Heterodyne or Beat Long wave Telegraph Receiving Set?

The Radio Amateur's Hand Book, by A. Frederick Collins is part of HackerNoon’s Book Blog Post series. You can jump to any chapter in this book here: [LINK TO TABLE OF LINK]. Chapter XIII - Heterodyne or Beat Long wave Telegraph Receiving Set

XIII. HETERODYNE OR BEAT LONG WAVE TELEGRAPH RECEIVING SET

Any of the receiving sets described in the foregoing chapters will respond to either: (1) a wireless telegraph transmitter that uses a spark gap and which sends out periodic electric waves, or to (2) a wireless telephone transmitter that uses an arc or a vacuum tube oscillator and which sends out continuous electric waves. To receive wireless telegraph signals, however, from a transmitter that uses an arc or a vacuum tube oscillator and which sends out continuous waves, either the transmitter or the receptor must be so constructed that the continuous waves will be broken up into groups of audio frequency and this is done in several different ways.

There are four different ways employed at the present time to break up the continuous waves of a wireless telegraph transmitter into groups and these are: (a) the heterodyne, or beat, method, in which waves of different lengths are impressed on the received waves and so produces beats; (b) the tikker, or chopper method, in which the high frequency currents are rapidly broken up; (c) the variable condenser method, in which the movable plates are made to rapidly rotate; (d) the tone wheel, or frequency transformer, as it is often called, and which is really a modified form of and an improvement on the tikker. The heterodyne method will be described in this chapter.

What the Heterodyne or Beat Method Is.--The word heterodyne was coined from the Greek words heteros which means other, or different, and dyne which means power; in other words it means when used in connection with a wireless receptor that another and different high frequency current is used besides the one that is received from the sending station. In music a beat means a regularly recurrent swelling caused by the reinforcement of a sound and this is set up by the interference of sound waves which have slightly different periods of vibration as, for instance, when two tones take place that are not quite in tune with each other. This, then, is the principle of the heterodyne, or beat, receptor.

In the heterodyne, or beat method, separate sustained oscillations, that are just about as strong as those of the incoming waves, are set up in the receiving circuits and their frequency is just a little higher or a little lower than those that are set up by the waves received from the distant transmitter. The result is that these oscillations of different frequencies interfere and reinforce each other when beats are produced, the period of which is slow enough to be heard in the headphones, hence the incoming signals can be heard only when waves from the sending station are being received. A fuller explanation of how this is done will be found in Chapter XV.

The Autodyne or Self-Heterodyne Long-Wave Receiving Set.--This is the simplest type of heterodyne receptor and it will receive periodic waves from spark telegraph transmitters or continuous waves from an arc or vacuum tube telegraph transmitter. In this type of receptor the detector tube itself is made to set up the heterodyne oscillations which interfere with those that are produced by the incoming waves that are a little out of tune with it.

With a long wave autodyne, or self-heterodyne receptor, as this type is called, and a two-step audio-frequency amplifier you can clearly hear many of the cableless stations of Europe and others that send out long waves. For receiving long wave stations, however, you must have a long aerial--a single wire 200 or more feet in length will do--and the higher it is the louder will be the signals. Where it is not possible to put the aerial up a hundred feet or more above the ground, you can use a lower one and still get messages in International Morse fairly strong.

The Parts and Connections of an Autodyne, or Self-Heterodyne, Receiving Set.--For this long wave receiving set you will need: (1) one variocoupler with the primary coil wound on the stator and the secondary coil and tickler coil wound on the rotor, or you can use three honeycomb or other good compact coils of the longest wave you want to receive, a table of which is given in Chapter XII; (2) two .001 mfd. variable condensers; (3) one .0005 mfd. variable condenser; (4) one .5 to 2 megohm grid leak resistance; (5) one vacuum tube detector; (6) one A battery; (7) one rheostat; (8) one B battery; (9) one potentiometer; (10) one .001 mfd. fixed condenser and (11) one pair of headphones. For the two-step amplifier you must, of course, have besides the above parts the amplifier tubes, variable condensers, batteries rheostats, potentiometers and fixed condensers as explained in Chapter IX. The connections for the autodyne, or self-heterodyne, receiving set are shown in Fig. 60.

The Separate Heterodyne Long Wave Receiving Set.--This is a better long wave receptor than the self heterodyne set described above for receiving wireless telegraph signals sent out by a continuous long wave transmitter. The great advantage of using a separate vacuum tube to generate the heterodyne oscillations is that you can make the frequency of the oscillations just what you want it to be and hence you can make it a little higher or a little lower than the oscillations set up by the received waves.

The Parts and Connections of a Separate Heterodyne Long Wave Receiving Set.--The parts required for this long wave receiving set are: (1) four honeycomb or other good compact inductance coils of the longest wave length that you want to receive; (2) three .001 mfd. variable condensers; (3) one .0005 mfd. variable condenser; (4) one 1 megohm grid leak resistance; (5) one vacuum tube detector; (6) one A battery; (7) two rheostats; (8) two B batteries, one of which is supplied with taps; (9) one potentiometer; (10) one vacuum tube amplifier, for setting up the heterodyne oscillations; (11) a pair of headphones and (12) all of the parts for a two-step amplifier as detailed in Chapter IX, that is if you are going to use amplifiers. The connections are shown in Fig. 61.

In using either of these heterodyne receivers be sure to carefully adjust the B battery by means of the potentiometer.

[Footnote: The amplifier tube in this case is used as a generator of oscillations.]

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Collins, A. Frederick. 2002. The Radio Amateur's Hand Book. Urbana, Illinois: Project Gutenberg. Retrieved April 2022, from https://www.gutenberg.org/files/6934/6934-h/6934-h.htm#chap13

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