The story of mysterious *French* truck drivers.

Although my job title has the word "Presales" in it, once in a blue moon I get involved in already deployed systems. Some years ago, this accounted for around 50% of my work and so I got to see and solve a fair degree of technical problems.

This is a true story. Although I have their blessing, I've none the less changed some details as I don't want to reveal who the end customer and Motorola Reseller are. So, no identification with actual persons (living or deceased) languages or places is intended or should be inferred.

Müller & Söhne (though that is not their real name) is a towing service provider based on the outskirts of a large city in South-Western Germany (this is not the real location or country). They are an approved service provider for several associations; insurers and leasing companies and operate across the whole region. 

Some years ago, they installed an analogue radio system so that they could save on their phone bill and avoid having to depend solely on [in those days] unreliable cellular service. The system consisted of two repeaters, both installed on hilltop locations that provided excellent coverage as well as mobile radios installed in all trucks.

The system had served them well for several years but around April of that year, they noticed that some French-speaking people were talking on their radio system. This occurred only for a few hours and surprised as they were, they did not take it any further.

Then again in May of that year, French-speaking people were once again heard talking on their channel. Some towtruck drivers tried calling these people back but they did not respond. They suspected that this was a convoy of trucks passing through the area, as these foreign transmissions disappeared within a few hours.

A few weeks later, French-speaking people were again heard talking on Müller & Söhne's one channel. This had now become a nuisance, as the number of jobs would increase around the same time as these people were heard, due to inclement weather.

As a precaution, they called the company who installed the radio system in; explained the situation and had them carry out a health-check on the repeaters. Of course, the system was working correctly and by the time the technician arrived there, the interference was almost gone. Bewildered, the dealer contacted the regulatory authority to ask them to investigate the encroachment. 

The regulatory authority began monitoring the channel but for the entire summer holiday season, no interfering transmissions were heard. After two months, they probably concluded there was no problem or had other matters to attend to.

October of that year was particularly unpleasant. After a hot and dry summer and unusually warm Autumn, Winter had arrived in full vengeance. And, together with several cold fronts, the French people appeared once again on the channel.

The owner of Müller & Söhne contacted the company who installed the repeaters and demanded the problem be fixed this instant. So the dealer came out once more and programmed the repeaters and radios to use PL (CTCSS). This, of course, had the desired effect and the French radio users were no longer heard but Max Schmidt (not his real name), Senior Technician at ACME Radio GmbH (not their real name) was not satisfied with that: They had installed the system and had a certain reputation to maintain.

Max made contact with Motorola Solutions and spoke to one of their support engineers (guess who). After describing the problem and some thought, the engineer asked Max when was the interference heard and for how long? Luckily, Max had kept records of when Müller & Söhne had called as well as some notes of when other instances of this symptom had occurred. Various other questions were asked and here are the results of that conversation:

What is the problem? Sporadic interference on Channel 2 from French-speaking people.

What other problems could we see but don't? None. Only the occasional interference on Channel 2 from French-speaking people.

Where is the problem occurring? <<City>> Germany; Radio System of Müller & Söhne on Channel 2.

Where else could the problem occur but isn't? Does not occur on Channel 1. Does not occur on any systems maintained by ACME Radio GmbH. It is not known whether any other systems maintained by other companies are affected**.

When does the problem occur? So far on 14 April between approximately 09h00 and 12h00; 4 May between approximately 14h00 and 20h00;  23 May between approximately 07h00 and 10h00; 8 October between approximately 06h00 and 11h00. Nothing, since PL was implemented - though this is only masking the problem.

When else could the problem occur but hasn't? Any other day of the year.

To what extent does this occur and is there any trend? No trend. Only occurs randomly at different times of the day and only for a few hours. The interference seems to get stronger then fade again after which it disappears.

Nothing was changed in the system that could explain the cause. The only distinction seemed to be the date and time. The engineer consulted the news for those days in the hope of finding an event that would explain the presence of French workers in Stuttgart. Whilst browsing newspapers from those days he noticed something curious. He then checked the historical weather data for that region and discovered that the occurrences of the reported interference coincided with an approaching weather system that brought a rapid change in weather.

Normally, radio signals above 30MHz are not reflected back to earth from the Ionosphere and such systems can only get as far as just beyond the visible horizon. Sometimes signals below 100MHz are affected by meteors striking the Ionosphere but this is a rare event and the effects only last a few seconds or minutes. Since these signals are not reflected back down the earth, they are only conveyed by ground-wave propagation and hence radio system coverage is comparatively smaller/different above 30MHz.

Occasionally, an approaching weather system will produce a layer of warm (sometimes moist) air on top of cold air. This warm air creates a higher refractive index which results in signals being bounced back down to earth and received way beyond the expected coverage area. This is what was happening with the channel 2 repeater of Müller & Söhne.

In the analogue television days, it was sometimes possible to receive television signals from distant locations. One famous hobbyist was Rijn Muntjewerff, who was able to regularly receive TV transmissions from as far as 1850km away using this phenomenon (known as ducting). The earliest and most famous incident was in 1938 when RCA Engineers watched the BBC all the way from Long Island USA.  

In the case of Müller & Söhne, the fix was to shift 12,5kHz up. The signal was, during the above weather conditions, making it all the way from central France to Müller & Söhne channel 2 repeater. 

The telecommunications regulator also took steps to coordinate frequency allocation with neighbouring countries and to ensure effective communication between them when cross-border interference occurs.