Radio Broadcast (Nov 1926-Apr 1927)

Radio Broadcast Photograph FIG. I It is not difficult to see why I bd at Plainfield, Vermont, "steps out." Maple boiled in paraffin forms the framework of this efficient transmitter. Note the standard equipment, General Radio con ' densers and other parts mentioned in this article IN THE April Radio Broadcast was described the portable shortwave transmitter that enabled 2 gy (the experimental short-wave station at the Laboratory of Radio Broadcast) to carry on communication with 9 ccq, 1000 miles away, with a plate input to a 20 1 -a tube of 0.04 watts, a record of 25,000 miles per watt. So many requests have come to the Radio Broadcast Laboratory for more complete directions for building this efficient set and so many readers have desired rules for tuning and operating it that the present article has been prepared. The portable transmitter, however, was one of those "long geared" affairs that, "placed end to end," would reach from the cellar to the garret, and truthfully was not a beautiful object. For transporting about the country in the rear of an automobile— for which it was designed — it was quite the thing, but for one's den, that is a different matter. Several of the readers of Radio Broadcast, however, have made a much better looking job of the small transmitters than that used at 2 gy and some of these are described in this article. For example, Mr. Roy L. Gale of Plainfield, Vermont, 1 bd, wrote: This is to inform you that I have constructed a dry-battery transmitter after the description of the one in Radio Broadcast for April. As a starter I worked Holland, Michigan, Rrookville, Ontario, and at 5-45 a.m. day before yesterday I worked Fredonia, Kansas, and a little later, a station at Cambridge, Illinois. 1 don't think this bad considering that 1 was using only about 350 volts on a five-watt tube. All report me as "strong and steady." Moreover, 1 haven't really learned to tune the thing yet, so I am expecting big success with it a little later. Old discarded BCL B batteries work fine on this rig. High Efficiency BTransmitter; ■ge T Capable of Long' mall Transmitters By KEITH HENNEY Director, Radio Broadcast Laboratory Again on April 9, Mr. Gale wrote: On Easter morning in broad daylight I worked 6 bil at Pomona, California. My input was about 25 watts to a VT2 so maybe this wasn't a very alarming record considering that I was using quite a bit of power, but there are two fifty watters near here who haven't done any better than this at any time of day or night. I attribute my good results to perfect insulation. Am using Pyrex insulators for the antenna. Several weeks after his first letter, Mr. Gale sent photographs of his set which appear in Figs. 1 and 2, and a description of his apparatus follows: The panel is Radion Mahoganite and the layout somewhat resembles that of Mr. Dixon's of Montana, a description of which appeared in Radio Broadcast some time ago. You will notice that the coils are well insulated, being supported by maple strips boiled in paraffin, mounted on a hard rubber strip, and this in turn supported by two General Radio insulators. The plate and grid condensers, leak, and sockets, are also supported in the same way. The choke is a Browning-Drake antenna coil. Weston meters are used throughout. General Radio 0.0005-mfd. tuning condensers, and sockets. Dubilier 2000-volt plate and grid condensers, 0.002 rnfd. in size. 1 use a 12 x 24 drilled window pane for lead-in and Pyrex antenna insulators. The antenna is a two-wire inverted L, 60 feet long, the counterpoise being a single wire 60 feet long, both rather high. A single vt 2 is used usually, but I have used two c-301-A's in parallel with good success. Was reported R5 dc in California when using about 25 watts. This in daylight, but not sunlight. 1 worked 6 bil at this time for 30 minutes. With a similar outfit using two 201-A tubes in parallel, Mr. Vincent Fertitta, 5 le, of New Orleans, has had excellent luck and the accompanying log shows what can be done with small power inputs. Note particularly that with an input of 13.5 watts he worked Italian 1 no. The average miles per watt of plate power expended for these transmissions is 84.5. Unfortunately Mr. Fertitta's photographs would not reproduce properly but the layout practically duplicates that of Mr. Gale, except that a transformer furnishes plate power instead of Mr. Gales's device of employing discarded B batteries. A SIMPLE MODEL JUST to show how simply one can build a similar transmitter, Figs. 3, 4, and the picture on page 34 give an idea of a 201-A transmitter built by Ferdinand Mann, an operator at 2 gy. This outfit, working from a DeWitt LaFrance power supply unit designed for broadcast receivers, and putting out about 200 volts at 30 milliamperes, puts 0.15 amperes into an average 40-meter antenna. This set, as well as those used by ibd and 5 le, use the Hartley circuit loosely coupled to the antenna. Fig. 5 is the circuit diagram of Mr. Mann's affair. The parts used in this small transmitter follow: 1 Benjamin Socket 2 13-Plate, G. I. Condensers 2 Dials Radio Broadcast Photograph FIG. 2 A front view of Mr. Roy Gale's transmitter which has communicated nearly 3000 miles using about 25 watts input. Mr. Gale operates station 1 bd at Plainfield, Vermont