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How to Calculate a J-Pole

The J-Pole is a simple omnidirectional vertical antenna mostly used for VHF radio frequencies measuring 30-300 MHz. As the name suggests, the antenna is in the shape of the letter J. The antenna's dimensions are directly related to the radio frequency wavelength for which it will give optimal signal transmission and reception. Various materials are used to make these antennas, including 1/2-inch copper pipe, aluminum pipe, copper wire and conduit tubing. However, the different materials will affect the length calculations of the antenna as well as where the antenna wire is to be connected to the antenna.

Things You'll Need

Hand calculator

Instructions

- 1
  Divide the J into four portions for doing the calculations. The longest upright piece is length A. The bottom of the J, which is a horizontal piece, is length D. Length B is the small upright portion of the J, and length C is the distance above the bottom of the J where the antenna cable is to be attached.
- 2
  Calculate the lengths by choosing the frequency in megahertz (MHz) for which you are designing the antenna. Use that frequency in the formulas for calculating the different parts. For length A, the formula for determining the length (in feet) is 705/MHz. For example if your frequency is 50 MHz, the length for A will be 705/50 = 14.1 feet. The formula for length B = 234/MHz. Length C and D are calculated by the same formula, 23.4/MHz.
- 3
  Adjust the calculated lengths to compensate for the velocity factor of the material of construction will affect your calculations. For 1/2-inch copper or aluminum tubing, the actual lengths will be 95 percent of the calculated values. For copper wire, the final lengths should be 82 percent of the calculated values. Use the appropriate velocity factor for the material you plan to use.

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