Antenna Length
The length of wire that should be used to construct a half-wave or quarter-wave antenna is the length of a half or quarter wavelength respectively of the frequency the antenna is intended to operate on. If you have the exam question gives you the wavelength of the frequency, (e.g. 6 meter half wavelength wire dipole), your most of the way there. Just divide the wavelength by 2 for a half wave antenna, or by 4 for a quarter wave antenna and convert to the units of length the exam question asked for, (inches for example). If the frequency is given, (e.g. antenna for 146 MHz), first calculate the wavelength using the formula:
There’s one last thing to notice. The wavelength equation above has frequency in the denominator. So, for a higher frequency radio wave, the wavelength is shorter. To adjust an antenna so that it works on a higher frequency, you just need to shorten it appropriately.
Example
For the exam question:
What is the approximate length, in inches, of a 6 meter 1/2-wavelength wire dipole antenna?
We get:
Then, there are about 36 inches in a meter
The closest answer to this number is 112 in.
Exam Questions:
What is the approximate length, in inches, of a quarter-wavelength vertical antenna
for 146 MHz?
A. 112
B. 50
C. 19
D. 12
What is the approximate length, in inches, of a 6 meter 1/2-wavelength wire dipole
antenna?
A. 6
B. 50
C. 112
D. 236
How would you change a dipole antenna to make it resonant on a higher frequency?
A. Lengthen it
B. Insert coils in series with radiating wires
C. Shorten it
D. Add capacity hats to the ends of the radiating wires
More About It:
The half-wave dipole antenna is one of the easiest antennas to build. It consists of two wires, each about 1/4 of the wavelength that will be transmitted/received as shown below. The wires should be supported horizontal to the ground, and ideally 1/4 wavelength above the ground. For in depth information on this antenna type, see Wikipedia:
http://en.wikipedia.org/wiki/Dipole_antenna
