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LF: Beacon signals / CW

To: [email protected]
Subject: LF: Beacon signals / CW
From: "James Moritz" <[email protected]>
Date: Tue, 30 Jan 2001 12:56:23 +0000
Organization: University of Hertfordshire
Reply-to: [email protected]
Sender: <[email protected]>
Dear LF Group,

I transmitted my usual beacon signal (5 x 110sec dashes and spaces, followed by a 400sec space containing 6wpm station ID) from about 2000 until 0240 last night, on 135.923kHz. Later, I received VA3LK - weakly around 0445, but good copy from 0650 - 0800, as observed by others. Although in the last week or so Larry's signals have probably been strongest at times between 0000 and 0300, they are more erratic and suffer greater QRN during this period. On balance, most consistent copy appears to be around dawn here.

Reading through some of the older radio literature, it is apparent that the term CW came about to distinguish it from the many other types of morse code telegraphy that existed at one time or another. Spark transmitters were apparently quite distinctive according to how the spark was produced, eg. quenched spark, synchronous and asynchronous rotary gaps, etc. The "musical" note of the resulting signal seems to have made up to some extent for the wide bandwidths, and lack of receiver selectivity. The Poulsen Arc transmitter had to use a type of FSK, in order to avoid switching the arc off. The only CW transmitters were HF alternators.

The early valve transmitters were often anything but CW; rectifiers did not come until later, and the transmitter was basically a big oscillator fed from whatever DC-ish supply was available. The only truly "continuous wave" signals would have been produced by battery supplies, but these were the exception. Most transmitters seemed to get HT power via some sort of rotating machine, with inevitable ripple, or even raw AC, so most of the transmitter signals would have been modulated to a greater or lesser extent, in frequency as well as amplitude. In those times, the "T" of an RST report would have had more meaning!

Even after well smoothed DC supplies were available, there seems to have been operational advantages to having additional modulation to the on-off morse keying. So there was modulated CW, and interrupted CW. I think MCW of a sort lives on in some MF beacons. MCW was also supposed to have advantages on HF under selective fading conditions. Also, a non-oscillating detector (eg. a crystal) could be used. But I expect some of the older subscibers to this reflector know more about this.

The last lot of transatlantic tests in the 1920s saw quite a bit of debate about the relative technical merits of new-fangled valve CW and trusty dependable spark transmitters. CW won the day that time!

Cheers, Jim Moritz
73 de M0BMU


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