Silence on 162 kHz

Pieter-Tjerk de Boer, PA3FWM

(This is an adapted version of part of an article I wrote for the Dutch amateur radio magazine Electron, April 2017.)

Near Allouis, in central France, a longwave broadcast transmitter has been operating since 1939, with a power of already 900 kW back then, covering all of France [6]. In the course of time it has operated on 182 kHz, 164 kHz and 163.84 kHz (that is exactly 10×214 Hz, a nice round number for digital dividers); since 1986 on 162 kHz, with up to 2 megawatt of power.

The French public broadcasting corporation needs/wants to reduce their expenses; therefore, their mediumwave transmitters were already switched off at the end of 2015, and at the end of 2016, the longwave transmitter was to suffer the same fate. However, they had apparently overlooked the fact that this transmitter is not just broadcasting the France Inter program in amplitude modulation, but also in phase modulation a time code, comparable to the (outside France) better known German DCF77 transmitter on 77.5 kHz, and similar services elsewhere in the world. Apparently, some 200000 clocks in France use this time code.

Exactly at the end of the year the amplitude modulation was switched off as planned, but the transmitter itself continues, carrying only the phase modulation. On a normal broadcast receiver one thus only hears silence. And switching off the amplitude modulation can't be much of a financial savings, but presumably course this money is now coming from a different budget...

Listening to 162 kHz in the Netherlands (or at least in the east of the country, e.g. via the Twente WebSDR), we do not hear silence. This is caused by the Luxemburg effect. The signals from Allouis to the Netherlands pass by Luxemburg, where still a powerful broadcast transmitter is operating on 234 kHz, the modulation of which is transferred in the ionosphere to Allouis' signal. And because Allouis itself no longer has amplitude modulation, this transferred modulation is very well audible, including the fact that it sounds muffled.

But of course, in view of Shannon's communication theory, one and a half megawatt is a bit much for transmitting a grand total of 1 bit per second, because that's all the time code is. No doubt they are going to think about how to proceed with this, e.g. whether the power and/or transmission hours can be reduced without causing problems for those clocks,



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