N = number of turns
L = inductance (uH)
r = coil radius (mm)

The longer the antenna (up to 987mm for 72MHz), the better it will work. Put as many bends in the antenna as you like as long as no bend exceeds 90 degrees and the antenna does not double back on itself.

If the antenna must be shorter than 987mm: On the horizontal axis at the bottom of the graph, find the position of the loading coil*. Go up vertically until you intersect the curve that matches the length of your antenna. Then move horizontally to the left and read off the load inductance on the vertical axis. Use the formula (483L/r)1/2 to compute the number of turns required for the loading coil.

*The position of the loading coil is the distance from the feedpoint divided by antenna length, in percent. The further the loading coil is from the feedpoint, the more range you will obtain. However, inductance increases rapidly as the coil approaches the end of the antenna. 40% to 60% is usually a good compromise.

Notes: The feedpoint is the end of the antenna that is attached to the receiver. A loading coil is an inductor that is inserted into a shortened antenna to compensate for the missing length. The inductor is a coil of wire. Make the diameter of the coil equal to twice its length. Where weight is critical, 40 gauge plastic-coated copper wire works nicely. The graph applies to the 72MHz band.