Radio broadcast .. (1922-30)

1 82 Radio Broadcast itTUINS'MDCC FIG. The four binding posts allow use of this circuit in various ways. For general use, post i goes to the antenna, and post 2 to the antenna post on the receiver the number of electrons which are trapped. This means that the average plate current will be correspondingly decreased, the telephones will be affected by this decrease in plate current. It is therefore necessary to bring the grid back to its original condition if the oscillations in the antenna have stopped. The grid leak which is shunted across the grid condenser permits the trapped electrons to "leak" out into the set. For further details see the interesting and helpful book, The Radio Pathfinder, by R. H. Ranger. TUNED R. F. AMPLIFIERS VERSUS UNTUNED R. F. AMPLIFIERS THERE is something to be said in favor of both of the methods of radio frequency amplification mentioned. The transformer system needs no adjusting, per stage. Excepting for the ordinary tuning of the input coupler controlling the received wave, there is no further adjustment of the tuner. In the tuned R. F. amplifier each stage must be tuned separately to resonance with the incoming wave. This operation is complicated and not always easy. But, when each stage has been properly adjusted, the reception is exceptionally good. One drawback of a set having tuned radio-frequency amplification is that the self-oscillations of the tubes make it a difficult set to control. This has been partly overcome by incorporating in the set, neutralizing capacities, resulting in a circuit similar to the standard neutrodyne set. The disadvantage in using untuned transformers is that they will not work with the same efficiency on all wavelengths. The wavelength at which best results are obtained is called the "peak." INTERFERENCE AND THE WAVE-TRAP IN A recent article by Mr. A. J. Haynes (November, 1923) this question of e iminating local interference was answered in detail. However, Fig. 5 shows the circuit diagram for a wave trap which will accomplish this elimination. The inside winding is a coil consisting of 45 turns of No. 24 D.C.C. wire wound on a 35" diameter tube, 2" long. The outside coil is wound directly over the first coil, being separated by a piece of linen or cambric cloth. It consists of 10 turns of No. 24 D.C.C. wire. Figs. 5 and 6 show the circuit, and method of coupling to a receiver. CHOOSING A RECEIVER THERE are several limitations that must be taken into consideration when choosing a receiver. Those of first importance are: amount to be spent; location, i.e., use of loop or outdoor aerial; purpose of its use, i.e., general broadcast reception for entertainment or experimentation. A one-tube reflex set is comparatively cheap, efficient in operation, and has the advantage of working a loud speaker for local reception. A threetube reflex will permit the reception of distant stations on a loud speaker, is selective in tuning and is generally conceded to be a standard type of home receiver. A three-tube regenerative set is also excellent although the novice experiences some difficulty at first in effectively operating it. A five-tube neutrodyne is an exceptional distancegetter, but on account of the multiplicity of controls is rather unwieldy to handle for one who knows nothing at all about radio. The super-heterodyne is the "high-powered" par excellence receiver of radio. Simple in operation and effective for long distance reception, this set can surely be called the highest type of radio receiver yet developed. Most super-heterodynes are designed for loop reception. The sets described will fit all sizes of pocketbooks and all have been described in past issues of RADIO BROADCAST. There is an advantage to building your own since the experience gained enables one to understand the workings of the set. If one is especially handy with tools, the building of a set is heartily recommended. In the May issue of the Grid a list of receivers that do not radiate, was given. We advise prospective purchasers of radio receivers to consult this list. VARIABLE CONDENSER ANTENNA INSIDE WINDING FIG. 6 ~LJOW to build a broadcasting station using two five-watt tubes, is the subject of an unusually complete how-to-make-it article wbicb will appear in this magazine for July. It is written by Ashley C. Dixon of Stevensville , Montana, owner of station KFJR and author of 'What Radio Means at a Rocky Mountain Ranch" in RADIO BROADCAST for January.