# Advanced Radio Theory for FCC Operator's ExamsOctober 1944 Radio News

 October 1944 Radio News [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged. Special Insert: Radio-Electronic Engineering Department

Advanced Radio Theory for FCC Operator's Exams

By Carl E. Winter

Outline of advanced elements with which prospective licensee must be familiar before attempting to pass the FCC exams.

Without adequate preparation, some trained electronic engineers would encounter a few hard nuts to crack in the Federal Communications Commission examinations for a Commercial Radio Operator License.

Fig. 1 - Condenser input filter.

Fig. 2 - Choke input filter.

Fig. 3 - Methods of neutralizing single and push-pull output stages.

A good percentage of the exam's questions apply to the basic principles of radio and offer little difficulty to ex-hams" and ex-"servicemen," but the premise upon which the FCC examinations are based is a series of "elements," each of which requires progressively more advanced technical knowledge.

In addition to "Basic Radio Theory," which appeared in the September issue of Radio News, understanding the practical aspects of the commercial operator's work and the application of radio principles to actual equipment in use are required in these advanced elements. Therefore, the following classifications, including the scope of subjects covered, may be of assistance to prospective licensees who are planning a preparatory course of study.

Mathematics

Ohm's Law problems involving application of the Law to circuit analysis occur frequently. These arithmetical problems are easily solved when a knowledge of when and where currents and voltages appear in any given circuit is obtained. You should know, for example, that a milliammeter connected between the center tap of a filament transformer and ground, will read the combined plate and grid currents of a simple triode stage. You should also be able to analyze other components of a circuit in similar fashion.

Ability to make mathematical computations pertaining to decibel gain or loss is required, particularly for applicants for Radiotelephone licenses. Ratios between average, effective and peak values of a sinusoidal wave should be memorized, as problems utilizing these values occur frequently.

Formulas for computing, the angle of lead or lag of current with reference to voltage in an a.c. circuit should be learned and the meanings of "leading power factor" and "phase difference" understood. Study of phase angles will be simplified if you remember that in an a.c. circuit, a series condenser causes the current to lead the voltage and that when inductive reactance predominates in the circuit. current lags the voltage.

The importance of the formulas for reactance and impedance should not be underestimated. The formula for determining impedance at resonance is essential and the values of reactance in series, parallel, and series-parallel circuits should be known.

Fig. 4 - Modulated carrier wave.

The student should not become too perturbed at the vast array of mathematical problems which will confront him when he opens his examination sheet. All these problems are based on specific formulas and should present no difficulty provided these formulas have been learned well.

Motor Generators

Questions pertaining to motor generators will be primarily mathematical and concern the "efficiency" and "regulation" of this equipment as well as for rectifier power supplies. "Efficiency" and "regulation" are not just terms but have definite mathematical formulas which are essential.

Rectifier Tubes

Possessing basic knowledge of vacuum tube principles simplifies the specialized requirement for rectifier tubes. The various types, and their respective advantages and disadvantages should be understood; and "inverse peak voltage" mathematical problems, in which sinusoidal wave ratios are applied, occur frequently.

Rectifiers

A knowledge of the principles of rectification is not sufficient for the examinations in the advanced elements. Formulas for the computation of ripple frequencies, the circuits for half-wave, full-wave, and bridge rectifiers should be learned. Mechanical rectifiers cannot be ignored for you may encounter a question or two pertaining to them.

Filters

Fig. 5 - Antenna directional characteristics.

The function of the filter network in conjunction with a rectifier circuit is another must for prospective licensees. The effects of inductances and condensers in filters (see Figs. 1 and 2), the characteristics of "swinging chokes" and the applications of bleeder resistances should be mastered.

An analysis of the differences between low-pass and high-pass filters, as shown in Fig. 6, paves the way for the study of attenuators, line pads, line equalizers (Fig. 7), and mixers. The functions and principles of these various networks should be mastered, and their methods of connection to, and effects upon, circuits understood.

Impedance matching, Fig. 8, and the proper termination of transmission lines should be studied, particularly by those interested in Radiotelephone licenses.

Crystals

As crystals play an important part in all radio equipment their function in stabilizing oscillators should be studied carefully. To assure adequate knowledge of crystal principles, the student should understand the meaning of, and terms used, in referring to crystal temperature coefficients and learn to solve mathematical problems which involve the crystal temperature coefficient in determining a transmitter's output frequency.

Oscillators

Fig. 6 - (A) Low-pass filter. (B) High-pass filter. The condensers and inductors are designed to attenuate the higher frequencies (A) and the lower frequencies (B).

Fig. 7 - Equalizers. Parallel resonance is depicted in (A) while (B) shows the equalizer as a series resonance type.

Fig. 8 - (A) T-type pad. (B) H-type pad. Input terminals of the attenuation networks connect to microphone Z1,while the output connects to amplifier shown as Z2.

Comprehension of the basic oscillator circuits such as Colpitts, Hartley, MOPA and tuned-plate tuned-grid is vitally necessary. Surprisingly enough, the Commission demands a thorough understanding of ancient spark and arc oscillatory circuits.

The multi vibrator, or relaxation oscillator, possesses peculiar characteristics of its own and should be studied as an individual type of circuit.

Electron-coupled, dynatron, and crystal oscillators should also be understood and the ability to draw any of the basic oscillator circuits from memory is an asset.

The student should know which oscillator circuits are particularly useful for generating harmonics and understand the several methods of keying a transmitter.

Inasmuch as many receivers utilize oscillators in regenerative circuits, the principle of regeneration should be understood. In regard to this, an understanding of the relationships of signal frequency, beat frequency and image frequency, as well as the ability to make simple computations pertaining to them, should be mastered.

Other types of receivers, such as, plate, grid leak, power, and diode detectors should not be neglected. Sufficient knowledge of receiver principles should be obtained to enable the student to answer trouble shooting questions. Learn what the super-heterodyne circuit is!

Amplifiers

A thorough understanding of amplifiers and their classes of operation is perhaps the most crucial part of the requirements. All types of amplifier circuits should be studied, and here again the ability to trouble shoot, at least on the examination sheet, is essential. Special emphasis is placed on the principles of modulating amplifiers and the student should stress modulator circuits in his preparation for the examination.

Neutralization

The purposes and methods of neutralizing a radio frequency amplifier; how to test for proper neutralization and the instruments used in such tests should be studied. Problems covering methods of neutralization (see Fig. 3) may occur.

Modulation

The heart of all radio broadcasting systems is modulation. This is a major subject in itself, and, broadly speaking, one can't know too much about it. Particular attention should be paid to high and low level, grid and plate modulation.

Although frequency modulation is with us, insofar as the FCC examinations are concerned, it remains something which the broadcast operator must prevent at all costs in his AM transmitter.

Investigate the causes of carrier and frequency shift and the results of over and under excitation! Learn exactly what overmodulation is and its effect upon transmitters! The ability to draw a carrier wave envelope and indicate on that drawing the dimensions from which the percentage of modulation is determined (Fig. 4) will be very helpful. Mathematical formulas for determining percentage modulation must be learned and trouble shooting in modulators is quite important.

The fluctuations and percentages of increase in antenna current and power during modulation should be understood. Possession of adequate knowledge of the principles of modulation will do much to clarify other principles of radio broadcast operation.

The foregoing outline might be considered the backbone of the knowledge required to pass the FCC examinations.

Getting back to mathematics, for a moment, formulas used in determining the resonant frequency of an antenna and the methods whereby these frequencies may be raised or lowered should also be learned. The directional characteristics of loop and vertical antennas must be known, as shown in Fig. 5. The formula for determining antenna resistance and current is simply an adaptation of basic Ohm's Law but should not be overlooked.

For the student who is interested in the Radiotelephone license, a knowledge of the respective advantages and disadvantages of lateral and vertical methods of transcription is required.

For those prospective licensees who are trying for a Radiotelegraph license a good deal of emphasis should be placed upon the operation and trouble shooting in auto alarms. This is a complex subject but the FCC is interested in knowing whether you can operate, adjust and maintain the two types of auto alarms in common use.

The basic principles of radio, and you will certainly know them if you succeed in passing the FCC examinations, will carry you through the complexities of auto alarms.

The most important thing for the student to remember when preparing for the FCC Commercial Operator's examination is that without a solid background of radio fundamentals, the examination cannot be passed! The student must review radio fundamentals until this basic material becomes second nature to him when he sits down to take that important examination.

Posted March 26, 2021