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How to Make Vertical Dipole Trap Calculations

Vertical dipole trap, or "Marconi" antennas, use the ground beneath the antenna as a reflective element. These antennas rise from the ground surface to a height specified by the target transmitting frequency. When the calculations are executed properly, this simple antenna design is effective. Calculating the proper lengths and impedance needed to achieve proper performance is based on standardized accepted equations, virtually guaranteeing proper performance.

Things You'll Need

Scientific calculator

Instructions

- 1
  Enter your target frequency in megahertz into the calculator. Divide this figure into 468.
- 2
  Multiply the resulting figure by .25 to determine your quarter-wave length. For example, dividing 3.8 (megahertz) into 468 equals 123.16. Multiplying 123.16 by .25 equals 30.79 (feet.) This is the length of each leg of the antenna.
- 3
  Use the resulting figure to construct your antenna to the proper height, remembering to keep it as isolated as possible from trees, buildings and other items that interfere with transmitted signals.
- 4
  Calculate the efficiency of the antenna by plugging in the value of the input resistance of the cable used to connect the antenna to the transmitter (measured in ohms.) Coaxial is 50 to 75 ohms, while twin-lead wires are normally 300 ohms. Divide 36 into this figure to determine efficiency. This shows you how much transmission power is used or wasted, respectively.

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