Radio transmission and reception of any variety requires an antenna to convert electrical currents from the transmitter into electromagnetic waves and vice versa. The type and dimensions of the required antenna are functions of the frequency of radio waves being transmitted or received. A dipole is one of the most basic types of radio antenna and is relatively easy to construct with basic materials and a modicum of knowledge of mechanical and electrical construction techniques, such as soldering. A dipole antenna can be used for nearly any radio purpose, including ham radio, television reception, Citizen’s Band, marine radio, etc. The only difference between a dipole for one radio service and a dipole for another will be the dipole antenna’s length.
As the name indicates, a dipole has two elements, which are usually constructed from copper wire. The wire may be insulated or uninsulated, but an important consideration is the mechanical strength of whatever gauge wire you choose to use. The two elements are separated by a piece of non-conductive material, known as an insulator. A feed line, which connects the antenna to the transmitter or receiver, is attached to the dipole’s two elements at the insulator as well.
The first step in designing your antenna is to figure out the length of the dipole’s two elements. As stated above, this is a function of the frequency of the radio service you are interested in using. Click here to see a chart listing the frequency ranges for common radio services. The total length of your dipole, to maximize its efficiency, must be calculated by the following formula: 468/frequency (in megahertz) = total dipole length in feet. You can use the aforementioned frequency chart along with the equation above to calculate how long your dipole should be from end to end (including both wire elements and the center insulator).
A quick trip to your local Radio Shack store should supply you with all the materials you need for your dipole antenna. Almost any wire will do, as long as it is think enough to support its own weight and that of the center insulators and feed line. Once you choose the wire, you can track down the antenna insulators. Radio Shack sells both ceramic and plastic insulators for wire antennas, either of which will work just fine. You could even make your own from any nonconducting material such as plexi-glass or PVC pipe, if you happen to have any of those sorts of materials around the house. You will also need to get some feed line for the antenna. For most purposes other than for CB and Amateur Radio, you should be able to get away with 75-ohm television coaxial cable or 300-ohm television twin lead wire. For communications, however, you will need to get 52-ohm coaxial cable that is made especially for communications applications, since this is what most communications radios require. Finally, you will need to get a connector to attach your feed line to the radio, which will vary based on the type of radio you have. The Radio Shack sales associate should be able to assist you in choosing specific products.
Now on to the construction techniques. The first step is to measure and cut the dipole’s wire elements. You should leave a little excess when you cut the wire so you will have some extra wire free to attach the elements to the supporting insulators. Twist the wire elements through the holes in the insulators, and solder with a soldering gun or high wattage soldering iron. It is crucial to have a high quality soldering tool to avoid cold solder joints that may be brittle and have a poor mechanical or electrical connection. After attaching the antenna elements to the center insulator, so the same with an insulator on each end of the antenna. You will use the end insulators to attach supports to your dipole so you can string the wire antenna up between trees or off a raised support.
After having attached the antenna elements to the insulators, you will need to strip back the insulation from your feed line so that you can attach the feed line to the dipole’s elements. Within the insulation of the feed line, you should find two conductors. One conductor should be attached to each dipole element, ensuring that there are no short circuits made between the two conductors. It is also a good idea to wrap the feed line around the center insulator as a type of stress-relief on the electrical solder joints. After the feed line is in place, solder the feed line’s conductors to the dipole elements, again ensuring that a good solder connection is made.
The last step is to attach the connector to the other end of the feed line so that you can attach your new dipole antenna to your radio. I consider this to be the most difficult part of any antenna construction job. Radio Shack sells a special coaxial cable stripper that is adjustable for different types of cable. Follow the directions that accompany both the cable stripper and the coaxial connector. It is crucial to avoid electrical shorts within the connector itself, so be very thorough with this portion of the dipole’s construction. The old fashioned solder connectors are much more reliable than the twist-on versions that are now available. They take more work to attach to the feed line, but are more than worth it because of the reliability of a good solder connection. This is especially important if you will be using your new dipole for radio transmission rather than mere reception. A short circuit in a transmitting antenna usually means a burned out transmitter that is very expensive to repair. The consequences are not as costly if you inadvertently cause a short circuit in a receiving antenna, but the antenna will be next to useless if you carelessly cause a short circuit at either end of the feed line.
One the above steps have been completed, all you must do is mount the antenna in its permanent location with supports from the end insulators and/or the center insulator and to attach the antenna to your radio with the feed line and its new connector. It is important to remember that a dipole antenna is bidirectional, though, and works best when transmitting or receiving perpendicular (at right angle) to the direction of the wire itself. You should factor this bidirectional quality of a dipole antenna when choosing how to orient the final product. Years of quality radio performance should be your result, if you have followed all steps carefully, and you should notice a marked improvement from other sorts of unidirectional antennas (such as telescoping rod antennas) that you are likely familiar with. This is due to the antenna’s energy being focused in two lobes on either side of the antenna rather than in a single radial pattern. Increased performance and better receiving/transmitting range is often the result of this “gain” over mono-element antennas. So happy antenna constructing, and may you enjoy your radio all that much more!
