Back                   Half-Wave Antenna Design

Contrary to popular belief, in simple theory one antenna can only truly allow a radio to transmit on one frequency. An antenna can be 'tuned' to work on other frequencies by using an antenna tuner or bridge such as this one. Many people do not realize this because many radios have mechanisms built in to overcome this problem without user intervention. It is important to note that when you 'tune' an antenna even by built in mechanisms you loose some of of the signal strength that can be delivered to your radio. In other words, you want to build an antenna to closely match the frequency you will most likely use.

One simple antenna for use on Amateur Radio frequencies is the Half-Wave dipole. This type of antenna is useful for frequencies between 1.8 and 30 MHz. A Half-Wave dipole is simply two pieces of wire that have an overall length of 1/2 the distance of a radio wave. The two wires are not connected in the middle, but rather each side ties to its own end of a feed-line such as coaxial or twin-lead cable. See diagram.

Use the calculator below to determine the overall length of a half-wave antenna. You will need to enter the frequency in Megahertz (MHz) which translates to millions of cycles-per-second. On an FM broadcast radio it is the same as the number that is on your radio dial such as 105.9 or 92.9. AM broadcast radios on the other hand use frequencies that are lower and measured in KHz or KiloHertz which means thousands of cycles-per-second. If your radio measures frequency in KHz then you will need to convert it to MHz for an accurate measurement of antenna length. You can do this by dividing the number in Kilohertz by 1000. For example if your radio station is on 880 in KHz, 880 / 1000 = 0.880 MHz.

Note: Antenna performance is measured in db or decibels of gain compared to an Isotrophic Radiator. An Isotrophic Radiator is a theoretical antenna that has a radiation pattern of a perfect sphere. It is not possible to construct an Isotrophic Radiator antenna, however it serves as a reference to measure other antennas performance against.

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