QST, vol. I, №6, May, 1916. The Audion as a Detector of Undamped Waves

QST, Vol. I, №6, May, 1916, p. 92-95

The Audion as a Detector of Undamped Waves

With introduction by the Editor, and description by C. D. Curtis and W. O. Horner.

INTRODUCTION

NUMEROUS articles have appeared on the oscillating Audion and various hook-ups have been devised, but in this article the purposes is to show the W. Ross McKnight (circuit and a simple connection which can be attached to an RJ5 DeForest Audion. The working principles of the Audion as a detector of high frequency oscillations is well-known to the amateur field. It is generally granted that the audion works on a "trigger" principle rather than as a rectifier. The working of an Audion with undamped waves may be understood by reference to a simple little analogy shown in Fig. 1.

On closing the battery circuit "E", a current is induced in the coil "L-1." If the windings of "L-l" are properly selected with regard to the magnet which forms its core, the induced current will cause an attraction of the diaphragm "D". This increases the resistance of the microphone "M" and results in a decrease of the current in the battery circuit. The resulting fall in current induces a current in "L-2" which acts through the magnet and repels the diaphragm. This time, the microphone resistance is decreased and once more the current in "L-3" is increased and the cycle starts anew. With a sensitive magnetomicrophone relay, a comparatively large current is set up in the battery circuit. This alternating current may be heard in the telephone "T."

This condition of affairs is nearly the same in an oscillating Audion hook-up. One circuit we may assume to be the secondary of the loose coupler, together with the grid and filament of the bulb. The second circuit is known as the wing circuit which consists of the inductance coil, filament, wing, telephone receivers, batteries, and condenser. The incoming signals set up oscillations in the grid circuit which repeat themselves in the wing circuit and thereby produce variations in the high potential battery which may be heard in the telephone receivers.

A more complete discussion of the question may be found in text books and in the Proceedings of The Institute of Radio Engineers. The short explanation which has been given will help the reader to understand what he is working for. The articles which follow by Mr. Curtis and Mr. Homer describe the apparatus and give a careful description of the workings. Every amateur who has done no work with the oscillating Audion now has an opportunity to read an article which is correct and written by those who have obtained results.

CONSTRUCTION
by C. D. Curtis

Superintendent of Schools, Pembina, N. D.

The Wireless Club in Pembina consists of about six members, all of whom have experimented with the Audion. The results for successful work I take pleasure in presenting to the readers of "Qst." Drawing No. 1 is simply a modified form of the McKnight hook-up. This combination has been used very successfully since last October.

One of the most important pieces of apparatus which one needs if he is trying to get into long distance receiving is a loose coupler, capable of receiving long waves. While the average undamped wave is in the neighborhood of 10,000 meters, we built our loose coupler capable of tuning to nearly 30,000 meters. Strange as it may appear, this loose coupler will also bring in 200 meter amateurs from all over the country. The primary and secondary of the loose coupler are respectively numbered L-l and L-2 in the drawing. The primary is 24" long and 8" in diameter, wound with No. 24 D.C.C. copper wire. The secondary is of the same length, but only 7" in diameter and wound with No. 26. The writer finds that 18 taps give sufficient variation in the tuning system. Coil L-3 is a duplicate of L-2, but it is optional as the circuit works with scarcely any difference, whether the coil is in or out. Coil L-4 is a small one, beins about a foot long and 5" in diameter. This coil is wound with No. 26 D.C.C. and has 12 taps.

No description need be given about the wood work of the loose coupler. Every amateur uses his own methods and his own ideas in that part of the apparatus. However, it might be well to say that the writer found it necessary to construct his own tubes which consisted of two thicknesses of cardboard of proper size, which were sewed together lengthwise. Two coats of shellac were given on the INSIDE to keep the cardboard from shrinking. After the winding one coat of shellac was put on the outside.

In the accompanying drawings the variable condenser marked C-2 can be exchanged for a fixed. The writer took eight sheets of heavy tinfoil, 2½" wide and 6" long with heavy brown paper as a dielectric. When the right capacity was found, this fixed condenser worked as well as a variable and once the right capacity was found it required no further changing. Besides, it will save one's pocketbook as it cost about ten cents and ten minutes' time.

The primary coil is very large but the writer finds that rarely more than four inches are used. All the Government stations using undamped waves may be heard on just one inch of primary winding. When coil L-3 is not used, every turn of the secondary coil, L-2, is brought into play.

All of coil L-4 is also used. Condenser C-1 is a common rotary which costs about $4.00.

HOW TO TURN AN RJ5 INTO AN OSCILLATING AUDION

Drawing No. 2 shows the writer's double hook-up on an RJ5 Audion detector. The following explanation describes this circuit. A cover has been removed and the wire leading from the negative terminal of the flash-light battery to the upper tuning post has been cut. Here the writer has inserted a small nickel plated switch on the outside of the box whose terminals are marked "1" and "2". The ends ot the cut wire are soldered to the posts of the switch on the inside of the audion cabinet. This little switch adds to the beauty of vour Audion and only costs about five cents and takes nearly a half an hour to insert. (Note: If you have an RJ4 model, cut the wire which leads from the positive pole of the illuminating battery to the first negative flash-light battery terminal. Then insert the switch as described above. This locates the switch in the upper right hand corner of an RJ4). In the RJ5 model the lower tuner post is the grid terminal, while the lower telephone post is the plate connection. A wire run to either D or P will give the same results. That is, either terminal may be used. In the RJ4, the upper telephone post is the plate connection while the left hand tuner cunnection on the back of the case is the grid terminal.

In the drawing, a single throw switch S-1, and a double throw switch at S-2 are shown. The switch at S-3 has already been described. For continuous or undamped waves, open S-3 and leave the two others closed. For spark stations, close S-3 and open both S-1 and S-2. One may observe that closing S-3 places the Audion in exactly the same hook-up as before the wire was cut. While opening S-1 and S-2 cuts out both condensers and the extra coil, L-4. If one wishes to use a variable condenser, across the secondary, it can be used for spark signals without interfering with the ing un continuous oscillations and they certainly have a fine flute-light note. Without the slightest inconvenience, I can work the switches and brighten up the bulb then NAA comes in with a loud, clear, distinct note. With the same facility, I can change to the other system. I am using the same continuous wave work. The writer does not use one, as he gets good results without. It is not shown in the drawing. For spark stations, it is necessary to burn the bulb much brighter than for continuous waves, and more voltage is required on the flash-light battery. This station is in exact opposition to that made by Mr. Cole in the February "QST"1. However, the writer finds that his experiments do not agree with the results suggested by Mr. Cole.

This hook-up has been tested by the writer every evening and day since last October. It works fine at the present writing. I hear the Government stations workbatteries, both high potential and illuminatins, also the same Audion bulb, with which I started last October. I have never lit the second filament except to test it tor a few moments. During experiments, I have tested out six bulbs and all oscillate quickly without the slightest sign of trouble. Germany has been clear here all winter, but I have not heard the German station for nearly a week and I don't suppose they will be heard again until Spring.

THE ROSS McKNIGHT CIRCUIT
By W. O. Homer, Cleveland, Tenn.

The writer is pleased to place before the readers of "QST" an efficient circuit for undamped waves originated by Ross Mc-Knight with whose permission, the circuit is shown. I am using a triple Audion amplifier, together with a two-step Multi-Audi-Fone, a loud speaking phone and horn. With this equipment I hear the NAA spark station and many high power undamped stations. Strong signals can be heard at two city blocks from the apparatus. The keen whistle note sounds like a tin whistle and by tuning condenser No. 11, it gives a musical note resembling a clarinet. I use double pole double throw switches so I can cut out the inductance coils and condenser in order to receive time signals from NAA. As soon as I have picked him up, I throw in a portion of No. 13 and 14 inductance which acts as an amplifier. Lastly, I finish up with condenser No. 7, and throw in the loud speaker which shouts Arlington's time so that everybody in my jewelry store hears it.

We have picked up the Radio Telephonic experiments conducted by the big Naval station at Radio, Virginia, on a wave length of about 5,000 meters, as well as the European stations. The Experimenter who wants one of the best and most sensitive stations should try out this circuit.


1 Статья Mr. Cole помещена не в февральском, а в мартовском номере "QST". (прим. составителя)