
Once you’ve cut the elements, you’re ready to begin assembling the antenna. If you purchased an HQ-1 center insulator, you would thread 3-in. of the antenna wire through one of the holes in the HQ-1 and wrap the wire around the post and wire, as shown in Figure 2. With a pair of pliers, hold the wire at C and solder the wire at D. Repeat these steps on the other side of the insulator with the other wire element.
Next attach the end insulators. Thread about three inches through one hole of the insulator and wrap the wire around itself. DON’T solder this wire, as you will probably have to adjust the length of the antenna.
To get the antenna up in the air, attach some rope to the other ends of the end insulators and then get the antenna as high up in the air as possible. Remember to connect the coax to the antenna before hoisting it in the air.
Once the antenna is in the air, you can check its SWR. The SWR is a measure of how well the antenna is tuned.
You can do this with an antenna analyzer, if you have one, or with a standalone SWR meter, or with the SWR meter found in most modern radios. If the SWR is above 2:1, you probably want to adjust the antenna length. If the SWR goes up as you increase frequency, that means the antenna is too long, and you have to shorten it. If the SWR goes up as you decrease frequency, then the antenna is too short and you have to lengthen it.
Once you’re satisfied with the SWR of your antenna, you may want to weatherproof the coax where it connects to the antenna. You do this by wrapping some COAX-SEAL around the PL-259 connector where it connects to the center insulator. If you do this properly, it will prevent water from getting into the coax and damaging the coax.
That should do it! When the 10m band is open, you’ll be able to work stations all around the world. It will be even more fun because you’ll be doing it with an antenna you built yourself.