Wire antennas have been in use since the earliest days of radio. They are simple to construct, easy to put up and fairly effective. The length of wire required for your antenna revolves around one simple question; namely, what is the wavelength of the frequency you will be using?
It's deceptively simple to find the wavelength at whatever frequency you happen to be using: just divide the speed of light in free space (300,000,000 meters per second) by the frequency in use (expressed in Hz, or cycles per second) and you have the wavelength in meters:
By all accounts, the most popular wire antenna in the ham radio world at frequencies below 30 MHz is the center-fed half-wave dipole. It consists of a straight wire one-half wavelength long, fed in the center, so that it is broken into two quarter wave length segments. You can use the formula given above to derive the length of these segments at your frequency. Theoretically, at this frequency such an antenna will look like a pure resistance at its feed point (the reactance will be zero) and will be easiest to match to a feed line. This condition is also known as "resonance".
Well, it's not quite that easy. Since you will probably be putting your antenna up near the surface of the earth, and not in outer space, there will be some adjustments to be made. The main lesson to be learned is that the resonant frequency of a piece of wire (i.e., its effective wavelength) is affected to some degree by nearly everything in its vicinity. Buildings, trees, other nearby antennas or wires and distance from the ground can all conspire to upset your calculations. The thickness of the wire compared to the wavelength is also a factor. Fortunately, most of these effects are minor and usually don't amount to more than a few percent.
The software package provided here is designed to help you in determining how much wire you need to put up in the air to get the energy from your transmitter started on its journey through the ether. The program allows you to select metric or English units for the final result, and frequency can be entered in either MHz (megahertz) or GHz (gigahertz). In the Mode box, you can choose whether you want to find the full wavelength or quarter wavelength. In this box you will also find a check box to reduce the free-space result by 5%, as this is a good start for a wire antenna in the real world.
Please note that the frequency number entered in the text box must be greater than 0.001. An entry smaller than this or a non-numeric entry will cause a warning to appear.
Also, remember that when putting any wire antenna into operation, there will always be a little trimming and tweaking required, so consider the result given here as a good starting point only -- or, you might just get lucky!Click here to download the Wire Antenna Size program