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Half-Wave and Quarter-Wave Antennas

 Half-Wave and Quarter-Wave Antennas

1. Basic Theory.

a. The antenna is part of the electrical circuit of the

center;">transmitter and receiver. As mentioned earlier, radio waves travel

in free space at 300,000,000 meters per second. Our antenna is not

in free space but erected over and near terrain features which affect

antenna length. For that reason, the physical length of the antenna

is shorter than the electrical length.

b. There are several factors which cause the antenna to be

physically shorter. As the diameter of the antenna wire increases,

the velocity or speed of the radio waves is slowed, decreasing

antenna length.


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.

SWR (standing wave ratio). Radio energy travels on a correctly
cut antenna in sine waves consisting of voltage and current. When
the antenna is the proper length the sine wave begins at one end of
the antenna and ends at the other end of the antenna. When the
antenna is not the proper length, too short or too long, the sine
wave doesn't match the antenna length, causing standing waves, or
reflected waves. High SWR could result in no radiated energy. It
also causes RF feed back, radio energy backing up making components,
mikes, key, etc., hot to the touch. SWR reading should be less than
1.5 to 1, but SWR reading up to 3 to 1 will work.




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