Radio Broadcast (May 1928-Apr 1929)

An Argument for Double Detection A SHORT-WAVE SUPER-HETERODYNE By ROBERT S. KRUSE IT WILL be recalled that Armstrong's form of double-detection receiver was devised to meet an emergency. The French army owned a large number of longwave amplifiers but desired short-wave amplification. Depending on the gullibility of the amplifiers, Armstrong devised a converter which would connect in the antenna circuit, hastily turning short-wave signals into longwave signals and passing them on to the amplifier. The amplifier trustingly accepted them as genuine long-wave signals and amplified them into something of presumable military value. In the enthusiasm of the moment, one assumes, there was invented the name "super-heterodyne" which has itself been amplified into something of unmistakable commercial value. This bit of radio history naturally brings double-detection to mind whenever one has a short-wave amplification problem on hand. In the broadcast band the problem no longer exists, other means of seemingly equal merit being universally known and somewhat less universally available. In the region below 150 meters, and in fact to some degree between 150 and 200 meters, the problem of satisfactory amplification is still present and one is somewhat amazed that the double-detection receiver does not have a larger foothold. The situation is probably historical. Until very recently the sub-200 meter region was almost wholly telegraphic; even the radiophone experimenters used the key to supplement their microphones. The commercial men who worked in the region were all message handlers. Naturally, therefore, the receivers were suited to telegraphic reception and for that purpose the double-detection principle has no alarming advantage between 25 and 200 meters; one can do nearly as well with a simple oscillating detector and audio amplifier. As soon as telephony and television invaded the shorter waves the story became different. The oscillating detector became merely regenerative and in that act lost most Given a short-wave receiver of the conventional regenerative detector type, how can it be improved ? Mr. Kruse turns it into a double-detection receiver, which is the "high hat" way of saying super-heterodyne. He adds a frequency changer, and an amplifier working at the frequency to which the desired signal is changed. This amplifier may be specially built or if, may be one's broadcast frequency receiver used as an amplifier, second detector, and audio amplifier. Mr. Kruse has another article on this double-detection business in. the office, and it will appear in a forthcoming issue of Radio Broadcast. — The Editor. of its sensitivity and all of its selectivity. The result is a receiver as primitive in principle (though not in circuit) as the infamous "single circuit" of a few years ago. Fig. I — The normal circuit of the i"Wasp,'> receiver One may turn to tuned r.f. stages, using the 222-type tube, but for several reasons this is not as simple as tuned r.f. in the 200-550 meter region. The best of these reasons is that one is trying to cover the huge territory between 14 and 200 meters and this is equivalent to 20,000 kilocycles! It is a troublesome task to cause a set of plug-in coils with gang tuning to "run together" with the cramped scales that result. One must either drop down to a single r.f. stage and two controls or else convert the signals to some more normal wavelength where they will be more amenable to amplification. The second scheme makes necessary the use of a doubledetection system. In this article the writer describes a doubledetection (super -heterodyne) receiver. It is made by adding a beating oscillator to an already existant short-wave receiver. The beat frequencies from this oscillator and receiver are amplified at a lower frequency, detected again, and then amplified. The Deled or -audio Adapter r I ''HE particular kit shown in the illustrations is chain-store distributed and, therefore, an excellent subject for manipulation since additional parts may be obtained easily. It happens also that the coils used are especially suited to the adaptation, although a "dodge" to make others perform similarly is also given. Referring to Fig. 1 we have the normal diagram of the set which is, for some mysterious, reason called the "Wasp"! It employs the usual regenerative detector, VTi, plus two stages of audio. Regeneration is controlled by means of a variable bypass condenser, G2. The tickler, T, and primary, P, are on the same plug-in form with the tuned secondary, S. 1 || * oB+ — 0 AnL First Grid Broadcast Receiver Fig. 3 — How the broadcast set is connected with the detectoroscillator february. 1929 page 262 •