SUBCOURSE EDITION

IS1143

 RADIO WAVE PROPAGATION

AND ANTENNAS  

INTRODUCTION

The most important element in a radio circuit is the antenna. You may have a powerful transmitter and a frequency, but without the correct antenna, communication will be less than desirable, if not impossible.

This subcourse provides information necessary to make the right choice for an antenna and how to select the correct frequency depending upon the environmental conditions.

Without an understanding of wave propagation, decisions on frequency selection and antennas could result in no communications.

Even though a frequency selection is made for you, without a thorough understanding of how the frequency was selected, you will not understand why you couldn't communicate or what frequency you should have used for a given radio circuit.

LESSON 1
IDENTIFY CHARACTERISTICS OF WAVE PROPAGATION

TASK:	Identify the characteristics of wave propagation.
CONDITIONS:	Given this lesson material, pencil, paper and without supervision.
STANDARDS:	Demonstrate competency of the task skills and knowledge by correctly responding to 70 percent of the multiple choice test covering identification of the characteristics of wave propagation. (This objective supports SM task number and title 113-596-7056, Direct Installation of a Doublet Antenna.)

Learning Event 1: GROUND WAVES.

1. In 1887, Henrich Hertz demonstrated that electromagnetic energy could be sent out into space in the form of radio waves. Radio waves travel at the speed of light in free space, 186,000 miles per second, or 300,000,000 meters per second. Free space implies that radio waves travel through empty space or a vacuum. In actual practice, radio energy travels slightly slower because of the presence of trees, hills, lakes, and the air it travels through. If we have a radio frequency of 1,000,000 cycles (1 MHz) per second, its full wave length is 984 feet. We will use the Greek letter lambda to represent wave length. V (velocity) will represent the speed of radio waves. F (frequency) represents the assigned frequency.

= V/F

Since: = V/F = 300,000,000 meters per second/1,000,000 HZ (1MHz)

= 300 meters = one wave length

one meter equals 3.2808 feet

converting into feet = 300 X 3.2808 = 984 feet = one wave length =

then one half wave length /2 = 984/2 = 492 feet

Figure 1. Simple radio communication system.

2. The Atmosphere. How do radio waves travel from the transmitter to the receiver? What effect does the atmosphere have on our radio energy? The answers to these and other questions will be answered as we discuss each facet of wave propagation. The atmosphere around us changes seasonally, yearly, daily, and hourly. The atmosphere is comprised of the troposphere, stratosphere, and the ionosphere.

Figure 2. Layers of the earth's atmosphere.

a. The Troposphere. The troposphere lies from the earth's surface to a height of approximately 6.8 miles.

b. The Stratosphere. The stratosphere lies between the troposphere and the ionosphere. It is also called the isothermal region. Its height is from 6.8 miles to 30 miles above the earth.

c. The Ionosphere. Because it is the furthest layer away, it is ionized the most by the sun. It extends from approximately 30 to 250 miles above the earth. The ionosphere has several layers which have varying levels of ionization.

3. Frequency Classifications. Not only does each atmospheric layer vary in ionization levels, but certain bands of frequencies have unique propagation characteristics. The lower frequencies have different characteristics from the upper frequencies. It is important to understand how each band of frequencies travels from the transmitter to the receiver.

Table1. Frequency band coverage.

*1kHz = 1 kilohertz = 1,000 hertz or 1 kHz

**1MHz = 1 megahertz = 1,000,000 hertz or 1 MHz or 1,000 kHz

***1GHz = 1 gegahertz = 1,000,000,000 hertz or 1 GHz or 1,000 MHz

Table 2. Frequency band characteristics.

4. Propagation in the atmosphere. There are two ways radio energy travels from the transmitter to the receiver: by means of ground waves or by sky waves. The ground waves travel along the surface of the earth. The sky wave travels from the transmitter to one of the ionospheric layers and is returned to earth. Long distance radio communication, depending on the frequency, can be by either ground or sky wave. The advantage of sky wave communication is that very little power is needed to travel long distances, say around 8,000 miles. In order to communicate by ground waves, a powerful transmitter is needed in order for the radio waves to travel the same distances. A combination of both ground and sky wave communication usually occurs. The earth's surface affects the radio energy coming in contact with it. Terrain features (jungle, desert, and large bodies of water) either aid or lessen the radio signal. Diffraction is the bending of the radio wave with the curvature of the earth. The only variable in a ground wave signal is the terrain over which it travels. There are many variables in a sky wave signal: the frequency, the ionospheric layers, the time of day, the season, and the sunspot cycle.

Figure 3. Principal paths of radio waves.

a. Reflection. A radio wave may be reflected. An example of reflection is shown in figure 4. A beam of light is shown into a mirror, almost all of the light energy is reflected. A radio signal is the same. Depending on the type of surface it contacts, the Signal will be either absorbed or reflected. Metal surfaces and bodies of water are good reflectors. Dense vegetation like that found in a jungle will absorb the majority of the radio energy. Notice in figure 4 that the beam of light is reflected at the same angle it entered the mirror. This is also true with a radio wave reflecting off the earth's surface.

Figure 4. Mirror Reflection.

b. Refraction. A radio signal that strikes an ionospheric layer is similar to the wave in figure 5. When a beam of light strikes a pool of water, the beam is bent slightly. This is what happens to a radio wave when it strikes an ionospheric layer. The signal is bent and is returned to earth. The terms reflection and refraction are used interchangeably.

Figure 5. Bending of light by refraction.

c. Diffraction. If that same beam of light is shown on an object, it will not cast a perfect shadow. The light rays tend to bend around the object and decrease the size of the shadow. This also happens to a radio wave that strikes an object such as a mountain. The radio wave tends to bend around the object. This is shown in figure 6.

Figure 6. Diffraction of wave around solid object.

5. Types of Ground Waves.

a. Radio waves that do not make use of the ionosphere are called ground waves. The received signal strength depends on how powerful the transmitter is. Terrain features the wave must travel over affects the received signal strength. The Earth's surface reduces the range of a ground wave signal. Mountains and jungles are bad terrain features. Sea water is the best terrain feature to transmit a radio signal over. Other bodies of water are also good, but not as good as sea water.

b. Figure 7 shows the various types of ground waves that a radio signal may take from the transmitter to the receiver. The signal may also be refracted by the troposphere. The ground wave is composed of a direct wave, a ground reflected wave, a surface wave, and a tropospheric wave.

Figure 7. Possible routes for ground waves.

6. Direct Wave Component. The direct wave is that part of the ground wave that travels directly from the transmitting antenna to the receiving antenna. The direct wave is limited to line of sight distances. To increase the range, increase the height of either the transmitting or receiving antenna.

7. Ground Reflecting Component. The ground reflected component is that part of the radio wave that is reflected before it reaches the receiving antenna. It may be reflected from the ground or from a body of water. When the radio wave is reflected, the phase is reversed. This could affect the reliability of communication. It could cancel out the radio waves that travel directly to the receiving antenna. To minimize the canceling effect, the antenna should be raised at either end.

8. Surface Wave Component.

a. The surface wave travels along the Earth's surface. It follows the curvature of the earth. When both the receiving and transmitting antennas are located close to the earth, the direct and reflected wave may cancel each other out.

Table 3. Propagation Characteristics of Local Terrain

b. The surface wave is transmitted as a vertically polarized wave. When using the surface wave, use a vertical antenna. A horizontal antenna transmits a horizontal wave which is short circuited by the earth. The better the type of local terrain, the further the signal will travel and not be absorbed. The range of the surface wave is determined by how powerful the transmitter is. An increase in power will increase the surface wave range. The range of the surface wave is also affected by the terrain features it must travel over.

9. Frequency Characteristics of Ground Waves.

a. The frequency used will determine which component of the ground wave will be used. If the frequency is below 30 MHz the surface wave will be used primarily. Between 10 and 30 MHz the local terrain features will determine which component of the ground wave will be used. At frequencies greater than 30 MHz the direct wave is primarily used because the local terrain features absorb the surface and ground reflected waves. Above 30 MHz, vertical or horizontal polarization may be used.

b. Frequency bands use certain components of the ground wave:

(1) The low frequency band (30 to 300 kHz) is used for moderate distance ground wave communication. A vertical antenna should be used in the low frequency band. The radio wave follows the curvature of the earth for several hundred miles.

(2) The medium frequency band (300 kHz to 3 MHz) is used for moderate distance communication over land and for long distance communication over sea water up to 1,000 miles.

(3) The high frequency band (3 to 30 MHz) is used for short distance communication. At these frequencies, the local terrain absorbs more and more of the signal as the frequency increases, decreasing the ground wave range. Long distance communications is possible using sky wave.

(4) The very high frequency band and higher bands (30 MHz and over) are used for line of sight communication. Only the direct wave component of the ground wave is usable. The range can be increased by raising the height of the antenna. Sky wave communication is not possible because the frequencies pass through the ionosphere and are not reflected.

Learning Event 1 Practice Exercise

Instructions	The following items will test your understanding of the material covered in this lesson. There is only one correct answer for each item. When you have completed the exercise, check your answers with the answer key that follows. If you answer any item incorrectly, review that part of the lesson which contains the portion involved.

1.   What is the speed of radio waves? A. 186,000 miles per second. B. 186,000,000 miles per second. C. 300,000 miles per second. D. 302,808 miles per second.

2.   The frequency range of the VLF band is-- A. .03 to 3 MHz. B. below .03 MHz or below 30 kHz. C. 30 to 300 MHz. D. above 3 MHz.

3.   Which of the following will not reflect radio energy? A. Sea water. B. Fresh water. C. Jungle. D. Metal buildings.

4.   Refraction is-- A. radio energy entering and leaving a layer at the same angle. B. similar to radio energy striking a mirror. C. similar to disfraction. D. radio energy bending upon entering a ionospheric layer and returning to earth.

5.   Which is the worst terrain feature to locate an antenna? A. Sea water. B. Wet soil. C. Desert. D. Jungle.

Learning Event 2: SKYWAVES.

1. Early radio communication was thought to be impossible over long distances. The reasoning, local terrain would absorb the radio signal. When trans-atlantic communication was accomplished, this opened up new questions. If the surface wave was limited, then how did communication take place? The conclusion made was that the earth was surrounded by something other than air. Two men, one an Englishman the other an American, suggested that a electrified layer above the earth reflected radio signals. Later experiments showed that more than one layer existed.

2. Formation of the Ionosphere: As shown in figure 2 the earth's atmosphere extends up to a distance of 250 miles. The level of ionization increases with height. The sun's rays combined with cosmic rays ionize these layers.

a. Ionization. The bombardment by the sun and ultraviolet rays charge the atoms in these layers. This action is called ionization.

b. Recombination. As the sun goes down and the intensity of the ultraviolet rays decreases, the ionization of the layers decreases. Just before sunrise, ionization is at its lowest point.

c. Source of ionization - the sun. The earth and the sun are composed of the same basic elements. The violent state of these elements on the sun keeps it in a constant of state of molten or gaseous condition.

There is only one principal ionized layer at night.

Figure 8.

IONOSPHERE STORMS

Definition:	Any marked or sudden deviation from normal conditions of height or frequency.
Effect:	Normally reliable frequency may become useless. Signal may weaken or "blackout".
Duration:	Several minutes to several weeks. Tendency to repeat every 27 days as the sun rotates.

Ionosphere storms usually originate in North and South Polar Regions.

Figure 9.

Figure 10. Solar eruption.

(1) Eruptions on the surface of the sun shoot hot gases from its surface up to a half million miles away. Spots of intense ultraviolet radiation are another disturbance noted. These spots are referred to as sunspots.

(2) The number of sunspots vary from year to year. The minimum to maximum sunspot cycle takes about 11.1 years. During periods of high sunspot activity, higher frequencies are usable. During low sunspot activity, lower frequencies must be used.

Figure 11. Sunspots.

(3) Dellinger fade. When the sun produces bright visible flares, the effect is felt immediately in the various ionospheric layers. Absorption of most radio frequencies is noted during this period. It is called the Dellinger fade. The lower frequencies are affected to a lesser degree.

3. Ionosphere Layers or Regions. There are four layers in the ionosphere. They called the D, E, F1, and F2 layers. All four layers are present during the daytime. At night, the F1 and F2 layers thin out and tend to merge into one layer - the F layer. The D and E layers disappear at night. These layers have less ionization. After the sun sets, recombination occurs and the layers disappear. The number of layers, their height, and level of ionization fluctuates. The ionization changes hour by hour, day by day, month by month, season by season, and year by year.

Figure 12. Layers of the ionosphere.

a. D Layer. The D layer is approximately 30 to 55 miles above the earth. This layer has the least ionization and therefore has the lease effect on radio frequencies. It is present during the day only. The height varies over the eleven year sunspot cycle. The D layer is approximately 6 miles thick.

b. E Layer. The E layer is approximately 55 to 90 miles above the earth. The E layer reflects radio frequencies up to about 20 MHz. The maximum one hop range of the E layer is 1,500 miles. This layer is present only during the day. The height of the layer varies during the eleven year sunspot cycle. The E layer is approximately 15 miles thick.

c. F Layer. The F layer is from 90 to 240 miles above the earth. The F layer is present only at night. This layer is created when the F1 and F2 merge. Because it is the most ionized, recombination takes place more slowly. The height varies over the course of the eleven year sunspot cycle.

d. F1 and F2 Layers. During the daylight hours, the F1 layer has a height of approximately 90 miles and is approximately 12 miles thick. The F2 layer has a height of approximately 160 to 220 miles and is approximately 15 miles thick. The F2 layer, being the closest to the sun, has the most ionization. The height of both layers varies over the eleven year cycle of sunspot activity.

e. Other layers. Other layers or clouds appear from time to time over the eleven year sunspot cycle. These layers appear near the E layer. Together, they are called the Sporadic E layer.

Figure 13. Effect of frequency on the critical angle.

Figure 14. Relationship between skip zone, skip distance, and ground wave.

4. Characteristics of the Ionosphere.

Critical frequency. Layer height will determine how far a radio transmission travels. In addition, the higher the frequency the greater the density of ionization that is required to reflect the signal back to the earth. The F2 layer will reflect higher frequencies than the F1 layer. The same will hold true for the F1 layer as compared to the E layer. The D layer will reflect frequencies below approximately 500 kHz. For each layer there is a maximum frequency which is refracted, but higher frequencies are not. This is called the critical frequency. Frequencies higher than the critical frequency will pass through the layer and will not be refracted. As a radio wave passes through a layer, it is partially absorbed. Figure 15 shows different frequencies striking different layers. Some will be returned, others will pass through. All frequencies below the critical frequency are refracted. Frequencies above the critical frequency pass through the layers.

Figure 15. Critical frequencies.

5. Regular Variations of Ionosphere.

a. General. The ionospheric layers exist because of the sun's activity. The sun's state of activity will determine, among other things, the critical frequency for each ionospheric layer.

Figure 16.

Table 4. Regular Variations of Ionosphere.

b. In general, because of the variations of ionization during the daytime, higher frequencies can be used. During the night, lower frequencies are used. The critical frequency for the F2 layer, which exists only during the day, is higher than that of the F layer. At night, the F layer is actually a combination of the Fl and F2 layers. It is common for stations in a net not to receive each other with the same signal strength. Layer density varies over the circuit path. It is common for one station to hear well and the rest don't. There are times when there is only one-way communication because of layer density variation. The layers vary in thickness from 6 to 75 miles.

Figure 17. Daily and seasonal variations in ion density.

c. Seasonal Cycle. As the earth tilts on its axis, the sun rays strike the layers obliquely. This will cause the northern half of all layers to be more ionized than the southern half because the northern hemisphere is tilted away from the sun. We can also see that there is a difference in layer height during the winter and summer.

d. Eleven Year Cycle. As stated earlier, the sunspot activity varies over an eleven year period. During a high sunspot activity, higher frequencies may be used. Longer distance communication may be also possible because of the use of higher frequencies. During low sunspot activity, lower frequencies must be used and shorter distance radio circuits can be expected.

e. Twenty-seven Day Cycle. The sun requires 27 days to rotate around its axis. While rotating, sun exposes different sunspot concentrations. This variation affects the layers, sometimes making F2 predictions difficult.

SINGLE HOP TRANSMISSION

Distance AB less than 2500 miles (4000 KM).

Figure 18a.

MULTIPLE HOP TRANSMISSION

Distance AB more than 2500 miles (4000 KM).

Figure 18b.

Figure 19.

IN THE HF BAND

-

Higher frequencies are bent less, that is, higher frequencies have more penetrating power.

Figure 20.

6. Irregular Variation of Ionosphere.

a. In addition to the regular variation of the ionosphere, there are temporary effects. Some of these are Sporadic E, sudden ionospheric disturbance (Dellinger fade), ionosphere storms and scattered reflections.

b. Sporadic E. The Sporadic E is a temporary phenomenon. It consists of an ionized cloud at a slightly higher height than the normal E layer. Why it appears and what causes it to move is unknown. It will reflect frequencies from 1.5 to 15 MHz. Its s appearance is frequent, especially in the middle latitudes. Not all stations in a net may experience the Sporadic E reflection.

c. Sudden Ionospheric Disturbance or Dellinger Fade. Ionization from a violent solar eruption travels down to the D layer. This causes an almost total absorption of all frequencies, above 1 MHz. This disturbance is called SID or Dellinger fade. This blackout of radio communication may last from a few minutes to several hours.

d. Ionospheric Storms. An ionospheric storm is caused by a severe disturbance of the ionospheric layers. The levels of ionization of the layers thin out, making reflections frequencies above 1.5 MHz difficult. Lower working frequencies are in order. These storms may last several hours to several days. These storms are caused by particle radiation from the sun. The storms will start normally after a sunspot group crosses the center of the sun.

Table 5. Irregular Variations of Ionosphere.

e. Scattered Reflections. Another irregular variation is the rapid change of ionization with height. A radio signal may be reflected by more than one layer. The received signal may arrive from several directions which will cause flutter fading.

7. Ionospheric Predictions. By the sounding of the ionosphere, predictions are possible. Long range forecasting can predict the optimum working frequency, maximum useful frequency, and lowest useful frequency.

Figure 21. Bright solar eruption.

Figure 22. Scattering of signal components of radio wave.

8. Sky Wave Propagation. Sky wave propagation is the reflection of radio waves from the various ionospheric layers.

Sky wave Propagation offers long range communication with very little power required. The most difficult question regarding sky wave propagation is what frequency to use. The HF (3-30 MHz) band uses ionospheric reflection most effectively.

a. Sky Wave Transmission Path. Figure 23 indicates the many varied paths a radio signal may take from the transmitter to the receiver. Notice that a receiving station located in the skip zone would receive no signal. Through proper frequency selection, antenna and antenna height determination, there will never be a skip zone. Notice also that from the point the radio signal leaves the transmitter to the point it contacts the earth is called the skip distance.

Figure 23.

(1) Sky wave modes. The distance the sky wave signal travels before it returns to earth depends upon the ionospheric layer used. When the signal strikes the earth, part of the signal is absorbed. The rest is reflected back to the ionosphere. This is repeated until the signal is too weak to be reflected either by the ionosphere or the earth. This is called a multi-hop transmission.

(2) Frequency. The problem as to what frequency to use is not an easy one to solve. As mentioned earlier, the higher the frequency, the higher the ionospheric density required to return the frequency to earth. Figure 28 shows radio signals of several frequencies. Some are returned while others are not. The 5 and 20 MHz signals are returned, while the 100 MHz signal is not. Notice that the 20 MHz signal travels further. While this may hold true for day time communication, it might not be true at night.

b. Maximum Usable Frequency (MUF).

(1) See Figure 23. For a given distance, there is a frequency in which any further increase in frequency will result in no communication. In other words, the station located in the skip zone does not receive a signal. The highest frequency that can be used between two points is the maximum usable frequency. As the distance increases the MUF increases.

(2) Care must be taken in selecting the frequency. Too high - it passes through the ionosphere or overshoots the receiver. Too low and it will be absorbed by either an ionospheric layer or the earth.

Figure 24. Average layer distribution of the ionosphere.

Figure 25. Skip zone.

Figure 26.

Figure 27. Relating reflected waves to distances along earth's surface.

Figure 28. Frequency versus distance for returned waves.

c. Lowest Usable Frequency (LUF). For a given distance, there is also a frequency which will be returned and which any further decrease in frequency will result in no communication. The decrease in frequency results in having all lower frequencies absorbed by the ionosphere or the earth. This is called the LUF.

d. Optimum Working Frequency (FOT). The frequency we select should be a compromise between the MUF and the LUF. With the fluctuations of the ionosphere, communication might not be possible using the MUF or LUF. We therefore choose a frequency that is lower than the MUF and higher than the LUF. This frequency is referred to as the FOT.

e. Signal Strength. There are several factors that affect the received signal strength. The orientation of the transmitting antenna, if possible, should be broadside to the direction of the receiving station (s). Likewise, the receiving antenna should be broadside to the transmitting station(s). As the radio signal passes through the layers, partial absorption takes place. Part of the signal is also lost when the signal is reflected from the earth. Fading is the rapid fluctuations of ionization of the layers, causing the signal to reflect off different layers.

LESSON 2
CALCULATE ANTENNA LENGTH

Learning Event 1: CHARACTERISTICS OF ANTENNAS.

Half-Wave and Quarter-Wave Antennas

1. Basic Theory.

a. The antenna is part of the electrical circuit of the 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. See Figure 29.

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.

Figure 30. Impedance along half-wave antenna.

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.

TRANSMISSION LINES

Introduction.

A transmission line is used to carry the RF energy from the transmitter to the antenna. There are times when the antenna is connected directly to the transmitter. Normally, however, the antenna is located some distance away from the transmitter. The transmission line should transfer the power with the least possible loss.

(1) Transmission lines dissipate power in three ways:

(a) Radiation. The transmission line radiates like an antenna, especially if its length matches the antenna.

(b) Heating. Any current flow results in heat. The greater the power the more heat is produced. To reduce skin effect, the cross sectional area of the center conductor is increased.

(c) Reflection. Radio energy emitted by the transmitter goes to the antenna in what we call traveling waves. If there is no load (antenna), the traveling waves are stopped abruptly. This causes the waves to be reflected back to the transmitter causing loss.

(2) Types of transmission lines.

(a) Single wire line. This is the simplest type of transmission line - a single wire connected to the antenna with the earth acting as the return path. Since there is only one conductor, the line is considered to be unbalanced. The disadvantage is that the line radiates much like an antenna, causing high line loss, because of no return path. The other disadvantage is that because of no return path, it is difficult to match the line to the antenna. An antenna tuning unit is required to match the transmitter to the line and antenna. However, there are times when the advantages of easy installation far outweigh the disadvantages. Some transmitters are broad enough to load across many types of transmission lines and antennas.

(b) Twisted pair. Two insulated wires (WD-1) can be used as a transmission line. It offers easy installation, but has high loss and should not be used above 15 MHz.

(c) Coaxial lines. When one conductor is placed inside the other separated by foam or plastic it transfers the RF power to the antenna with a minimum of loss. There is some loss as the frequency is increased. To offset this, the cross sectional area of the center conductor is increased. This is the best transmission line to use, because it has the least power loss.

BASIC FEEDER SYSTEMS

Introduction.

The transmission line transfers the RF power from the transmitter to the antenna. There are two general types of transmission lines: resonant (tuned) and nonresonant (untuned).

(a) Resonant feeder line is the same length as the antenna. It is rarely used in tactical applications.

(b) A nonresonant transmission line is one that has an SWR of less than 1.5. In order to achieve this, the impedance of the antenna and the transmission line must match. An antenna tuning unit is used in some applications to match the transmitter to the line and antenna.

(1) Single-wire feed. A single wire can be used as a nonresonant feed line. Because the impedance of a single-wire feed is 500 to 600 ohms, a point on the half-wave antenna must be selected that will match the impedance of the line. The antenna impedance varies from 2500 at the end to about 73 ohms in the center. A point 14 percent from the center of the antenna will provide the 500 to 600 ohms required (A of figure 31). To reduce radiation or coupling make sure the single-wire feed is at right angles to the antenna. A good electrical ground connection is also required to provide a return path to the transmitter.

(2) Twisted-pair feed. WD-1 can be used in an emergency to provide a feed line from the transmitter to the antenna (B of figure 31). The impedance requirement of a twisted pair is 70 to 80 ohms. The center of the half wave antenna provides that impedance. This type of feed should be used only as a last resort because of the very high power loss.

Figure 31. Single-wire and twisted-pair feed systems.

(3) Coaxial line feed. A coaxial feed provides a two conductor line which offers the least line loss of all practical field feed systems.

BASIC RADIATION PATTERNS

Introduction.

An antenna radiates energy in a particular pattern in free space. It is useful to examine these radiation patterns. It is possible to design an antenna system to provide us with the best possible communication.

(1) Radiation types and patterns.

(a) An example of a source that radiates in all directions is the sun. This type of radiator is called an isotropic radiator. If we could measure the sun's radiation as we move around it in a circle, we would find it was the same all along the circle.

Figure 32. The sun as an isotropic source of radiation.

(b) Another type of radiator is called anisotropic. An example is a flashlight. The light beam radiates only a small portion of the total space around the flashlight. If we move in a circle around the flashlight, we find the level goes from zero to maximum then back to zero again.

Figure 33. Flashlight as anistropic source of radiation.

(2) Dipole antenna radiation.

(a) The terms dipole and doublet are used interchangeably. Both are used to indicate a basic half-wave antenna.

(b) Radiation pattern of a doublet. The doublet is the simplest form of an antenna. The radiation pattern is similar to the flashlight. See figure 34. There is a vertical as well as a horizontal radiation pattern. As you can see, the pattern is in the form of a doughnut. Whether it is seen from the side or from the top, the pattern is full.

Figure 34. Development of vertical and horizontal plane polar patterns from solid radiation pattern.

(c) By looking at figure 35, you can see that the antenna can be mounted either vertically or horizontally. The radiation patterns are similar. The difference is that a horizontal antenna radiates horizontally in two directions, while a vertical antenna radiates horizontally in all directions. Figure 36 indicates the beam width and relative power patterns.

Figure 35. Radiation pattern of dipole (half-wave) antenna.

Figure 36. Beam with measured on relative field strength and relative power patterns.

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 also learned, 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.

Figure 37. Reflection produced by ground plane.

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.

Looking At The Looking At The Antenna Broadside Antenna From The End

Figure 38.

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.

Figure 39. Solid pattern produced by horizontal half-wave antenna located a half-wavelength above ground.

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.

Figure 40. Vertical-plane radiation patterns produced by vertical half-wave antennas.

Figure 41. Solid patterns produced by vertical half-wave antenna located a half-wavelength above ground.

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. See Figure 42. 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 radiation 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.

Figure 42.

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.

Table 6. Ground Material Conductivity.

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.

6. Polarization.

a. The band of frequencies we use will determine the best polarization. At low and medium frequencies, vertical polarization should be used. This will take advantage of the surface wave which travels vertically. A horizontal antenna has a horizontal wave that will be short-circuited and will travel less than a vertical wave at the same frequency. The disadvantage of using a vertical antenna at these frequencies is that a sky hook will have to be used to hold the antenna up. For example, a 2 MHz antenna that is a quarter wave long is 117 feet. It would not be possible to erect a practical field antenna 117 feet high. We, therefore, would be forced to use a horizontal antenna. We would be forced to make a compromise - like it or not. At frequencies above 3 MHz, the polarization is immaterial. However, for a sky wave, a horizontal antenna should be used. For a ground wave, a vertical antenna should be used. The disadvantages of a vertical antenna are that it radiates in all directions. Also, if its a whip, a high loss occurs caused by the loading coils trying to compensate for the whip being too short.

b. The choice of whether an antenna is vertical or horizontal, in some cases, is out of our hands. If we are mobile or mobile at a halt, obviously, the only choice is a vertical antenna. Likewise, if we are in a jungle area, our choice must be horizontal. A desert or arctic location also presents a challenge of how to install a mast section to support a horizontal antenna. In most cases, most of our nets are of short distance (0 to 35 miles). This makes communication difficult because you can't communicate by ground wave only, nor can you communicate by sky wave only, especially if the antenna is a whip. For short distance sky wave a horizontal antenna should be used erected a quarter wave or lower above ground. Lower antenna heights can be used with some degradation of the transmitted signal. If a whip is used for sky wave then it should be bent at a 45° angle.

Learning Event 1 Practice Exercise

Instructions	The following items will test your understanding of the material covered in this lesson. There is only one correct answer for each item. When you have completed the exercise, check your answers with the answer key that follows. If you answer any item incorrectly, review that part of the lesson which contains the portion involved.

1.    Which of the following affect the physical length of an antenna? A. Terrain features. B. Wire cross section. C. Insulators. D. All the above. 2.    The most efficient antenna length is-- A. eighth of a wave. B. quarter of a wave. C. half a wave. D. one wave. 3.    SWR is caused-- A. by an antenna cut to proper length. B. terrain features. C. ground conductivity. D. by an incorrect antenna length. 4.    Which of the following makes the best feed line? A. Twisted pair. B. TV lead in. C. Single wire. D. Coax.

Learning Event 2: COMMON ANTENNAS.

HORIZONTAL ANTENNAS

1. Doublet. The doublet antenna is the most common HF antenna used by the military. The doublet usually comes in kit form. The kit