So, is that contest over? I do not think so, not yet, not for me..
The Radio Club of Hartford (affiliated) recently conducted a very interesting contest in the building of C.W. sets. Several months ago the rules were drawn up and about a dozen members entered. The idea was to devise a simple inexpensive C.W. set of low power, preferably operating from 110 volt lighting current, to supersede the spark coil in the small stations about town. The sharper wave and greater distance with reduced interference made this very desirable, and the Radio Club of Hartford is to be commended for instituting steps that can well be followed by other cities in the reduction of QRM.
One of the members of the club offered a silver loving cup as a grand prize, and in addition there were five 5-watt power tubes offered by the club. The rules called for the award of the prizes to the men having the highest scores on the following basis:
|Overall electrical efficiency||50%|
|Ingenuity in construction||15%|
|Economy in cost||15%|
The awards were made at the final meeting of the club before closing for the summer, and eight contestants were on hand with their sets. These were of every imaginable description, but mostly following the general idea of a small base bearing a vertical panel carrying the controls, with the apparatus behind.
Considerable ingenuity was displayed in the source of power. Several of the sets used step-down ("toy") transformers on the 110 volt supply, the low potential current thus obtained being used both to light the filament and to operate a spark coil with regular interrupter, the secondary voltage of which was dropped by a shunt condenser and then fed to the plate of the tube. These sets of course would operate from a storage battery equally well.
It was a condition of the contest that the operating wave length should not exceed 200 meters, and thru faulty design only two sets were able to achieve this - those submitted by J. C. Randall, 1ANQ, Dist. Supt., and F. H. Schnell, 1MO, our Traffic Manager, who is incidentally vice president of the club. Both of these sets, however, were able to get down to 180 meters, altho readings were taken at 200. A phantom antenna was used, consisting of a 12-ohm resistance and a mica condenser of .0005 mfd. capacity, in series with a Jewell thermo-couple ammeter.
Mr. Randall's set was an experimental one, very neatly mounted on a large drawing board, and connected up very similar to the schematic hook-up. It used a 50-watt Radiotron excited by an Acme 250-watt transformer and achieved an over-all efficiency from supply mains to antenna of 13.65%. The transformer was probably too large for the tube for utmost efficiency, and the iron losses seem to have been relatively large, whereby 1ANQ was handicapped in his competition with 1MO.
Mr. Schnell's set made an over-all efficiency of 18.82%, putting 1.12 amps. in the 12-ohm antenna at 200 meters while drawing 80 watts from the line; antenna watts, 15.1. His set was declared the winner, altho the decision was very close, and he was awarded the cup.
Photographs and the hook-up of the winning set appear herewith. It uses a single 5-watt Radiotron and achieves its 15 watts in the antenna with 750 volts on the plate, the plate current being 46 m.a. At the input the plate remains unchanged in color.
The oscillating circuit is the one described by Mr. Whittier in the July QST. The inductance is a Tuska No. 181, built into the panel, and the transformer which has three windings, was Built to order by Thordarson. Inside the inductance the tickler may be seen, which consists of 25 turns of bell wire on a form 2 1/2" in diameter. This is shunted by the variable C1, the tuning of which is critical. The grid variable, C2, might just as well be a fixed condenser of .0003 mfd. C3 is a mica by-pass, capacity not critical, preferably about .002. C4 and C5 by-pass the radio component across the two halves of filament windings, and are small paper condensers of .001 mfd. The grid leak, R, is an ordinary graphite potentiometer sector used in series as a variable resistor.
Mr. Whittier uses the key in series with the grid leak, but in this set such operation caused a continual "gurgle" when the key was idle which was very distressing for local work, altho it disappeared at a distance. To overcome this the key is now placed at the bottom of the inductance as shown. The general arrangement of the set will be apparent from photographs.
This set gives the familiar "one-side-of-the-cycle" note which when heterodyned is not at all unpleasant. During June 1MO has worked 2KL, Redbank, N. J.; 1UN, Manchester, Mass.; 2UD and 2AJW, New York; etc., which speaks for the carrying powers of the C.W. in summer.
The cost of this set, including tube, was $34.00.
I found the following note about this transmitter in Dow's "The C.W. Manual" book, 1922:
One of a number of circuits employing alternating current for both plate and filament supply is shown in Fig. 15. Here a single transformer having two secondries is used.
The mid-point of the filament winding is tapped to eliminate the "hum" which would be caused by connecting the grid to one end of the winding. The transformer secondaries are bridged as illustrated in the diagram by small condensers designed to withstand the impressed electromotive forces. The correct values of these capacities will be found to be approximately .002 microfarad in the case of the one bridging the plate supply secondary and almost any value greater than 0.2 in the case of the others.
Care must be exercised in using capacities across a supply of high potential alternating current, as a power circuit involving high currents is liable to result. These currents while termed "wattless," result in large copper losses in the windings. The circuit of Fig. 15 illustrates the method referred to earlier in this chapter, of obtaining the required grid input by a variable coupling device having a fixed amount of wire in the circuit at all times. L, is the input inductance and is coupled to L, by either the sliding tube or variometer method. It is bridged with a capacity C, to facilitate adjustment. The conventional capacity grid leak resistance method of maintaining a negative grid potential is used.
This circuit may be termed an ideal one in so far as simplicity is concerned. It oscillates easily, and is very efficient, but it possesses the disadvantage of utilizing only one-half of the alternating current cycle. During the part of the cycle when the plate is negative no oscillations are produced.
Dow, Jennings Bryan. 1922. The Cw Manual: Design and Construction of Radio Telegraph and Telephone Transmitting Equipment. Pacific Radio Publishing Co.