��Popular Science Monthly
��Preventing Interference of Signals by Amplifying
AN interesting [patent on a highly selective arriingement for use at a radio-telegraph recei\'ing station is No. 1,173.079, issued in 1916 to E. F. W. Alexanderson. Realizing that the prin-
���Fig. 1. Inductively coupled circuits in a series of magnifying relays
ciple of selection bj' tuning to wave- length may be applied several times in the same group of instruments, the inventor has arranged a .series of tuned radio-frequency magnifying relays, as shown in the diagrams. The circuits look complicated, but are not very hard to manipulate. Anyone who has two or three audion bulbs, preferably of the double-plate "amplifier" type, will be able to try out a series of experiments along the lines of this invention.
Referring to Fig. i, it is seen that the antenna i passes to earth through the transformer primary 2 and series tuning condenser 3. The secondary circuit is tuned to the desired incoming wave- length by means of condenser 8, and connects via wires 5 and 6 to the grid G and filament C of the first ani|)lifying tube/. Battery 9 is in the grid circuit. so as to adjust the relay to its best magnifying condition, and battery B is used to heat the filament (or cathode) C. Wing circuit battery 1 1 has one terminal attached to the filament and the other, through the primary of transformer 12, to the wing or anode .1. The i)osili\e side of II is connected to A; the proper polarity of the other two batteries of the first tube is to be found by trial.
The second amplifying tube // is connected in the same way. Condenser 15 serves to tune the secondary of transformer 12; luiKeries 16 and 17 take the places of and il, respectively. The plate circuit from 17 to .1 includes the primary coil of the third transformer
��18, whose secondary is tuned sharply to the incoming waves by means of con- denser 19. Wires 25 and 26 run to the grid and filament of the third vacuum tube ///, which is arranged to rectify and "detect" the desired signals instead of merely amplifying them. It will be noted that the grid circuit contains a small series condenser 27, which is shunted by a variable high resistance 28 and a polarizing battery 29; it is through the co-operation of these three elements that the third bulb is adjusted to rectify the signal waves and so to produce pul- sating response-currents in the telephone recei\er 33 and condenser 34.
It is not necessary to use inductively coupled circuits as shown in Fig. i. If the tubes are interlinked by suitably designed and tuned auto-transformers, as in Fig. 2, the same results will be ob- tained. By comparing the two diagrams the similarity of the various parts ma>- be seen; the main difterence lies in the substitution of single coils and condensers such as 55 and 60 for the two-coil couplers and capacities typified by 12 and 15. As indicated by the switch in Fig. 2, the telephone condenser 62 (or 34) is not essential.
The high degree of tuning anticipated by the use of this entire arrangement is gained by the successive selectivity of a series of tuned circuits. If each tuned circuit is adjusted to cause a response to the desired signal ten times as loud as to that which is causing interference, and if the desired signal is amplified five times in intensity by each relay
���Fig. 2. linked
��Here the tubes arc inter- by tuned auto-transformers
tube while the interfering signal is not amplified, it is evident that the final response will contain little of the un- desired disturbances. In a normal rei-eiwr, lia\ing the same sclcctiveness for iiiiiiviilual steps, and not amplifying,