CHAPTER 20
THE ANTENNA
IT'S MORE THAN A PIECE OF WIRE

You may think a radio transmitter's antenna is just a length of wire running from the foremast to the mainmast, and that any dumb-bell can rig one. A receiver's antenna may be that simple, but that is not quite true for a transmitter antenna. An ANTENNA IS a piece of wire. It is cut to the PROPER LENGTH and CORRECTLY installed so that it will RADIATE EFFICIENTLY the energy delivered to it from the transmitter. The word "EFFICIENTLY" is the word you want to note well. ANY WIRE carrying an a.c. radiates electromagnetic energy-remember the HUM that your receiver picked up from a 60-cycle power line? And the static from a neon sign driven by an induction coil?

The power line and neon sign are not EFFICIENT RADIATORS because they were not designed to radiate energy. The power line carries energy from the power plant to your motor or light bulb, while a neon sign is built to produce light.

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THE DIPOLE

The BASIC ANTENNA is a DIPOLE-a WIRE with a length equal to HALF A WAVE LENGTH. If a station is operating on a wave length of 100 meters, the dipole to be used at that wave length will be-

100 / 2 = 50 meters, or about 164 feet.

A transmitter operating on a wave length of one meter (300 mc.) will require a dipole 1/2 meter long-about 20 inches.

IMPEDANCE OF A DIPOLE

First of all, you must remember that an antenna carries a.c. Therefore the antenna will have inductive reactance as well as RESISTANCE. In a dipole, the impedance is MAXIMUM at BOTH ENDS, and MINIMUM at the

CURRENT AND VOLTAGE IN A HALF-WAVE ANTENNA

If a feeder line from the transmitter is connected to the center of a DIPOLE, the antenna will operate as if you set

During one half of the alternation, the electrons will flow from right to left, figure 138B. On the next half-alternation, the generator will make the electrons flow in the opposite direction, figure 138C.

In an antenna, as in any other circuit, the flow of electrons is the GREATEST where the IMPEDANCE is LEAST.

What's the voltage along an antenna? Voltage is always GREATEST where the IMPEDANCE is the HIGHEST. Thus you will find the HIGHEST VOLTAGE at the ENDS of the dipole, figure 138D. During one half of an alternation, the left end of the dipole will be MAXIMUM NEGATIVE, and the right end will be POSITIVE. On the next half alternation, the POLARITY of voltages is reversed.

If the antenna extends EXACTLY one-quarter wave length on each side of the generator, the REBOUNDING or reflected ELECTRONS from the negative end of the dipole will return at the proper instant to reinforce the movement of other electrons already moving in that direction. But if the antenna is GREATER or LESS than one-quarter wave length on each side of the generator, much of the energy will be lost in the collision of electrons trying to flow in TWO directions at the same time.

ELECTROMAGNETIC FIELD SURROUNDING A DIPOLE

A dipole suspended out in space away from the influence of the earth would be surrounded by an ELECTROMAGNETIC FIELD the shape of a DOUGHNUT, as shown in figure 140. You see that no radiation takes place at the ENDS of the dipole. If the antenna is mounted vertically,

Set the dipole PARALLEL TO the surface of the earth-the field is the shape of a doughnut standing on edge. The GREATEST FIELD STRENGTH is along a vertical line PERPENDICULAR to the dipole.

ELECTROSTATIC FIELD SURROUNDING A DIPOLE

High voltage at each end of the dipole produce an ELECTROSTATIC FIELD which is at maximum strength at the ends of the dipole. But if the antenna is shorter or longer than a half-wave length, the electrostatic field strength will be greatest at the point where the voltage is maximum.

The electrostatic field is always present with an electromagnetic field. One cannot exist without the other. In most cases, only the electromagnetic will be discussed, but remember, the electrostatic is always there too.

STANDING WAVES

The electrostatic and electromagnetic fields surrounding an antenna each form STANDING WAVES. The two types of standing waves are as dissimilar as current and voltage. The electrostatic field is 90° out of phase with the electromagnetic field. The presence of an ELECTROMAGNETIC field can be shown by the glowing of a MAZDA lamp-loop in the presence of the field, while a NEON lamp will glow in the presence of an electrostatic field. The points along an antenna where the magnetic fields are MAXIMUM are called CURRENT LOOPS. The points where the electrostatic fields are maximum are called VOLTAGE LOOPS.

If you move a NEON bulb along an r.f. transmission line, the bulb will glow each time a voltage loop is reached. If the transmission line is several wavelengths long, several voltage loops will be spotted.

You can determine the wavelength of your transmitter approximately if you measure the distance between the loop points, since each loop is exactly one-half wavelength from the other.

ELECTRICAL LENGTHS AND ACTUAL LENGTHS OF ANTENNAS

An ideal antenna, one completely free from the influence of the earth, would have an ACTUAL LENGTH exactly equal to its ELECTRICAL LENGTH. For instance-an ideal half-wave antenna for use with a 100-meter wavelength would be 50 meters long.

Since no antenna is completely free from the influence of the earth, the PHYSICAL length of an antenna is approximately 5 percent shorter than its ELECTRICAL length. A half-wave antenna for a 100-meter station will be 50 meters minus 5 percent or 47½ meters long.

The physical length of a half-wave antenna for frequencies above 30 mc. can be calculated from the frequency by using the following equation-

LENGTH (feet) = (492 x 0.95) / frequency, in megacycles

The number 492 is a factor for converting meters to feet. The correction factor, 0.95, is 100 percent minus the 5 percent loss due to the effect of the earth.

THE HERTZ ANTENNA

Any antenna that is one-half wavelength long is a HERTZ ANTENNA, and may be mounted either vertically or horizontally. The great length of HERTZ antennas makes them difficult and costly to build to handle low frequencies. Consider the problem of constructing a half-wave antenna

Because of the construction difficulties and costs, you will find that half wave antennas are seldom used with broadcasting transmitters operating at frequencies below 1,000 kc. But half-wave antennas are widely used with high-frequency communication transmitters. A half-wave antenna for a 30 mc.-10 meters-transmitter will be only a little over 16 feet long.

THE MARCONI ANTENNA

The MARCONI ANTENNA is also known as the QUARTER-WAVE ANTENNA, and the GROUNDED ANTENNA. Figure 142 illustrates the principle of a Marconi antenna

The advantage of using a Marconi antenna can be seen when you compare a length of 426 feet for a Marconi to 851 feet for a Hertz antenna at 550 kcs.

The quarter-wave antenna is used extensively with portable transmitters. On an airplane, a quarter wave

ANTENNAS OF OTHER LENGTHS

Occasionally you'll need an antenna of some other length than one-quarter or one-half wavelength. You'll see some of the usual lengths in figure 143.

Figures 143A and 143C are examples of CURRENT FED antennas, while figures 143B and 143D are VOLTAGE-FED. The expressions VOLTAGE-FED and CURRENT-FED refer to the points along the antenna where the power is applied. In the CURRENT-FED antenna of figure 143A, the power k delivered to the antenna at the point of HIGHEST CURRENT. The antenna of figure 143B is VOLTAGE-FED, the power being applied to the point of HIGHEST VOLTAGE.

CORRECT THE ELECTRICAL LENGTH

After the antenna has been erected, you .may find that its physical length is greater or less than its electrical length. If a grounded antenna is less than one-quarter wavelength, there will be a CAPACITIVE effect at the base, and an INDUCTANCE must be added in series to increase the ELECTRICAL LENGTH, as in figure 144A.

When the physical length of an antenna is GREATER than its correct electrical length, the antenna will have excess INDUCTANCE. In this case it will be necessary for you to add a CONDENSER in series with the antenna to SHORTEN its electrical length, as in figure 144B.

ANTENNA TUNING CIRCUITS

You will have to change the ELECTRICAL LENGTH of the antenna each time you change the FREQUENCY of the transmitter. Since you can't climb up the superstructure and chop off a piece of the antenna each time you

Transmission lines used with frequencies below 300 mc. are of four general types-the OPEN TWO-WIRE system, the COAXIAL CABLE or CONCENTRIC LINE, the TWISTED PAIR, and the SHIELDED PAIR.

Figure 145 shows an open two-wire transmission line. Wires are held rigidly in a parallel position by INSULATED SPACERS. For 20 mc. and lower, a spacing of at least six inches is desirable. For frequencies higher than 20 mc. a spacing of four inches is best.

Figure 146 is a drawing of COAXIAL CABLE or a CONCENTRIC LINE. It consists of a copper tube with a copper wire extending down the length of the tube. The wire is held centered in position in the tube by INSULATED SPACERS

Higher operating efficiency is obtained by filling the tube of the CONCENTRIC LINE with NITROGEN under several pounds of pressure. But a pressurized line is often a source of trouble. Vibrations caused by gunfire or rough sea may cause leaks which allow the pressure to drop. If this happens, the efficiency of the line will drop.

The TWISTED PAIR and the SHIELDED PAIR are not commonly used as transmission lines. Both types are shown in figure 147. The twisted pair is the least efficient. The

RESONANT AND NON-RESONANT TRANSMISSION LINES

Transmission lines are either RESONANT or NON-RESONANT. A RESONANT line has characteristic STANDING WAVES, while a NON-RESONANT line does not.

Remember the STANDING WAVE is the result of a certain amount of energy being REFLECTED BACK along the transmission line. Imagine a transmission line so long that NONE of the energy sent out by the transmitter ever reaches the end of the line. Naturally, since none reaches the end, none can be reflected back.

But no line is that long, so why not string up a line of convenient length and connect a device to the far end that will ABSORB ALL the energy traveling down the line? Since all the energy is absorbed, none is left to be reflected back. This gives you a NON-RESONANT line. To do this, the IMPEDANCE of the ABSORBER matches the IMPEDANCE of the ANTENNA. The absorber will collect all the energy

A RESONANT LINE does NOT have its impedance matched to the impedance of the antenna. This type of line is actually an ANTENNA whose length is some multiple-1, 2, 3, etc.-of a QUARTER wavelength. You fasten one end of the line to the antenna, the other end to the transmitter.

RESONANT lines are usually OPEN TWO-WIRE SYSTEMS, while the NON-RESONANT line may be TWO-WIRE, a CONCENTRIC, a SHIELDED, or TWISTED PAIR.

YOUR JOB AND ANTENNAS

You may never be called upon to rig an antenna, or even change an installation you are using, but the knowledge of what an antenna is, and what it does will help you in the tuning of your transmitters.

Remember the antenna's job is to radiate, in the form of electromagnetic energy, as much as possible of the energy delivered by the transmission lines from the transmitter. To do this, the antenna must be correctly built and correctly installed. But more important as far as you are concerned-the transmitter must be correctly tuned and coupled to the antenna. That is your job.