Radio Broadcast (May 1928-Apr 1929)

JANUARY, 1929 RADIO BROADCAST 183 No. 131 Radio Broadcast's Home Study Sheets January, 1929 Calibrating a Radio W avemeter A RADIO laboratory, regardless of how -^small it may be, cannot get along without a wavemeter or frequency meter. Such a meter generally consists of a coil, a condenser, and a dial. If it is part of an oscillating tube circuit, so much the better. It can, then, be used as a source of signals from which a receiver may be adjusted to a desired frequency. A good frequency meter can be made from coils such as the General Radio Company Series 277 which have the dimensions shown in Table 1 . When attached to a tube, as shown in Fig. 1 , with or without a grid current meter, a very useful frequency standard may be had. The problem is to calibrate it. Calibrating such a meter is a very interesting and instructive experiment. LIST OF APPARATUS EL7 — £ 1.5 ma. Mi Plug Plug ■-o < } 1 45 + I— lllllh 1. An oscillating wavemeter as in Fig. 1. 2. An oscillating detector tube, tuned to some known frequency in the broadcast band. (See Fig. 2.) 3. An audio amplifier connected to the output of the oscillating detector. 4. A pair of headphones connected to the output of the amplifier. FIG. I TABLE I Coil Turns Size Wire 1 )iam Length of Winding Inductance 277-A 15 21 2H' If" 0.014 mh 277-B 30 21 2J1' U" 0.055 mh 277-C 60 21 2Ji" 23!" if" 0.217 mh 277-E 90 27 if" 0.495 mh PROCEDURE Connect up the wavemeter and the oscillating detector and place within a foot or two of each other (See Figs. 1 and 2). Connect the detector loosely to an antenna and pick up a known broadcast station. By means of a vernier condenser, or a fine adjustment on the tuning condenser, tune the detector to "zero beat" with the broadcasting station. Move away the antenna coupling coil slowly and see if the beat note — which should be as near zero as is possible to hear in a quiet room with one stage of audio — changes. If so, adjust the tuning again until no sound is heard. The broadcasting station and the local generating receiver are tuned to the same frequency. In the Laboratory a 610 kc. station was used. Now use the broadcastband coil for the wavemeter, and make its tube oscillate. Couple the wavemeter and the detector inductances fairly closely together, perhaps by winding a turn of wire about each and connecting the turns together. Turn the wavemeter dial slowly, and mark down on a piece of paper when beat notes are heard in the telephones. A very loud note will be heard when the two circuits are in exact resonance (it may be necessary to decrease the coupling to get the exact dial setting), and another loud note will be heard when the wavemeter is tuned to the double frequency — or half the wavelength — in our case at 610 and 1220 kc. Between these points several other much weaker "squeaks" may be heard. Put them down but mark the strong ones with an asterisk. Then use a smaller wavemeter coil and repeat. Put down the squeaks again marking the loudest. At least two loud notes should be heard, the second and the fourth harmonic, in our case, the 1220 and 2440 kc. points. Repeat for as many coils as are to be calibrated. If the coils are wound so that each smaller coil has half as many turns as the preceding one, the beats will occur at the same place on the dial. That is, if we pick-up 610 kc. at 85 degrees on one coil, we ought to look for 1220 kc. at about 85 degrees on the next smaller coil, and so on. Now prepare a table like that in Table 2, in which the numbers along the top are secured by multiplying the detector frequency by whole numbers, say from 1 to 5, and in which the vertical columns represent the upper figures divided by whole numbers. Thus in our table, the detector frequency is 610 kc. Twice this gives 1220 kc, three times 1830 kc, etc. Reading down, one half gives 305, one third gives 205, etc Then from this table make a list of the various frequencies that may be looked for in our calibration, viz., 610, 763, 813, 915, 1016, etc 1 2 TABLE 3 2 4 5 6 1 610 1220 1830 2440 3050 3660 2 305 610 915 1220 1525 1830 3 202.5 406 610 813 1016 1220 4 152.5 305 457 610 763 915 What actually happens as we tune the wavemeter is as follows. We are listening in the oscillating detector circuit. It is generating not only a 610 kc. current but multiples of this frequency as well, harmonics they are called. These additional frequencies are much weaker than the fundamental, 610 kc. When we tune the wavemeter to 1220 kc, its fundamental (1220 kc.) beats with the second harmonic of the detector (1220 kc.) and so we get a squeak. It is also possible for the second harmonic of some frequency to beat with the third of another, producing a beat frequency of 610 kc. For example, a beat occurs when the wavemeter is tuned to 763 kc, that is because 763 kc. is the fifth harmonic of 152.5 kc, and 610 kc. is the fourth harmonic of 152.5 kc. (see Table 2). We now have data showing points on the wavemeter dial where we heard beat notes, and a list of frequencies at which beat notes should occur. How can we identify and properly label the points? Let us consider the broadcast-band coil, which in the Laboratory is tuned by placing the condenser across only half the coil so only a small part of the band is covered. We set down the figures as in Table 3, and subtract the dial settings as in Column 2. We heard strong beats at 10.2 and 85 degrees on the dial. We guess that these are respectively the 1220 kc. and the 610 kc points. Now we note that from 10.2 to the next point is 24 degrees, and that from this point to the next at 47 is a difference of 13 degrees. If we consider 13 degrees as a unit, we see that there are 6 units between 1220 kc. and 610 kc that is about 100 kc. per unit. So we put down 1220 kc. as the 10.2 degree point, subtract 200 kc. for the next and get 1020, (which is two units distant), subtract one unit or 100 kc. for the next and get 920 kc, and so on. Now we look at our table of expected beat notes and find that 1220, 1016, 915, 813 and 610 are to be expected. We car. then attach these frequencies to the above points. We can get the frequencies of the other coil points in exactly the same way. If we wish we may use another method of computing roughly what the beat frequencies are, and then check them against our table of expected beats as before. We note that between 10.2 and 85 degrees — a difference of 75 degrees approximately — a difference of 610 kc. exists, or a difference of about 8 kc. per degree. Then the difference between 10.2 and 34 should give 23.8 X 8 kc. or about 190 kc, that is from 1220 to 1220-190 or 1030 kc. Actually our table shows the frequency to be 1016 kc. TABLE 3 Dial degrees Diff. Units Diff. f Approx. f Exact 10.2* 1220 1220 34.0 23.8 2 1020 1016 47.0 13.0 1 920 915 60.0 13.0 1 820 813 85.0* 25.0 2 610 610 •Indicates points on dial where loudest beat notes are received. PROCEDURE Either set up the apparatus and calibrate it as suggested, or complete the data in Table 2. Plot the frequencies against dial setting. Transfer these frequencies to meters and make a calibration of wavelengths in meters against dial setting. Make a table similar to Table 2 but calculate the beats in terms of wavelength in meters. Calculate the inductance of the coil (Home Study Sheet No. 2 July 1928) and from it calculate the condenser capacities at various settings and plot. As a check on the above data, pick up another broadcast station whose frequency is known and repeat the calibration. See how nearly the calculated points and calibration curve check each other. PROBLEMS 1. Do you know why an oscillating vacuum tube produces harmonics? 2. If the nearest approach to the actual "zero beat" you can attain is 100 cycles at 1000 kc. what percentage accurate is your calibration? Why cannot frequencies below about 100 cycles be heard in the receivers? 3. Remembering that wavelength in meters is proportional to the square root of L times C, what is the ratio of capacity when the wavemeter is set at the second and then the third harmonic? That is, suppose the capacity setting of the wavemeter for the second harmonic is C degrees. What will it be for the third? Do you see a way to check your calibration by this method? Note: Readers may send the answers to these questions to the Editor to be checked. Phones