Basic Radio is a free introductory textbook on electronics based on tubes. See the editorial for more information....



Problems And Questions

Author: J.B. Hoag

PROBLEMS AND QUESTIONS

1.1 — A 1000-volt battery is connected between the filament and plate of the vacuum tube shown in Fig. 1 B. Compute the velocity and energy with which the electrons strike the screen. Compare your computed velocity with that given in the graph of Fig. 1 C.

1.2—A moving electron has an energy of 500 e.v. What is its velocity in cms. per sec. and in miles per hour?

1.3 — Ten coulombs equal one " e.m.u." of electricity. From the numbers given in Sec. 1.2, compute the ratio of e.m.u. to electrostatic units. Compare with the velocity of light and with the velocity of radio waves. This comparison led Maxwell to the prediction of radio waves in 1864.

2.1 — A No. 14 copper conductor is 200 feet long, a second No. 14 copper conductor is 600 feet long.

2.1a — How will the resistance of the second compare with the first?

2.1b — If the first has a resistance of 0.516 ohm, what is the resistance of the 600-foot length?

2.1c — In general, how does the length of a conductor affect its resistance?

2.2 — The theory of metallic conduction tells us that the resistance of a conductor decreases as the cross-section or area of the conductor increases. (The mathematician calls this an inverse relationship.) No. 10 copper wire has a cross-section of 10,400 circular mils (abbrev. CM.), while No. 16 has a cross-section of 2,600 CM. (Both values are approximately correct.)

2.2a — How should the resistance of 500 feet of No. 16 copper wire compare with that of 500 feet of No. 10 copper wire?

2.2b — If the resistance of 1,000 feet of No. 10 copper wire is 1 ohm, what is the resistance of 500 feet of No. 16 copper wire?

2.3 — Copper wire is the kind most frequently employed in radio apparatus. Discuss several reasons why this metal is used.

2.4 — A resistor of 4 ohms is used in the filament circuit of a vacuum tube to limit the current to 0.3 ampere.

2.4a — What is the voltage drop across the resistor?

2.4b — What is the power in watts dissipated in the resistor?

2.4c — If the battery voltage is 6.2 volts, what is the voltage applied to the tube?

2.5 — Two 10,000-ohm, 1-watt resistors are connected in parallel.

2.5a — What is the effective resistance of the combination?

2.5b — What is the wattage dissipation of the two in parallel?

2.6 — Two 5,000-ohm, 2-watt resistors are connected in series.

2.6a — What is the effective resistance of the combination?

2.6b —How many watts can the combination dissipate?

2.7— A 6,000-ohm, 10-watt resistor is connected in series with a 4,000-ohm, 10-watt resistor.

2.7a — What is the resistance of the combination?

2.7b — If 40 milliamperes flow through the combination, determine the voltage drop across each and the power dissipated in watts by each resistor.

2.8 — The two resistors of 2.7 are connected in parallel and 40 ma. flow into the combination.

2.8a — What is the resistance of the combination?

2.8b—What is the voltage drop across the parallel circuit?

2.8c — What current flows in each branch?

2.8d — How does the sum of the branch currents compare with the current flowing into the combination?

2.8e — What is the dissipation in watts of each resistor and of the combination?

2.9 — A 5,000-ohm, 10-watt variable resistor is used in a certain experiment to regulate current.

2.9a — What is the maximum safe current when using all the resistance ?

2.9b — What is the maximum safe current when using 2,500 ohms of the rheostat? (Hint: Half the resistance means half the wattage rating. Why?)

2.10 — What is the voltage drop across the rheostat under the conditions stated in 2.9?

2.11 — Show the most economical connection for the filaments of three type-807 tubes (6.3 volts at 0.9 amp.) and 1 type-6C6 (6.3 volts at 0.3 amp.) if they are to be operated off 32 volts d.c. Determine the proper values of all resistors employed.

2.12 — The battery supply-leads to a transmitter have a total resistance of 0.0125 ohm and carry a current of 65 amperes, (a) Calculate the voltage drop in the leads, (b) Calculate the power loss in the leads.

2.13 — Four resistors are placed in parallel. The first has a resistance of 36 ohms, the second a resistance of 30 ohms, the third a resistance of 16 ohms, and the resistance of the fourth is unknown. The current through the entire circuit is 7 amperes, through the 16-ohm resistor it is ,2.5 amperes. Determine the value of the unknown resistance and the currents through the branches.

2.14 — The combined resistance of two resistors in parallel is 750 ohms. One has a value of 1,200 ohms. What is the other resistance?

2.15 — A broadcast station increases its power from 50 kw. to 500 kw. Determine the power increase in decibels.

2.16 — An amateur has a transmitter which produces an antenna current of 1.3 amperes. He rebuilds his transmitter and then obtains an antenna current of 3.1 amperes. Determine the gain in decibels. The antenna remains the same.

2.17 — An audio transformer has a 3-to-l turns ratio. If 0.4 volt is applied to the primary, determine the db. gain of the transformer.

2.18 — The output of a receiver is 2 watts into a 600-ohm load. A second receiver with the same input delivers 5.3 watts into a 2,000-ohm load. What is the output level of the first receiver with respect to the second ? What is the output of each receiver with respect to 6 mw.?

2.19 — An amplifier has a rating of 12 watts output and a gain of 60 db.

Calculate: (a) output level, (b) input level in db., (c) input power in watts,

(d) If the input and output impedances are 250 ohms, determine the voltage

gain in db., (e) the power gain in db.

2.20 — A carbon microphone has an output of 10 millivolts and a condenser microphone has an output voltage of 0.5 millivolt, both measured across equal impedances. Determine the level of the condenser microphone with respect to the carbon microphone.

2.21—A thermocouple-galvanometer has a scale of 100 equal divisions. Its resistance is 4.5 ohms and full scale deflection current is 125 ma. Determine the current when the meter deflection is 35 divisions.

3.1 — What is the practical unit of capacitance?

3.2 — When is the microfarad used in radio work? What is the symbol?

3.3 — When is the micromicrofarad used? What is the symbol?

3.4 — Why are air dielectric condensers physically so much larger than equivalent capacitance mica or paper dielectric types ?

3.5 — Show various symbols which would appear on a schematic diagram for a variable condenser and a fixed condenser for the following types: variable air, variable air split-stator, compression-type variable mica condenser, fixed mica or paper condenser, fixed electrolytic condenser.

3.6 — What is the practical unit of inductance?

3.7 — When are the millihenry and the microhenry used?

3.8 — What is the symbol for a fixed air-core inductance, an air-core tapped inductance, an iron-core inductance?

3.9 — What types of windings for inductances are employed principally for radio work?

3.10 —What is meant by the term " Time Constant "?

3.11 — What is the time constant for an R-C circuit of 0.05 μfd. capacitance and 0.5-megohm resistance?

3.12— What is the effect on the TC of increasing either R or C?

3.13 — When testing iron-core filter inductances with an ohmmeter (a milli-ammeter, resistor, and low-voltage dry battery in series), it is frequently observed that the needle on the meter changes quite slowly. Comment on this.

3.14a — Two coils, well shielded from each other, are connected in series. One has an inductance of 250 μh., the other an inductance of 120 μh. What is the inductance of the combination?

3.14b — The coils of problem 3.14a are connected in parallel. What is the effective inductance of the combination ?

3.15 — A condenser of 600 pF. is connected in parallel with one of 400 pF.

3.15a — What is the capacitance of the two together?

3.15b — If these condensers are charged across a source of 100 volts, what is the charge in coulombs in each?

3.16 — The two condensers of problem 3.15 are connected in series across the same 100-volt source. (The leakage is negligible.)

3.16a — What is the capacitance of the series combination?

3.16b — What is the charge in each condenser?

3.16c — What is the potential across each condenser?

3.17 —Show by means of a sketch how you would connect four 0.002-μfd. condensers to obtain a resultant capacitance of 0.002 μfd. This arrangement is sometimes used in transmitters. State several reasons why it is a useful device.

4.1 — A 4-pole alternator rotates at a uniform speed of 600 r.p.m. Determine the output frequency in cycles per second (Hz).

4.2 — An a.c. motor operating on 60 Hz rotates at the rate of 3,600 r.p.m. Determine the number of poles.

4.3 — What is the least number of poles that an a.c. synchronous motor can have to operate on 60 Hz?

4.4 — What is the effective value of a sinusoidal a.c. voltage which can produce the same degree of lamp brilliance as a 120-volt d.c. source produces? What is the maximum value of this a.c. voltage? What is the average value of the a.c. voltage averaged over the complete cycle? Why?

4.5 — Make a sketch showing an a.c. voltage in phase with an a.c. current.

4.6 — Make a sketch showing an a.c. voltage leading an a.c. current by 30°.

4.7 — Make a sketch showing an a.c. current leading an a.c. voltage by 60°.

4.8 — A radio-frequency ammeter, measuring sinusoidal current, reads 11.5 amperes. What is the peak value of this r.f. current?

4.9 — A peak-reading voltmeter reads 156 volts. Determine the effective value of this voltage, which is known to be sinusoidal.

4.10 — Make a sketch of a wave which has a peak value of 100 volts but whose effective value cannot be 70.7 volts.

4.11— Make a sketch of an a.c. current whose value, averaged over a cycle, cannot be zero.

4.12 — Make a sketch of an a.c. wave whose peak and effective values are identical.

4.13 — Make a sketch of a series of unidirectional pulses, each a half sine-wave in shape, and indicate as nearly to scale as you can the average value.

5.1 — A 5-μfd. condenser is used as an a.f. " bypass " condenser. What is its reactance at 100 Hz and at 4,000 Hz?

5.2 — What is the reactance of a 0.01-μfd. condenser at 456 kHz? At 3 MHz?

5.3— Determine the reactance of a 3-henry choke coil at 120 Hz and a 120-μh. coil at 3 Mhz.

5.4 — Coils for use at radio frequencies are often " doped " or dipped in wax to impregnate them. What effect will this have on the coil Q (= Xl/R) and the distributed capacitance?

5.5 — A coil has a reactance of 40 ohms and a resistance of 30 ohms. Determine the impedance of this coil.

5.6 — A condenser of 10-μfd. capacitance is connected in series with a 100-ohm resistor across a 50-volt, 1,000-cycle source. Find the circuit impedance, the circuit current, the voltage drop across the condenser, and the voltage drop across the resistor.

5.7 — A vacuum tube has both d.c. and a.c. in its output. Show by a sketch how a coil and a condenser could be used to separate the a.c. from the d.c.

5.8 — A coil has an inductive reactance of 100 ohms and a resistance of 10 ohms. Determine the impedance of the coil. How does this value of impedance compare with the coil reactance ?

5.9 — Under what conditions may the resistance of a coil or a condenser be neglected in comparison with its reactance value ? (Hint: the answer to problem 5.8 will help.)

5.10 — What factors influence the voltage ratio of an iron-core transformer? What is the relation between the primary current and the secondary current in a voltage step-up transformer? Express the voltage and current ratios as a statement that the two ratios are equal.

5.11— Since the magnitude of impedance can always be expressed by \Z\ = E/I, state why a transformer may be regarded as an impedance-changing device.

5.12 — A transformer is wound with 20,000 primary turns and 200 secondary turns. What is the impedance transformation ratio of this transformer?

5.13 — A certain vacuum tube requires a load impedance of 7,000 ohms and is to operate a 10-ohm permanent-magnet type loudspeaker. What is the proper turns ratio for the " matching transformer " ?

5.14 — The voltage across a 2.5-ohm voice coil with a 400 cycle-sinusoidal voltage is 1.5 volts r.m.s. (a) How much power is supplied to the voice coil? (b) What is the peak value of the a.c. voltage?

5.15 — Why is an ordinary low-frequency a.c. ammeter or a.c. voltmeter useless at radio frequencies? What type of meter is used at radio frequencies? What is the principle upon which it operates? Make a neat sketch of the scale of a 0-3 ampere radio-frequency ammeter showing the positions of the 0-, 1-, 2-, and 3-ampere positions.

6.1—A series circuit has a 253-μh. inductance and 100-pF. condenser. The radio frequency resistance of the coil is 8.2 ohms. One volt is applied to the circuit.

6.1a — What is the resonant frequency of the circuit?

6.1b —What is the circuit Q?

6.1c — What is the voltage appearing across the condenser at resonance?

6.1d — What is the voltage across the coil at resonance?

6.2a — If the coil and condenser of problem 6.1 are connected in parallel, what is the resonant impedance of the combination ?

6.2b —1,000 volts at the resonant frequency is applied to the parallel circuit. What is the current through the coil and what is the current through the condenser ?

6.3— A parallel circuit, resonant at 1,000 Hz, is required. A 0.2-μfd. condenser is available. What value of inductance will be required for this circuit?

6.4 — The output circuit for a transmitter has an inductance of 4.2 μh. in one branch and a capacitance of 120 pF. in the other branch. When the circuit is properly loaded, the resistance in the inductive branch is at 10 ohms.

6.4a — At what frequency is the circuit resonant ?

6.4b — What is the impedance of the parallel circuit when loaded?

6.4c — When the load is removed from the circuit, the resistance in the inductive branch becomes 0.8 ohm. What is the impedance of the circuit unloaded?

6.5 — A coil has an inductance of 180 μh. and a resistance of 10.2 ohms at 1,200 kHz. Determine the coil Q at this frequency.

6.6 — A coil has an inductance of 2.5 henries and a resistance of 2,000 ohms at 400 Hz. Determine the Q of this coil.

6.7 — Determine the resonant frequency of a coil of 200 μh. in series with a 0.0002-μfd. condenser.

6.8 — What value of coil is required to resonate with a 25-pF. condenser at a frequency of 15 MHz?

6.9 — The minimum capacitance in a tuned-circuit, tuned by a variable condenser to cover a band of frequencies beginning at 1,700 kHz, is 40 pF. The maximum capacitance is 400 pF.

6.9a — Determine the inductance of the coil required.

6.9b — Determine the frequency range covered by the coil and condenser.

6.10 — A " wave trap " may be either a series or a parallel resonant circuit. When such a series trap is connected across the input of a receiver, and tuned to an interfering signal, the interference is considerably reduced. A 10- to 250-pF. variable condenser is available. Determine how much inductance would be required to trap out an interfering signal on 1,420 kHz and discuss the action of the wave trap. Comment on the reasons for your choice of L and C values.

6.11 — A certain antenna circuit looks like a 20-ohm resistance and a 200-ohm capacitive reactance at a frequency 3,105 kHz. Determine the inductance required to resonate the antenna circuit. If this circuit has induced in it from the transmitter a voltage of 50 volts, what will the antenna ammeter read at 3,105 kHz? How much power will be delivered to the antenna?

6.12 — Why are quartz crystal resonators used in receiving apparatus? By means of two curves, compare the selectivity of an ordinary tuned circuit with that obtained when the crystal filter is introduced.

6.13—What factors affect the sharpness of resonance of a tuned circuit? What determines the value of the current at resonance in a series resonant circuit? Why is a high ratio of L to C desirable in a tuned circuit? What factor or factors control the practical limit of the L/C ratio?

6.14 — Under what conditions does a series circuit present minimum impedance which is entire resistance? At what frequencies will it present inductive reactance and at what frequencies will it present capacitive reactance? At what frequencies is the current almost solely determined by the reactance?

6.15 — Under what conditions does a parallel circuit present maximum impedance entirely resistive? At what frequencies will it present inductive reactance? At what frequencies will it present capacitive reactance?

6.16— Any inductance coil has distributed capacitance. Can you show that any coil will have an apparent inductance greater than its true inductance at certain frequencies, that it may appear as a pure resistance at certain discreet frequencies, and as a capacitance at other frequencies ?

7.1 — When a coil used at radio frequencies is placed inside a shield: What is the effect on the coil's inductance? What is the effect on the coil's resistance? What is the effect on the distributed capacitance ?

7.2 — At a.f., the use of non-magnetic materials for coil shielding is practically useless, while at radio frequencies the use of non-magnetic materials such as copper or aluminum is required. Explain why this is so.

7.3 — An r.f. shield with seams of high resistance running lengthwise of the shield is very detrimental, while the same type of seam running around the shield has little effect on the shielding. Account for this behavior.

7.4 — Indicate by a diagram how you would connect coils and condensers to make a π-type low-pass filter, a high-pass " T "-type filter.

7.5 — What class of filter would you employ to separate a zero frequency (d.c.) component from a number of a.c. components?

7.6 — What class of filter would you use to separate an r.f. component at 3,500 kHz from an a.f. component at 5,000 Hz ?

7.7 — Show by a diagram how a low-pass π-type filter, with cutoff frequency of 3,000 Hz, could be combined with a high-pass π-type, with cutoff of 2,000 Hz, to pass a band of frequencies approximately 1,000 Hz wide. Explain why the effect is " band pass."

7.8 — Show by the use of a diagram how a low-pass T-type filter and a high-pass T-type could be connected to eliminate a band of frequencies from 1,500 Hz to 2,000 Hz but pass all other frequencies from 100 to 5,000 Hz. Indicate the approximate cutoff frequency of each type and explain the operation.

7.9 — When is more than one filter section required ?

7.10 — If one filter section provides an attenuation of 60-to-l of an unde-sired voltage, how much attenuation will two similar filter sections provide?

7.11 — A low-pass filter working from 5,000 ohms into 5,000 ohms is to cut off at 4,000 Hz and be of the π-type. Determine the values of L and C required.

7.12—Determine the constants for a T-type high-pass filter to cut off at 5,000 Hz working from 100,000 ohms into 100,000 ohms.

8.1a — What is the distance in feet from " crest-to-crest " of a wave-length of 4,000 meters, 500 meters, 200 meters, 40 meters 7.5 meters and 20 centimeters?

8.1b — Determine the frequency in kHz for each of the above wavelengths.

8.2 —What is the wave-length in meters of a 2,000-kHz wave? Of a 7-MHz wave, of a 120,000-kHz wave, of a 5,000-MHz wave?

8.3a — A certain antenna acts in such a way that each meter of its length is equally effective in having a voltage induced in it by a passing wave. If it has induced in it a voltage of 72 μv. and has a length of 8 meters, what is the field strength?

8.3b — If this antenna is connected to a receiver, what voltage will appear at the receiver input posts if the field strength of a broadcast station at that locality is 0.4 millivolt per meter?

8.4 — What is the effect on the operation of an antenna when its electrical wave-length is the same as that of a passing wave?

8.5 — Suppose that a transmission line is 4 wave-lengths long and that energy is sent down the line. The end of the line is open circuited. Comment on the distribution of voltage and current on the line for this open circuit condition.

9.1 — What part of the radiated wave energy is a low-frequency radio telegraph or a broadcast station particularly interested in? Why?

9.2 — Explain why the ionosphere is useful for radio communication.

9.3 — What approximate frequency band is useful for long-distance, low-power, daytime communication? Night-time? Do the seasons affect the useful frequencies ? Illustrate.

9.4 — How can fading in the broadcast band, at distances approximately 70 miles from the transmitter, be explained?

9.5 — How does fading probably occur on the high-frequency bands where only the sky-wave is received ?

9.6 — State several striking differences between transmission on 7,000 kHz and 60,000 kHz.

10.1a —Under what conditions will the anode current be independent of anode voltage in a high-vacuum tube ?

10.1b — Under what conditions will the anode current be dependent on the anode voltage ?

10.2 — What is meant by " space charge " ? In the presence of space charge, where is the most negative space in the tube ?

10.3 — What type of cathode is most commonly used in receiving tubes and some low-powered transmitting tube types? What type is most frequently used in most glass envelope transmitting types and what type is used in high-power types? Discuss the probable reason for this.

10.4 — Which type of filament can be damaged by overload? What can be done in some cases to repair this condition? What is this process called?

10.5 — Distinguish between direct and indirect heating of the cathode. What class of tubes mostly employ indirect heating of cathode. Where would you expect to find practically all types directly heated? What advantages are possessed by indirect cathode heating?

10.6 — What is meant by " cathode leakage " ? In what tube types would it be found?

10.7 — Suggest a possible condition whereby a meter inserted in the anode circuit might indicate a direction of electron flow opposite to that ordinarily expected. Account for this behavior.

10.8 — Why must thoriated tungsten filaments be operated at specified voltage whereas the filament voltage on the other types is not so critical ?

11.1 — A half-wave, single-phase rectifier tube is used with a condenser-input type of filter. The r.m.s. value of the voltage is 117 volts. Why is the peak inverse voltage applied to the tube considerably greater than the peak value of the peak a.c. voltage from the supply source? What is the greatest value that the peak inverse voltage could attain?

11.2 — A full-wave, center-tap single-phase rectifier uses a transformer and an 80-type tube. The rectifier operates into a π-type condenser-input filter. The r.m.s. secondary voltage per half is 300 volts. Represent the load by a 5,000-ohm resistor.

11.2a — Make a diagram showing the apparatus properly connected.

11.2b — Trace the current flow for each alternation and show that the current in the load is unidirectional.

11.2c — Make a diagram to show why the current through each half of the rectifier is peaked and explain why it flows for less than half the time per alternation.

11.2d — Explain in your own words the purpose of each filter element (i.e., choke and condensers).

11.2e—State why the output voltage will increase as the load current decreases and why the output voltage decreases as the load increases.

11.2f — What factors will influence the regulation of the output voltage?

11.3 — Why is a three-phase rectifier system preferable to a single-phase full-wave rectifier system in higher power units?

11.4a — What are the two principal types of vibrators ?

11.4b — Why is the polarity of the supply voltage very important for one type but unimportant for the other?

11.4c — Make a schematic circuit diagram for each type and explain the operation.

11.4d — State the important points in the care and maintenance of a vibrator power supply.

11.5 — D.C. core saturation of the transformer secondary is a limitation in the operation of a half-wave rectifier. How is this limitation removed in the full-wave connection?

12.1 — In general, how would you determine the cutoff bias for a triode tube?

12.2a — From the characteristic curves for a typical triode tube, determine the amplification factor graphically.

12.2b — Also determine the a.c. plate resistance.

12.2c — From these two, determine the transconductance.

12.3 — What feature of the Ip-Ep characteristic curves indicates whether the tube has relatively high or low plate resistance?

12.4 — What is meant by the " linear portion " of the characteristic?

12.5 — Under what conditions may the so-called " static " characteristic curves for a tube be considered as the " dynamic " characteristics ?

12.6 — Which of the tube constants p., rP, gm, gives the maximum voltage amplification most readily ? Why ?

13.1 — State the characteristics of " Class A " operation.

13.2 — What are the important factors affecting the input capacitance of a triode tube ?

13.3 — One stage in a two-stage resistance-coupled amplifier has a measured gain of 60, and a second has a gain of 12. What is the total voltage gain of the amplifier ?

13.4a — A d.c. meter inserted in the plate circuit of a Class A amplifier, with proper bias and excitation, will show negligible change in reading. Explain.

13.4b — If the bias is excessive, the meter will show an increased reading with normal excitation. Explain.

13.4c — Similarly, with too low bias, the meter will show a decrease with normal excitation. Explain.

13.5 — Make a diagram, with time as horizontal axis, to show the instantaneous relations in the grid and plate voltage. Use a sinusoidal a.c. component of excitation and show (not necessarily to scale) the average bias voltage, the excitation voltage, the average plate voltage, the instantaneous plate voltage, the average and instantaneous plate current. From this diagram, show that the a.c. voltage in the plate circuit is 180° out of phase with the a.c. grid voltage.

13.6 — Explain in your own words why the omission of a cathode bypass condenser will decrease the gain of a stage.

13.7 — What is the essential difference between Class A power amplifier tubes and Class A voltage amplifier tubes ?

14.1 — State in your own words how you distinguish a Hartley type oscillator circuit from a Colpitts type.

14.2 — What is the principal function of an oscillator?

14.3 —What is the effect on oscillator operation of an excessively high value of grid-leak resistor?

14.4 — A pendulum clock has a pendulum, a main spring, and escapement movement. An oscillator circuit comprises a tube, an L-C circuit and a " B " battery. Pair up the analogous parts.

14.5 — Make a schematic diagram of a tuned-plate Hartley-type oscillator employing a 6J5GT tube and a series feed of the power supply. (This circuit is often found in superheterodyne receivers.)

14.6 — How is the feedback voltage controlled in the Hartley, Colpitts, and tuned-grid tuned-plate types of oscillators?

14.7a — How can a two-stage resistance-coupled amplifier be converted into a multivibrator?

14.7b — What are some uses of the multivibrator? How can the frequency of a multivibrator be controlled ?

14.7c — Why do the values of R and C in the grid circuits of a multivibrator influence the output frequency ?

14.8a — Under what conditions do you think the use of a push-pull oscillator desirable ?

14.8b — When is a single-ended oscillator more advantageous than a push-pull oscillator? Justify your statements.

15.1— What are the characteristics of a tetrode tube which make it superior to a triode as an r.f. amplifier? Explain.

15.2 — Why is the pentode superior to both the triode and tetrode as an r.f. amplifier? Explain.

15.3 — Show by means of the characteristic curves for a triode and a pentode why the a.c. plate resistance of the pentode is much greater than the triode.

15.4 — Discuss the difference between a variable-μ pentode and a sharp cutoff pentode and state where each type would be most advantageously employed.

15.5 — What advantages does the beam tube possess over other tube types? Where are beam tubes principally employed?

15.6 — List several combination tubes and state the uses to which each type may be put.

15.7 — What is the effect of gas in a high-vacuum type of tube? How can the presence of gas in such a tube be detected and what is the principal remedy for dealing with a gassy tube?

16.1 —A carrier wave on 3,000 kHz is amplitude modulated with a 1,000-cycle tone.

16.1a — State the frequencies appearing in the output.

16.1b — What are these additional frequencies called?

16.1c — What determines the width of the transmission band required for a radio telephone transmitter using amplitude modulation ?

16.2 — A carrier with peak voltage of 50 volts is amplitude modulated. The peak r.f. voltage at the modulation crest is 90 volts, at the trough is 10 volts.

16.2a— Make a diagram to scale showing the modulated wave.

16.2b — Determine the percentage modulation produced.

16.2c — What would be the crest voltage for 100 per cent modulation?

16.2d — What would be the trough voltage for 100 per cent modulation?

16.3 — Why must the r.f. driver stage for a grid-modulated Class C amplifier have good regulation?

16.4 — Why cannot suppressor-grid modulation be employed with a triode tube?

16.5 — Suppressor-grid modulation is similar to control-grid modulation of a triode. Explain.

16.6 — Cathode modulation is said to embody simultaneous grid and plate modulation. Explain why this is true.

17.1 — In the diode detector, there are at least three components appearing across the diode load resistance when an amplitude-modulated signal is demodulated. Consider a carrier on 2,500 kHz with tone modulation at 1,000 c.p.s.

17.1a — What are these three components?

17.1b — If the carrier has a value of 20 volts peak and the modulation is 50 per cent, what is the peak value of the 1,000-cycle tone appearing across the diode load resistance ? Show by a diagram why this is so.

17.2 — Explain why the grid-leak detector is more sensitive than the plate detector.

17.3 — By a diagram show how the plate detector rectifies the signal applied to it.

17.4 — Why does the tuned circuit supplying the plate detector have better selectivity than the same circuit applied to a diode detector?

17.5 — What is the principle upon which the regenerative detector operates ?

17.6 — Make a schematic diagram of a regenerative detector using a pentode tube. Control the regeneration by controlling the screen voltage.

18.1 — Draw the circuit of a relaxation oscillator using a glow-tube, and a graph of the voltage across the condenser vs. time.

18.2 — A small wheel, with one spoke painted white, is rotated by a motor. When illuminated by a Strobotac whose flashing rate is 3,600 per min., two stationary white spokes are seen, diametrically opposite each other. Explain. What is the r.p.m. of the motor?

18.3 — Mercury vapor is used in the larger thyratrons. What advantage and what disadvantage does it have in comparison with the argon used in the very small tubes ?

18.4 — What precautions must be followed in the operation of gas-filled tubes and in the design of their circuits to prevent damaging them?

19.1 — Draw diagrams such as those of Figs. 19 E and F to illustrate a few of the possibilities of applying a.c. on the grid, the shield-grid, and the plate of a gas-filled tetrode.

19.2 — Draw the circuit of a full-wave rectifier using gas-filled two-electrode tubes and state the precautions which must be taken in its operation.

19.3 — Why does a tuned output transformer in an inverter circuit yield comparatively pure sinusoidal output voltages?

20.1 — Describe several applications of photocells, stating the type of cell which would preferably be used in each case.

20.2 — Draw a circuit for photocell control of an a.c. operated thyratron. Hint: see Phase Shifters.

20.3a — Ten foot-candles are received on a surface 2 ft. from a source. What is the candlepower? What will be the illumination at twice this distance?

20.3b — What will be the light flux in lumens passing through an area of 1 sq. in. at a distance of 4 ft. from the source in problem 20.3a ?

20.3c — What is the total flux in lumens from the source in problem 20.3a?

20.3d — The source of light in 20.3a is used to illuminate a photocell of area 1 sq. cm. and sensitivity of 40 microamperes per lumen. If the cell is at a distance of 1 ft., what is the output current from the cell?

20.4 — Under what conditions does a photo-multiplier tube prove to be especially advantageous?

21.1 — State at least two precautions which must be followed in order to prevent damaging a cathode-ray tube.

21.2 — If the electric deflection sensitivity of a cathode-ray tube is 0.1 mm. per volt, how long a line will appear on the screen when the 110-volt a.c. supply is connected across the deflection plates?

21.3 — State two methods of changing the brightness of the light on the screen of a cathode-ray tube.

22.1a — How could the CRO be used as a voltmeter? Would it read peak or r.m.s. volts?

22.1b — How could it be used as an ammeter? Show by means of a diagram how you could observe the shape of the charging current to a storage battery from a half-wave Tungar rectifier.

22.2— What is the main reason for the use of a linear sweep-circuit? What is the principal limitation of the linear sweep-circuit in most modern oscilloscopes?

22.3a — How can Lissajou figures be employed to check the frequency of an audio-frequency oscillator? Make a diagram to show the connections of the apparatus employed.

22.3b— Could the same scheme be used to check the frequency of a crystal oscillator against a standard frequency? Explain.

22.4 — A square wave of 100 c.p.s. is applied to VERT, plates while a linear saw tooth of 200 c.p.s. is applied to the HOR. plates. Show by a diagram the pattern resulting.

22.5 — Develop graphically the pattern resulting when two equal but 90° out-of-phase voltages are applied to the VERT, and HOR. deflection plates of the CRO.

23.1 — Distinguish between a linear circuit element and a non-linear circuit element. Give an example of each.

23.2a — How can amplitude distortion be produced in a Class A amplifier? How is it minimized ?

23.2b — How can frequency distortion be produced in a Class A amplifier?

23.3 — What is the approximate upper limit of distortion permissible in triodes ? Pentodes ?

23.4a — What is meant by Class AB operation?

23.4b — Distinguish between Class AB1 and Class AB2 amplifiers.

23.5 — Class C amplifiers operate with the highest power output and plate efficiency of all types. Explain how this is accomplished.

23.6 — The plate current in a Class C amplifier is rich in harmonics. Explain how these harmonics are greatly reduced by the use of a tuned " tank circuit."

24.1 — Make a schematic diagram of a Class A voltage amplifier using a cathode bias and a resistance-capacitance coupling to the following stage. Indicate approximately suitable values for the circuit elements. Use a pentode tube.

24.2 — In the resistance-capacitance coupled amplifier, the grid blocking condenser must have a very high leakage resistance. Tell why this is so.

24.3 — In most applications of tubes as amplifiers, the Tube Manual makes the statement, " The grid leak must not exceed 1 megohm," or some similar value. Can you suggest why the grid-leak resistance must not exceed a certain value ?

24.4 — The value of the actual plate voltage available at the plate of a resistance-capacitance-coupled pentode amplifier must be maintained at a fairly high value, thus limiting the maximum plate loading resistor. Explain why this is true.

24.5 — Why are plate and grid filters essential in high-gain amplifiers ? Make a diagram to show an amplifier with grid and plate filtering.

24.6 — Resistance-capacitance-coupled amplifiers are commonly called " resistance-coupled " amplifiers. Differentiate between these and true resistance-coupled amplifiers, both in their circuits and in their applications.

25.1 — What is the equivalent electrical circuit of a transformer-coupled amplifier?

25.2 — What is the controlling factor in frequency distortion of a transformer-coupled amplifier?

25.3 — Compare the single-button carbon, dynamic, and crystal microphones as to (1) relative output, (2) output impedance, (3) frequency range, (4) cost, (5) maintenance, (6) ruggedness.

25.4 — What is meant by the term " pre-amplifier " ? When is the preamplifier used ?

25.5 — Make a diagram of a typical two-stage speech amplifier which could be used with either a SB carbon or a dynamic microphone. Include a gain control which is practical.

25.6 — What is meant by " packing " in a carbon microphone? How would such a condition occur, how would you test for it, and how can such a condition be remedied?

26.1 — Compute the effective gain of a degenerative (and of a regenerative) feedback amplifier whose voltage amplification is 50, when the feedback voltage is 1 per cent of the output voltage.

26.2 — Discuss the tendency of feedback amplifiers to go into oscillation.

26.3 — Compare the circuit of a regenerative feedback amplifier with that of a multivibrator.

26.4 — State the advantages of a feedback amplifier which has both regeneration and degeneration.

27.1 — How does a radio-frequency amplifier differ from an audio-frequency amplifier ?

27.2 — Why are radio-frequency amplifiers as used in receivers usually operated Class " A "?

27.3 — What is meant by " sharpness of resonance "?

27.4 — What is a band-pass radio-frequency amplifier?

27.5 — How does the amplification and selectivity of a radio-frequency amplifier vary with frequency ? Why?

27.6 — What is the effect of regeneration in a radio-frequency amplifier?

27.7 — What class of radio-frequency amplifier is generally used for transmitting purposes?

27.8— What is the average efficiency of radio-frequency power amplifiers?

27.9 — Why is it necessary to neutralize a triode radio-frequency amplifier both in receivers and transmitters?

27.10 — What are the two principal types of neutralizing circuits employed in practice? Make a diagram of a typical amplifier for both types.

27.11 — State, in the proper order, the steps necessary to neutralize a triode amplifier.

27.12 — How would a sensitive thermocouple milliammeter be used in the neutralizing process? Could a CRO be used as an indicator of neutralization? If so, show how it should be connected to the amplifier being neutralized ?

27.13 — A Class C, r.f. amplifier supplies 140 watts to an antenna with a plate efficiency of 70 per cent. If the d.c. plate voltage is 1,250 volts, what is the d.c. plate current?

27.14 — Distinguish between plate efficiency and power amplification ratio. What class of amplifier has the highest efficiency ? What class has the highest power amplification ratio ?

28.1 — In a plate-modulated Class C, r.f. amplifier what is the source of the energy for modulation?

28.2a —The plate input to a Class C amplifier unmodulated is 200 watts. How much audio power is required for complete modulation?

28.2b— If the plate efficiency of the amplifier in problem 28.2a is 75 per cent, how much power will be supplied to the antenna if the coupling circuit is 95 per cent efficient?

28.2c — How much power is present in the radiated sideband energy when the modulation in 28.2b is 100 per cent?

28.3 — Itemize the required conditions to be fulfilled for a Class C amplifier to be capable of practically distortionless modulation characteristics.

28.4 — Why do the readings of the plate current and grid current meters in a Class C plate-modulated amplifier remain substantially constant regardless of the presence or absence of modulation?

28.5 — Show by means of a block diagram how to use a CRO in checking the modulation characteristics of a telephone transmitter.

28.6 — State the cause and remedy of three defects that can produce " downward modulation " in a radio telephone transmitter.

28.7a — Using the Heissing " common choke " method of plate modulation, with the same d.c. voltages on the modulator and modulated amplifier plates, why is it impossible to obtain 100 per cent modulation without severe distortion?

28.7b — How is the condition of 100 per cent modulation obtained in practice when the above scheme is used ?

28.8 — A Class C, r.f. amplifier is to be plate modulated 100 per cent using the common choke method from a Class A modulator, type 845. For a plate supply voltage of 1,250 on the 845, the RCA Trans. Tube Manual states that the undistorted power output is 24. watts when working into a load of 16,000 ohms. Find:

28.8a — The proper value of plate voltage for the r.f. amplifier.

28.8b — The proper plate current for the r.f. amplifier.

28.8c — The value of the dropping resistor between the modulator and the modulated amplifier.

28.8d— What size and voltage rating bypass condenser would you recommend for bypassing the resistor of problem 28.8c?

28.8e — What current rating and inductance would you recommend for the modulation choke?

28.9 — When using a tetrode or pentode modulated amplifier why is it advantageous to modulate the plate and screen simultaneously?

28.10 — How is grid modulation of a Class C amplifier accomplished?

28.11 — What factors determine the degree and linearity of modulation in a Class C amplifier?

28.12 — What methods of coupling the modulator to the modulated amplifier are commonly used? What are their relative advantages and disadvantages?

28.13— A Class C modulated amplifier operates with Ep equal to 2,000 volts, antenna current unmodulated being 6.4 amperes. If the plate voltage is reduced to 1,250 volts, what is the unmodulated antenna current for the new plate voltage condition?

28.14 — With a d.c. grid current of 15 ma. average value, what grid-leak resistor will be needed to furnish an operating bias of 75 volts ?

28.15 — A Class B audio amplifier has an output transformer with taps at 5,000 and 10,000 ohms. It is to be used to modulate a Class C amplifier. The

28.15a — What Class C amplifier input will this amplifier modulate completely ?

28.15b — Under what Class C amplifier operating condition would you use the 5,000-ohm tap and when the 10,000-ohm tap?

28.16—Discuss the necessary relationships which must exist between the modulator and the modulated amplifier in a Class C plate modulation system for optimum operation of both.

28.17 — Compare plate modulation, grid bias, and suppressor-grid modulation methods with respect to their practical advantages and disadvantages.

29.1 — Make a workable schematic diagram of an electron-coupled oscillator using the Hartley circuit. Indicate the parts of the circuit which determine the frequency and the parts associated with the " work " circuit.

29.2 — In the practical version of the electron-coupled circuit, approximately where should the cathode tap be placed from the ground end ?

29.3a — Why is the use of a voltage divider advisable in supplying voltage for the screen grid of the electron-coupled oscillator tube rather than the use of a series resistance?

29.3b — What advantages does the electron-coupled oscillator possess over other types of self-controlled oscillators ?

29.4a — What characteristic of the quartz crystal makes it excellent to control the frequency of oscillators?

29.4b — Why are the low temperature-frequency-coefficient quartz crystals superior to other types in crystal-controlled oscillators ?

29.5 — An X-cut crystal has a temperature-frequency coefficient of —18 Hz per MHz per C, and is operating in an oscillating circuit at 1,410 kHz at a temperature of 50C. What is the frequency if the temperature changes to 85C?

29.6 — An AT-cut crystal has a thickness of 1.27 mm. Determine its approximate operating frequency.

29.7 — What defect of the standard quartz crystal circuit does the Tri-tet circuit overcome? Explain why this is so.

29.8a — Name five important uses of oscillators.

29.8b — Comment on the need in certain cases for a high order of frequency stability in an oscillator, whereas in certain other cases the power output may be paramount.

29.9a — What phase relation must be observed between the grid and plate voltages ? How is this obtained in a " tickler " feedback oscillator ?

29.9b — What is the usual order of plate efficiency in an oscillator ?

29.9c — A 203A tube requires 7 watts to supply the grid losses and this tube delivers 120 watts to its tank circuit. The plate voltage is 1,000 volts and the plate current is 170 ma. The tank circuit absorbs 10 watts in inherent losses. How much power will be supplied to the load? What is the plate efficiency?

29.10 — A power-type Hartley oscillator is to be used in a transmitter for the master oscillator stage and is to cover a frequency range from 250 kHz to 550 kHz. What considerations should be made with respect to obtaining reasonable efficiency and frequency stability?

30.1 —What alterations should be made in the circuit of Fig. 30 A to change it from a square-wave to a pulse generator?

30.2 — Explain in detail how a multivibrator oscillates.

30.3 — Plan the circuits of a " fathometer," wherein a succession of short sound pulses sent to the bottom from a ship are reflected back and picked up with a microphone. Compute the time delay between the direct and reflected pulses when the depth is 100 fathoms.

30.4 — Compute the virtual height of the ionosphere when the echo delay time of the pulse method is 0.001 sec.

30.5 — What type of circuit needs to be added in front of the circuit of Fig. 30 I to use it as a frequency meter for alternating current?

30.6 — Draw the circuit of a triple coincidence counter using photocells at the input. Explain how it might be used to determine the direction of a distant light source.

30.7 — In how many steps will the condenser of Fig. 30 L be charged for each discharge if the input pulse rate is 20,000 and the output rate is 4,000?

30.8 — Draw the circuit suggested at the end of Sec. 30.13.

30.9 — Draw a time-delay circuit operating on the principle that it takes time for a condenser to fill up through a resistor.

31.1a — Why do modern transmitters employ an oscillator and one or more stages of power amplification rather than having the oscillator work directly into the antenna?

31.1b — What factors govern the number of stages a transmitter will require?

31.2 — What factors govern the choice of a buffer amplifier to be used as a driver stage?

31.3 — What is the essential difference between r.f. amplifiers in transmitters and r.f. amplifiers in receivers? Why should this difference exist?

31.4— An r.f. amplifier is supplied in the plate with 200 ma. at 475 volts. If the plate efficiency is 65 per cent, what power will be supplied to the tank circuit ?

31.5a — What condition must be fulfilled in the plate circuit if the amplifier tube is to operate with maximum efficiency for a given set of bias, excitation and supply voltage values?

31.5b — How is this condition realized in practice?

31.6a — What does the term "parasitic oscillations" mean?

31.6b — What operating characteristics of an amplifier leads one to suspect the presence of parasitics ?

31.6c — At what frequencies do parasitic oscillations occur? What is responsible for their presence? Why may they appear to be entirely absent in low-power stages yet pronounced in higher-power stages ?

31.7a — Why is it unnecessary to neutralize a frequency multiplier stage?

31.7b — Why is it necessary to operate a frequency multiplier at a high bias voltage?

31.7c — What tube types make better frequency multipliers than others? What tube types should follow a multiplying stage?

31.8 — A crystal oscillator operates on 2,200 kHz. It is desired to have the output of the transmitter on 13.2 MHz. Explain how this could be done.

31.9a — Explain what is meant by "high-level" modulation.

31.9b — A transmitter has a crystal-controlled oscillator, two buffer stages and a power-amplifier output stage. The second power-amplifier stage is plate modulated. What class of telephone transmitter is this? What class of amplifier operation must the P.A. stage be?

31.10 — State the steps necessary to place a high-level telephone transmitter " on the air."

31.11 —Explain the procedure to adjust properly a Class B, r.f. amplifier for proper operation as an amplifier of a modulated wave.

31.12a — Why cannot a Class C amplifier be used to amplify an amplitude-modulated wave?

31.12b — Why is Class B the type of operation for amplification of a modulated wave?

32.1a — What determines the usable sensitivity of a receiver?

32.1b — What noise voltages are generated in a receiver?

32.1c — Why is the first stage in a receiver the important stage from the standpoint of inherent noise?

32.2 — What simple test could you make to determine whether the noise is being generated in the first tuned circuit or the tube ?

32.3 — Why is a receiver with at least one stage of radio-frequency amplification less noisy (even though more sensitive) than a receiver in which the first tuned circuit feeds the mixer tube directly?

32.4a— Discuss the selectivity requirements for a receiver to be used for telegraphy, communication telephony (150-3,000 Hz), normal broadcasting (100-10,000 Hz), and high fidelity broadcasting (30-15,000 Hz).

32.4b — What part (or parts) of the receiver is (or are) mainly responsible for the selectivity ?

32.4c — Is the use of a narrow band-pass filter in the audio section of the receiver ever employed? What is the advantage of such a scheme?

32.5 — Discuss a method to measure the over-all fidelity of a receiver. State what equipment is required and how you would conduct the test.

32.6 — A superheterodyne receiver is tuned to 2,738 kHz. The intermediate frequency is 475 kHz. To what frequency is the demodulator circuit tuned?

32.7 — A superheterodyne receiver has an i.f. of 465 kHz and is tuned to 1,712 kHz. What is the frequency of the high-frequency oscillator circuit ?

32.8 — A superheterodyne receiver is tuned to 1,450 kHz. It has an i.f. of 456 kHz, and is experiencing " image" interference. Determine two possible " image " frequencies to cause this interference.

32.9a — State cause and effect of space-charge coupling in a converter tube.

32.9b — At what frequencies in the range of an all-wave receiver is this trouble most likely to be serious?

32.10 — State the principal reasons for the development of the pentagrid converter tube.

32.11 —What is the principal reason for the use of a separate oscillator tube when used with a pentagrid mixer tube ? What defect will the use of a separate oscillator not eliminate?

32.12 — What are the principal problems to be considered in the design of an oscillator to be used in the frequency conversion system of a superhet receiver?

32.13 — If the gain of an r.f. stage is 5, mixer gain is 15, and an i.f. amplifier per stage is 100, and two i.f. stages are used, what is the over-all gain?

32.14—What are three conditions which must be fulfilled if two tuned circuits are to track?

32.15 — The high-frequency oscillator of a superheterodyne receiver is almost universally operated at a higher frequency than the received signal. Why is the procedure justified economically?

32.16a — A broadcast receiver with one tuned circuit between the antenna and the mixer grid receives a local broadcast station (assigned frequency 1,500 kHz) at both 1,500 kHz and 570 kHz. The receiver's i.f. is 465 kHz. Explain why this occurs.

32.16b — How could this defect in the receiver operation be remedied?

32.17a — What apparatus is required to align a receiver?

32.17b — A receiver is to be aligned. The aligning frequencies are 2,850 kHz and 1,750 kHz. The receiver i.f. is 456 kHz and has one r.f. stage, a mixer stage, and a separate h.f. oscillator stage. Make a schematic diagram of the circuits to be aligned, indicating the aligning condensers.

32.17c— Outline, step-by-step, the alignment process.

32.18 — In many multi-band superheterodyne receivers, there is no provision for oscillator alignment at the low frequency end of the higher frequency bands. Account for the reason why these are not required.

32.19a — What are the important requirements of an a.v.c. system?

32.19b —What is meant by "straight a.v.c"? "delayed a.v.c"? "amplified a.v.c"?

32.20a — What is the principal defect of an a.v.c. system which has too small a Time Constant?

32.20b — What is the principal defect of an a.v.c. system which has too great a Time Constant ?

32.21 — Why do not a.v.c. systems, of the type ordinarily used, work satisfactorily for the reception of telegraph signals?

32.22 — State two important results in the use of r.f. amplification ahead of a detector stage in a t.r.f. receiver.

33.1—What are some advantages of f.m over a.m.? Why is narrow-band f.m. (±15 kHz) better than wide-band (±75 kHz) for communication circuits?

33.2a — How might a condenser microphone be used with a low-power oscillator to produce a frequency modulated signal ?

33.2b — What are the methods employed commercially to produce frequency modulation?

33.3 — What is meant by a reactance tube modulator?

33.4— A reactance tube modulator produces a total swing of 2 kHz about a mean frequency of 4,800 kHz. How may this frequency-modulated oscillator be used to produce a signal on 38.4 MHz and what will be the total deviation?

33.5 — If the highest modulation frequency is 4,000 Hz, what is the deviation ratio of the above transmitter?

33.6 — Why is it not possible to employ a CRO to check an f.m. transmitter? What class of amplifier operation would be used in all stages of the f.m. transmitter? Why can an amplifier tube be more completely utilized in an f.m. transmitter than in an a.m. transmitter?

33.7a — What variation will be observed in the d.c. plate and grid current meters on an f.m. transmitter? Explain.

33.7b — What variation will be observed on the antenna current ammeter? Explain.

33.8 — Compare the modulator power needed for an f.m. transmitter as contrasted with the modulator power needed for a.m. transmitters.

33.9 — Outline a method for measuring the deviation linearity of an f.m. transmitter. State the equipment required and show by means of a block diagram how it would be connected.

33.10a — To receive frequency-modulated signals, the tuned circuits of the receiver must be much wider than for the reception of amplitude-modulated signals. How is this accomplished in the f.m. superheterodyne and what circuits are considered ?

33.10b — What other differences exist in the superheterodyne receiver for the reception of f.m. as contrasted with a.m. receivers?

33.11a — What is the purpose of the limiter tube in an f.m. receiver? Why is a cascaded two-stage limiter superior to a single limiter stage ?

33.11b — How is limiter operation accomplished in the tube operation?

33.12 — Explain why no special a.v.c. circuit is required in the f.m. receiver.

33.13 — A receiver designed for reception of wide-band f.m. (±75 kHz swing) can be used for the reception of narrow-band f.m. (±15 kHz), but a narrowband receiver will not receive wide-band signals without excessive distortion. Comment on the results to be expected with either type receiver when receiving signals for which it is not designed.

33.14 — The input signal which will saturate the limiter (or second limiter) will determine the ultimate receiver sensitivity. Why is this so ?

33.15 — State the method you would use and the equipment required to adjust a frequency modulated receiver for proper operation.

34.1 — A ship fixes its position by cross-bearings on two known transmitting stations. As a check, another bearing is taken on a third transmitter. The three lines do not intersect at the same point. What would you do?

34.2 — Would you ground or open all other antenna circuits when taking a bearing with a direction finder? Why?

34.3 — A direction finder has just been installed aboard a ship. State the procedure for its calibration.

34.4 — Why are marker beacons installed at frequent intervals in mountainous country ?

34.5 — Why does the accuracy of the A and N beacon increase as you approach the transmitter?

34.6 — What advantage does the vertical antenna radiator system possess over the crossed-loop antenna system ?

35.1 — A power amplifier tube is designed to work into a load of 4,000 ohms. The antenna resistance at the point of coupling is 24 ohms. The P.A. output circuit is a parallel resonant circuit inductively coupled to the output which consists of a coil and condenser in series-with the antenna circuit connected conventionally. Explain clearly how the 24 ohms at the antenna are transformed into a 4,000-ohm load for the tube.

35.2a — What is the difference between a resonant and a non-resonant transmission-line system?

35.2b — Why does any transmission line which is terminated in its characteristic impedance look like a line infinitely long to the source of energy ?

35.2c — What is the effect on transmission-line operation if a transmission line is not terminated in its proper impedance? How could a line be checked to determine if the line termination is correct?

35.3 — What is the difference in the operation of a line which is an odd number of quarter wave-lengths long as compared with one which is an even number of quarter wave-lengths ?

35.4 —An ungrounded antenna is operated on its eighth harmonic at a frequency of 21.2 MHz. Determine the antenna length required.

35.5 — Make a diagram of the voltage and current distribution on a grounded antenna which is being operated on its third harmonic. Also make a diagram for the voltage and current distribution on an ungrounded antenna operated on its third harmonic. If both antennas are operating on the same frequency, how do their physical lengths compare?

35.6 — If a concentric line is to have a characteristic impedance of 75 ohms and the inner conductor is 1/4 inch in diameter, what is the proper value for the inner diameter of the outer conductor?

35.7 — State several uses of transmission lines for both resonant and non-resonant applications.

36.1a — A λ/4 wave antenna has a natural wave-length of 120 meters. What is the natural frequency of this antenna?

36.1b — If the antenna of problem 36.1a is operated on a frequency of 2,200 kHz, what kind of reactance will the antenna present at the coupling point?

36.1c — Similarly, if the antenna is operated at 2,800 kHz what reactance will appear at the base?

36.1d — What kind of loading is required to make the antenna of problem 36.1a resonant at 2,200 kHz? At 2,800 kHz?

36.2 — A resistance of 2,500 ohms is to be matched to a 100-ohm resistance by means of a quarter-wave transmission line at a frequency of 8,210 kHz. Determine the proper value of the line's characteristic impedance and the line length. If the line is to be constructed from No. 10 wire, what is the proper value for the line spacing?

37.1—A u.h.f. transmitter is to supply a half-wave antenna at a frequency of 240 MHz. The transmitter will be crystal controlled. Determine the approximate length of the antenna, taking into account the decreased velocity of the waves at this frequency.

37.2 — Show by block diagram the circuit you might use for the transmitter of problem 37.1 if the highest frequency at which the crystal will oscillate is not greater than 10,000 kHz, even at harmonic operation of the crystal.

37.3a — Outline briefly the limitations of regenerative oscillators and amplifiers at the ultra-high frequencies.

37.3b — Discuss briefly the present approaches to the solution of these limitations.

37.4 — State four distinct uses of transmission lines at ultra-high frequencies.

38.1 —A positive-grid oscillator tube has a cylindrical anode whose diameter is 1 cm. and whose potential is the same as that of the filament. What grid voltage will be needed to generate 50-cm. waves? What would be the wavelength if the anode diameter were 2 mm.?

38.2 — What is to keep the grid of a B-K tube from overheating under electron bombardment?

38.3 — The magnetic field intensity applied to an oscillating magnetron is 2,000 oersteds. What wave-length is being generated?

38.4 — An electron has been accelerated by a 200-volt battery. How far will it travel during one period of a 50-cm. wave?

38.5 — State in your own words why a cavity resonator is used to tune microwaves.

38.6 — Compute the cross-sectional dimensions of cylindrical and rectangular wave-guides which will just admit 50-cm. waves of the Eo and H01 modes of vibration, respectively.

38.7 — Why is a wave-guide built of galvanized iron superior to one built of plain iron sheeting?




Last Update: 2011-03-27