CALCULATING ANTENNA LENGTH

No matter whether you plan on using a dipole, end fed or sloping wire antenna, it is essential in all radio communications that you use the correct lengths of antenna wire for the frequency you're using to allow for an optimum working distance. The precise antenna length varies slightly from the equation below dependant on the resistivity of the wires used and environmental conditions, but in general it provides an accurate guide as to the length you should use.

Free Space Equation

C = Speed of Light (in meters per second) Approximately 300,000,000 m/s
f = Frequency (in Hertz)
λ = Wavelength (in meters)

The "Free Space Equation" allows us to work out the wavelength needed within a vacuum environment (where environmental conditions and resistivity of wire don't exist). From the equation triangle you can work out either the frequency or wavelength (the speed of light remains constant).

Wavelength (m) = Speed of Light (m/s or m s^-1) / Frequency (Hz)
Frequency (Hz) = Speed of Light (m/s or m s^-1) / Wavelength (m)

e.g. When we need to find out the wavelength for an antenna working on 145MHz (145,000,000 Hz) we do the following equation.

Wavelength (m) = 300,000,000 (m/s) / 145,000,000 (Hz)

Since we're working with MHz we can simplify this by removing the last 6 zeros from both the frequency and speed of light:

Wavelength (m) = 300 ('megameters' per second) / 145 (MHz) = 2.07m

Wavelength (m)
	Frequency (MHz)

So How does this help us?

Well when we consider that all the antennas you learn about are taught as ratios of wavelength e.g. ½ λ Dipole or ¾ λ end-fed it becomes very simple from then on to work out the length you need.

When using clansman braid however, in order to take into account its resistivity it's best to refer to the antenna length plate affixed to radios such as the UK/PRC320 as the maths will have already been done for you.

Steve Fraser of Duke of Yorks Royal Military School has produced a small program which calculates the correct length of wire that you need depending on the type of wire and the frequency. It is available here as a "zip" file. Please save the file to your computer in order to use it.