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How to Calculate Antenna Feed Line Length

An antenna feed line connects a radio transmitter or receiver to an antenna. The distance depends on the physical relationship between the two and the objects that may obstruct running the cable. One of the most common mistake engineers and ham radio operators make is to assume the necessary length equals the distance from the transmitter to the tower plus the height of the antenna on the tower. After ordering the cable, they find out that they must circumvent one or more objects and don't have enough. Because feed line cable is difficult to splice, many such mistakes can be quite costly.

Things You'll Need

Sketch pad
Tape measure

Instructions

- 1
  Make a diagram of the layout to include the radio equipment and the tower.
- 2
  Plan a proposed route and walk through it making special note of places to pass through walls and objects that you need to bypass.
- 3
  Draw a proposed route on the sketch, making a separate line for each straight section in vertical, diagonal or horizontal directions -- no matter how short. If you need to bury the line, include the depth underground.
- 4
  Measure each distance represented by a line and write it near the line in the drawing.
- 5
  Write all distance into a column and add them up. Double check to make sure you didn't miss one.
- 6
  Add a 10 perfect "fudge factor" to your total. For example, if your measurements equal 110 feet, add 11 feet and order 121 feet of cable.

Ham Radio