A Radio Course for Physical Science Teachers

Kenneth Wright

Central Michigan University, Mt. Pleasant, Michigan

Because many students in the upper grades of elementary schools,and in junior and senior high schools are keenly interested in radio?teachers who can develop that interest perform a real educationalservice. There have been few courses, however, to give teachers lack-ing a background in radio the requisite practical experience andtheory in this subject. At Central Michigan University we have de-veloped such a radio course, for we could find no time in either ourregular physics or physical science courses for adequate instructionin radio.

Central^ course may be taken by physics students as an electiveonly, but it may be used by the physical science student as a part ofhis minor. It is also cross-referenced with the industrial arts depart-ment. It provides two hours credit. The course time nominally is forone hour of lecture and three hours of laboratory a week. Sometimes,of course, some of the lab time is used for lecture; and enthusiasticstudents and an equally enthusiastic instructor may well spend morethan three hours in the lab. This is more easily possible if the courseis given in the evenings as ours is. Another very real advantage ofevening hours is that townspeople not regularly enrolled in schooloften wish to enroll in such a course. The ideal instructor for such acourse is probably a ^Ham55 who is always strong in electronics. Thecourse does, in fact, prepare would-be "Hams^ to pass the test neces-sary to receive the radio amateur^s license.The equipment used in Centrals course has been obtained from

surplus or from old radios donated by friends and acquaintances.The tubes used are from these sources. Stores which take radios inon trades are glad to get rid of those trade-ins for very nominal sums.Laboratory courses are often very expensive because of the costlyequipment; but, as has been noted, this radio course could be addedto the science curriculum without disturbing the departmentalbudget.At present, we have three main laboratory projects: one is to build

a power supply; two, to build an amplifier; and three, to build a radio.We are planning to add the building of a small oscillator or transmit-ter. Even though you may have commercial power supplies available,it is desirable that the students learn to build their own, so that theycan construct these units to supply the electrical power to operateother devices. The main requirement for a power supply for elec-tronic equipment is that it furnish D. C. at several hundred volts tothe plates of the tubes. Rectification or changing of A. C. to D. C. may

643

644School Science and Mathematics

be achieved by a number of different devices, of which the most com-monly used is the high-vacuum diode tube. Instruments useful forthe experiments are the multi-meter, audio-generator, oscilloscope,and vacuum tube voltmeter.The students work in groups of two. Set up the apparatus as shown

in Fig. 1, using transformers from surplus or from old radios. Eachgroup is given a transformer; and by the use of an ohmmeter, the 110volt leads, high voltage leads, and low voltage leads are determined.The leads with the highest resistance are the high voltage leads. Theresistance R should be 20,000 ohms or higher, with a wattage of atleast twenty. Almost any rectifier tube may be used. We used a 5Y3since that was what we had. If an oscilloscope is available, connect itacross A B. Measure the voltage across A B using a multimeter. Dis-connect the lead from one plate of the tube, forming a half-waverectifier. Check with the oscilloscope and multimeter as before.

6 +

TOJ R

A

FIG. 1. Full-wave rectifier

It is evident that the output of the rectifier system is far from beingpure D. C. The output is actually a pulsating D. C., a D. C. with asuperimposed A. C. wave called a ripple. As we are striving to get apure D. C. output from our rectifier system, it is obvious that we

_^ +

C AFIG. 2. Filter-one capacitor.

A Radio Course for Teachers 645

must remove the ripple from the output waveform. This is accom-plished by filtering out the ripple component by connecting a filtersystem to the output of the rectifier tube. A filter system is a circuitof capacitors and inductors. Connect a capacitor across the line fromC to D in Fig. 2. The size of this capacitor should be around 10 uf.Use the oscilloscope and meter across A B as before. Connect a chokecoil of about 10 henries as shown in Fig. 3. This gives you a chokeinput filter. Again use the oscilloscope and meter. Connect the secondcapacitor as shown in Fig. 4, resulting in a capacitor input filter.Check as before with the oscilloscope and meter.

/-Tyy^? D

R

C AFIG. 3. Filter-choke input.

^ mryr\ P B’’ � I s. ^

T T r+

F C AFIG. 4. Filter-capacitor input.

If you plan to use the power supplies for the amplifiers and radios,they should be built on separate chasis. Since we have regulatedpower supplies available at Central, we build ours bread-board styleand tear them down after we have learned how to construct them.

After this project, we spend another three weeks in the lecture roombefore we are ready to build the two tube amplifier. We assemble theamplifier on a chassis 7 X 11. It is constructed on one end of the chas-sis, leaving room for the r.f. section when we build the radio. The de-sign of the resistance capacitance coupled amplifier and the values ofthe different parts are calculated before the students come to thelaboratory. The schematic diagram is shown in Fig. 5.

646School Science and Mathematics

FIG. 5. Resistance-coupled amplifier

Parts List for Fig. 5.

Ci, �2 .01 ufCs, Cg 25 ufC4 .1 UfRi .5 megohmsR2 800 ohmsRs 100,000 ohmsR4 50,000 ohmsRB .5 meg potentiometerRe 300 ohms 10 wattRT 4000 ohms 20 wattTi Output transformerVi 6SJ7¥2 6V6

The tubes used are one 6SJ7 and one 6V6. By using the characteris-tics of the tubes from the tube manual and from the value of thepower supply, the values of the load resistor, the screen grid droppingresistor, and the cathode resistor may be determined. The values ofthe capacitors may also be determined or may be given to the stu-dents by the instructor. Almost any output transformer is satisfac-tory.

After the amplifier is completed, many checks may be made. Atfirst the plate voltages, screen grid voltages, and grid bias voltagesare measured. If an audio oscillator is available, introduce a signalinto the input, and with an oscilloscope across the output, observethe wave form. Check the gain at various frequencies, and notewhether the gain decreases at high frequencies. After making thesechecks, connect a record player across the input and observe themusic. Some groups may wish to experiment by adding a tone con-trol.To construct the radio, more stages must be added, at least one

radio frequency stage to amplify the r.f. signal and a detector stage

A Radio Course for Teachers 647

to demodulate the signal. The output of this stage is coupled to theaudio amplifier previously described. The r.f. section and detectorstage are shown in Fig. 6.The antenna circuit is inductively coupled to the grid circuit of

the first tube by the air-core radio frequency transformer T. Thesecondary of this transformer is tuned by the variable capacitor Ci.Unfortunately a single-tuned radio frequency stage does not provideeither the amplification or selectivity necessary to receive a weakdistant station without interference from a strong local one. One or

more r.f. amplifier stages may be added, but in our experience one

stage of r.f. is adequate if we use a regenerative detector.The regenerative detector stage is a 6SJ7. Regeneration is con-

trolled by the 250K potentiometer in the screen circuit of the 6SJ7.This varies the amount of signal fed through Ls and 1.4 and to the gridof the tube. The r.f. gain is controlled by the 25K. control in thecathode return of the first r.f. tube. The antenna coil is wound on afour prong, 1^ inch low loss plug in form and the detector coil iswound on a six-prong form of the same type. All coils are wound with

-6+

FIG. 6. R.F. Section and detector stage.

Parts List for Fig. 6.

Ci, �2Cs, �4, C?�5, CgRiR&R2RaR4Li.LsL2,L4L5ViV2

Gang Capacitor .00036 uf.luf.00025 uf25,000 ohm potentiometer250,000 ohm potentiometer100,000 ohms2 megohms250,000 ohms15 turns100 turns20 turns6SK76SJ7

648 School Science and Mathematics

No. 32 enameled wire. All windings are in the same direction.If you should be dissatisfied with the selectivity of the radio and

wish, therefore, to add one or more r.f. stages, this can easily be doneby arranging them one after the other. Thus, each amplifier tubeboosts the signal right where the preceding stage left off. This alsoimproves the selectivity of the radio.

If any of you are old enough to remember your own excitement atmaking a crystal set that really worked, you can well imagine thesatisfaction the students get when their radios are completed. Forthe students, especially for the girls�and there will be some�thisis the real climax of the course. To the instructor, too, this providessome of the fun of teaching. To him, however, the goal and the realsatisfaction come from knowing that the students understand the^why55 of each part of the circuit. These students, in turn, as teacherscan develop their own courses for junior and senior high schools.

NSTA REGIONAL CONFERENCES

The National Science Teachers Association announces eight regional confer-ences for the fall of 1968 and a spring conference to be held in June, 1969.

Dates, locations, and area chairmen for the conferences are:Northern Regional, Winnipeg, Canada, August 27-29, Kenneth H. Charles-

worth, 936 Riverwood Avenue, Winnipeg.Southeast Regional, Charleston, South Carolina, September 26-28, Clarence

Chestnutt, Jr., Department of Biology, Baptist College at Charleston.North Central Regional, Chicago, Illinois, October 3-5, Sister Mary Ivo,

BVM, Archdiocese of Chicago School Board.Northeast Regional, New York City, New York, October 3-5, Bernard M.

McFadden, 3798 Regent Lane, Wantagh, New York.Northwest Regional, Portland, Oregon, October 10-12, Gene Doty, Hillsboro

High School, Hillsboro, Oregon.South Central Regional, Little Rock, Arkansas, October 10-12, A. C. Brewer,

School District of Springfield R-12, Springfield, Missouri.Midwest Regional, Denver, Colorado, October 31-November 2, Kenneth W.

Horn, E.S.E.A. Center, 2525 West Sixth Avenue, Denver.Eastern Regional, Charleston, West Virginia, November 7-9, Llewellyn S.

Cole and Eloise A. McElfresh, Kanawha County Schools, Charleston.Southwest Regional, Las Vagas, Nevada, November 14-16, Lester G. Burg-

wardt, Basic High School, Henderson, Nevada.Pacific Regional, Honolulu, Hawaii, June 12-18, 1969, Walter Luke, 2042

Aamanu Street, Pearl City, Hawaii.