FROM TELEGRAPH TO COHERER CW - THE EARLY DAYS


No matter where you look in the evolution of electronic communication equipment, advancements along the way have generally been incremental. Even before the use of electricity in one way or another, humans started (perhaps) sending cryptic messages over distances using flares. By 1792 the visual telegraph in France covered most of the country with 556 stations. Prior to radio the U.S. fashioned the pony express and followed that in a decade with a telegraph system. Note that tele and graph, from the Greek meaning distance and write, make up our word telegraph. The electric telegraph was developed my Samuel Morse in 1837 and Alfred Vail contributed the signaling alphabet for it. By 1861 the overland telegraph connected the west coast to the east coast, effectively snubbing out the pony express. That's where this note comes in, starting with a look at the basic telegraph circuit.

Figures 1A and 1B display the send-only and two-way circuits. You might think of the sender-only circuit as similar to a key with code practice oscillator and speaker or headphones. When you close the key, current flows through the circuit loop, thereby causing the coil to attract (pull down) the balanced hammer onto the sounder rod. This action is accompanied by a "click" when the metals meet. When you release to key, the coil loses its current and releases the hammer, causing a "clack." A short duration between the click and clack can be considered a DIT and a longer duration (~ three times as long) can be considered the DAH. Hence Morse code can be send and heard locally.

Now consider the two-way circuit, Figure 1B. Two keys, two sounders (coils), and a battery make up the single circuit loop. Assuming that the key at the far end is left closed, pressing the key at the near end is all that is needed to close both sounders, with both producing a “click.” While some early short-distance links used a pair of wires to complete the loop from end to end, earth ground was soon used as the return wire to reduce wire cost. As with any change, however, the developers soon discovered that the resistance of the earth was much greater than any wire. As a result larger batteries had to be used so sufficient current was available to operate the sounders. A local-only loop might use a few volts; however, ground returns required several hundred volts, thus increasing the cost of the venture.  


two-way telegraph


As an aside, early systems used a roll of paper and a pencil to “mark” the dots and dashes received. It was soon recognized that the operators could copy the code sent by just listening to the clicking and clacking of the control relays. Naturally the relays, with minor modifications, became known as sounders.

CW WITH EARLY RADIO

Now let’s take a look at the technology upgrade from wire telegraphy to CW communication with early radio equipment. As you likely recall spark was once king. Over time spark transmitters were improved until sparks where generated at a near continuous rate. The RF received at a distant point could then be considered CW. You can imagine with the decade of work just completed on telegraphy that many during this invention crazy era searched for ways to click and clack via radio. The discovery of the coherer enabled this to happen. 

Recall that the coherer is a two-lead device with a small internal gap between its leads that is filled with metal filings. Once shaken and in the absence of an RF field, the impedance across the coherer is moderately high. When the coherer is hit with a short RF signal of sufficient strength across its leads, the filings align and provide for a low resistance path between the external leads. Once the coherer it tapped again the filings re-scramble and the resistance between the leads is again large. 

coherer CW

One early circuit solution that appeared - similar to that for the wire telegraph – is shown in schematic form in Figure 2. Early experimenters figured out how to use the coherer and a telegraph sounder together. Let’s identify the players and then we’ll describe the action of the circuit. A tuned antenna is shown at upper left, including C1 and L1 to assist in resonating it. The coherer, D1, is shown at bottom left in series with the antenna. The de-coherer – or bell clapper if you will – is shown close by at “a” and is extended by a rod to the hammer of the buzzer coil, L2. The usual telegraph coil is shown at middle right and is labeled L4. Its clapper in turn is connected to the sounder coil, L5, with the click-clack hammer at top right. L3 is simply a radio frequency choke. The local battery is shown as B1 at bottom right. 

Let’s walk through the circuit, starting with the buzzer at L2. With power applied from the battery the buzzer runs continually just like an old doorbell assembly. The contacts at “b” are initially closed so L2 is energized. This causes the de-coherer to strike the coherer at “a” and open the contact points at “b.” Once that occurs the spring on the hammer (not shown) – or rotary switch if you will – pulls the contact back up and the operation starts all over again. Typically the cycle takes a few milliseconds. 

Now let’s concentrate on the telegraph relay and sounder action. Without an RF signal at the antenna of sufficient strength and with the coherer turned off by the buzzer action, the resistance of the coherer will be high, thus keeping the telegraphy coil and the sounder from coming on. When an RF signal of sufficient strength arrives at the antenna the coherer reacts by lowering its resistance on average a sufficient amount to draw current through L4 and turn on the telegraph coil. This action in turns causes the contacts at “c” to close and create a “click” at the sounder.  Once the RF goes away, the de-coherer gains the upper hand and turns the coherer off with the help of the buzzer. In effect, the sole purpose of the buzzer is to cause a “clack” at the sounder once the RF DIT or DAH is complete. This was just one of many arrangements invented by the new breed of RF minded telegraphers. 

Once headphones became available, of course, the coil in the phones replaced the telegraph coil and sounder, leaving a receiver with an antenna, coherer and buzzer with de-coherer. It might be interesting to try copying code using a tuned antenna and a 2N7000 MOSFET driven by a 555 timer along with a pair of headphones attached via an RFC. 72. WØXI.