PRACTICAL HALF-WAVE ANTENNAS
1. Introduction.
a. We have discussed how to calculate a half-wave. Now, let's
discuss the patterns half-wave antennas make. We have shown in
figures 34 and 35, the radiation pattern of an antenna in free space.
Since our antennas must be erected over earth, the patterns created
are different.
b. The ground has the greatest effect on the medium and high
frequency antennas which are mounted fairly close to it in terms of
wavelength.
2. Ground Effects.
a. If a horizontal antenna is erected some distance above ground,
its radiation pattern is as shown in figure 37. Notice that some of
the energy travels directly to the distant station. Notice also that
part of the energy strikes the ground directly in front of the
antenna. As we have learned earlier, phase reversal takes place and
may cancel out the direct wave if the ground-reflected wave and the
direct wave arrive at the same time and are out of phase. If they
arrive in phase, the ground reflected wave adds to the direct wave,
making it stronger. As the height of the antenna is increased, the
ground reflected signal either adds to the direct wave or creates a
null. This action results in a series of radiation lobes. As we
have alsolearned, radio energy goes into the earth before it is reflected.
The conductivity of the earth will determine how deep the signal will
penetrate and how much of the signal is reflected.
3. Ground-Affected Radiation Patterns.
a. Reflection factor. If we assigned the direct wave a value of
1 and the ground reflected wave a value of 1, then the maximum signal
we could have would be 2. As we see from Figure 38, there are
varying vertical angles of maximum and minimum radiation lobes. The
number of lobes vary as the height of the antenna above ground is
increased.
b. Horizontal half-wave antenna. Let's apply the reflection
factor to a horizontal antenna erected at distance above ground.
Notice figure 38. Patterns A, C, E, and G are the vertical radiation
patterns. Patterns B, D, F, and H are the vertical radiation
patterns at right angles to the antenna. Figure 39 shows a better
picture of the radiation produced. Both figures 38 and 39 show a
half-wave antenna.
c. Notice that in figure 38, as the height is increased from a
quarter wave length above ground, the lobe divides into two lobes.
Notice also that the number of lobes equal the number of quarter
waves. At four quarter waves or one wave length above ground, there
are four lobes. Notice also that for odd quarter wave heights above
ground the major lobe is at 90 degrees.
d. Vertical half-wave antenna. Ground reflection also affects
vertical antennas. See figures 40 and 41. Notice that a vertical
antenna erected 1 quarter wave above ground has two lobes. As the
height is increased, the number of lobes increases. An antenna 1
wave length in height has 6 lobes.
e. It now can be seen that the ground reflection factor and the
antenna height play a major role in the radiation of radio energy.
In later sections we will see that we can select a particular antenna
height for a certain distance of transmission. For example, for
short distances the antenna height should be less than a quarter
wave. For long distance communication, the antenna should be a half
wave or more in height. We can improve the ground reflection through
the use of a counterpoise or radial ground. This increases the
conductivity of the earth and lessens the energy lost going into the
earth.
4. Changes in Radiation Resistance.
a. The radiation resistance at the center of a half-wave
horizontal antenna erected in free space is 73 ohms. The actual
resistance of the same antenna erected over varying ground
conductivity and heights is zero to approximately 100 ohms.
b. The change in resistance occurs because of the
ground reflected wave. It occurs in the following manner: Let's say
that a given power is applied to an antenna in free space. The
resistance is 73 ohms because there was no ground
reflection. But, suppose that the same antenna is erected at a given
distance above the ground. The ground reflects part of the energy
back to the antenna, adding to the existing current and lowering the
resistance. It is assumed that the ground reflected wave was in
phase with the direct wave; therefore, adding to the original
current. If the two waves are not in phase, the overall current is
less, resulting in a higher radiation resistance.
c. The change in radiation resistance of a vertical half-wave
antenna is much less than that of a horizontal antenna. The maximum
resistance is 100 ohms at the center of the antenna at a height of a
quarter-wave above ground. It decreases to about 70 ohms at a height
of a half-wave length.
5. Effects of Practical Grounds.
a. Up to this point we have discussed the reflection factor over
a uniform high conducting ground. As we can see from table 6, the
conductivity varies over different types of ground. How does this
affect a reflected signal? Instead of having a maximum reflection
factor of 2 (1 from the direct wave and 1 from the ground reflected
wave), we might have the direct wave only. This could occur if the
antenna was erected over a poor conducting ground. In addition,
incomplete nulls might be produced. This would happen if the
reflected wave was in phase with the direct wave and both waves not
of equal amplitude. Also, the reflected wave could be absorbed by
the earth.
b. Frequency effects. Not only does the ground affect the
radiation pattern, it has a pronounced effect on certain frequencies.
At low and medium frequencies, the radio waves go into the earth to a
depth of about 50 feet. The lower the conductivity, the further the
wave goes into the earth. At high frequencies, the wave penetrates
to a depth of about 5 to 10 feet. Ground absorption is considerable
for takeoff angles below 12 degrees. As the frequency is increased,
the ground reflected wave is further absorbed until only the direct
wave is left. The radiation resistance over imperfect ground is less
than it is over a good conducting ground.
c. Antenna height. The question of how high an antenna actually
is above ground is not an easy one to answer. Since the wave goes
into the earth, it is difficult to determine the true height of an
antenna. We can make any ground a better reflecting conductor by
using a counterpoise or radial ground, to create a definite starting
point.
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