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How to Calculate for a Radio Wave Antenna

Antennas must be the proper length to work efficiently. If an antenna is too long or short, the standing wave ratio (SWR) will rise above acceptable limits. The SWR measures the amount of radio frequency (RF) energy that is reflected back to the radio instead of going out the antenna. The formula for calculating the length of a 1/2 wave antenna is: 468/f (MHz) = length of antenna in feet. The "f" is the frequency in megahertz. The formula is calculated for a 1/2 wave antenna, because full wave antennas are often too long to be practical. A full wavelength is the distance between two crests of a radio wave.

Things You'll Need

Calculator

Instructions

- 1
  Calculate the center frequency of the antenna to calculate. Add the lowest frequency to the highest frequency and divide by 2. For example, the center frequency of an amateur radio that transmits and receives between 1,800 and 2,000 MHZ is 1,900 MHZ [(1,800 + 2,000)/2]. Radios usually display the operating frequency in megahertz. If not, hertz is converted to megahertz by dividing any given frequency in hertz by 1,000. If you do not know whether your display is in hertz or megahertz, check the manual.
- 2
  Enter 468 in your calculator and press the "divide" key.
- 3
  Enter the center frequency in megahertz and press the "equal" key. This is the length of your 1/2 wave antenna in feet.

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