Figure 29. Effect of antenna circumference on wave
velocity.
c. Another factor that affects antenna length is the feed line
that connects the transmitter to the antenna.
The insulators also
affect antenna length. This is called end effect and is compensated
by making the antenna 5 percent shorter. Thus to find antenna length
you use the formula
N = number of half waves
L = .95(492/F) or said another way L = N-.05(492)/F
L = 468/F
(F = frequency in megahertz)
d. The half-wave antenna is the shortest antenna that a
transmitter will load efficiently.
This is called a resonant
antenna. Resonant means that the electrical length matches or equals
the physical length of the antenna. The purpose of the antenna is to
radiate as much of the power of the transmitter as possible.
e. Impedance.
Half-wave antennas fed in the center have an
impedance of 73 ohms.
Half-wave antennas fed at the end have an
impedance of 2500 ohms.
Off-center fed antennas normally have an
impedance of 500 to 600 ohms.
47
Previous Page
