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Whip antenna parameters

Question

What creating antennas from wire, what is length and size of the wire to be used?

Answer

The length of the antenna should be measured from the point where it leaves close proximity to the ground, or from the input/output. If a whip is mounted on a box, and connected with simple wire, then that wire becomes part of the antenna.

In the case of the whip, there must be a connection to a ground, even if the ground plane area is nothing more than circuit traces and a battery. The whip and ground plane combine to form a complete circuit. The electromagnetic field is set up between the whip and the ground plane, with current flowing through the field, which completes the circuit. Ideally, a ground plane should spread out at least a quarter wavelength, or more, around the base of the whip, ground planes can be made smaller, but this will affect the performance. The ground plane area must be considered when designing an antenna.

The length is inversely proportional to the frequency and may be calculated by: wavelength in cm = 30,000 / frequency in MHz. Thus, for a quarter wavelength:

l (cm) = ¼ ( 30,000 / freq (MHz) )

The formula should only be considered as a course measurement however since the length may actually be shorter if the whip is overly thick or wide, has any kind of coating, or is not fed close to the ground. It may also need to be longer if the ground plane is too small.