JOVE RJ1.1, Assembly Manual

8

Components

The antenna is composed of several types of components (Figure 1) including

wire, coaxial cable, connectors, insulators, rope, supports, and hardware.

Figure 1a and 1b. Antenna Parts.

Wire – the copper wire is used for the antenna elements. We are constructing two

identical half-wave dipole antennas and phasing them together with feed line. The entire
length of the dipole is therefore, equal to the length of 1/2 of the wavelength (

λ

 ) of

radiation to be detected. Thus each side of the dipole antenna is 1/4 wavelength long (See

Schematic on previous pages). Since the Radio JOVE receiver is tuned to the frequency

of 20.1 MHz (M=mega=10

6

), the wavelength is 48.968 feet (14.925 meters). A useful

formula for calculating the half-wavelength for an "ideal" dipole in free space for a

specific frequency is:

λ

/2 (in feet) = 492 / frequency (in MHz)

or         

λ

/2 (in meters) = 150 / frequency (in MHz).

For practical antennas, however, the measured values are smaller than the “ideal” values.

This is a result of resistance in the wire and end effects of the dipole. These two

properties effectively shorten the length at which the wire will resonate or most

effectively receive radiation at a frequency of 20.1 MHz. To calculate the "practical"

half-wavelength of antenna use the formula:

λ

/2 (in feet) = 468 / frequency (in MHz)

or         

λ

/2 (in meters) = 142.5 / frequency (in MHz).