are ~/4(one-fourth wavelength) at the operatingfrequency. Together, of course, the sections make theeffective length of the antennaL/2(one-halfwavelength) at the operating frequency.One feature of the dipole antenna is that it does notneed to be connected to the ground like other antennas.Antennas shorter than a half wavelength must use theground to achieve half-wave characteristics. The half-wave antenna is already long enough to radiate thesignal properly.Because of sophisticated antenna systems andtuning processes, half-wave antennas can beelectrically achieved aboard ship. Thereforewavelength is becoming less and less the criteria fordetermining the types of antennas to be used on ships.Dipole antennas can be mounted horizontally orvertically, depending upon the desired polarization, andcan be fed at the center or at the ends. Because it isungrounded, the dipole antenna can be installed aboveenergy-absorbing structures.QUARTER-WAVE ANTENNAA quarter-wave antenna is a grounded antenna thatis one-fourth wavelength of the transmitted or receivedfrequency. You will hear the quarter-wave antennareferred to as a “Marconi antenna.” The quarter-waveantenna is also omnidirectional.As we mentioned earlier, a half-wave antenna is theshortest practical length that can be effectively used toradiate radio signals into free space. The naturalquestion, then is, “How do we use a quarter-wavelengthantenna if a half-wavelength is the shortest length thatcan be used?” The answer is simple.Figure 2-20.—Direct and image signal of a quarter-waveantenna.Figure 2-21.—Current distribution in a real antenna and itsimage.Two components make up the total radiation froman antenna. One component is that part of the radiatedsignal which leaves the antenna directly. The other is aground reflectionthat appears to come from anunderground image of the real antenna (figure 2-20).This image is sometimes called the mirror image and isconsidered to be as far below the ground as the realantenna is above it.Figure 2-21 shows basic current distribution in areal and image antenna. There are certain directions inwhich the direct wave from the real antenna and thereflected wave from the image are exactly equal inamplitude but opposite in phase. Conversely, there areother directions in which the direct and reflected wavesare equal in amplitude and in phase. Therefore,depending on the direction and location of the point atwhich the field strength is measured, the actual fieldstrength may be (1) twice the field strength from the realantenna alone, (2) zero field strength, or (3) someintermediate value between maximum and minimum.It is this “real” and “image” radiated field that forms thebasis for using quarter-wavelength antennas.This reflected-energy principle is very useful in thelower frequency ranges, although ground reflectionsoccur in the high-frequency range as well.The antenna does not always need to be placed atthe Earth’s surface to produce an image. Anothermethod of achieving reflected images is through the useof ground planes. This means that a large reflectingmetallic surface is used as a substitute for the ground orEarth. This method is frequently used in the VHF/UHF2-19
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