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.

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