A while ago Wireless Waffle added an FM DX logbook listing reception of far distant (a.k.a. DX) FM radio stations which have been received in the UK at various times. Reception of such stations has also been discussed before in particular with reference to sporadic-E propagation.

FM stations being received over a long distance by sporadic-E tend to be very strong, and can often overwhealm reception of local stations. Those being received through tropospheric ducting are often somewhat weaker.

It was a surprise, therefore, on a recent drive around London's orbital motorway, the M25, whilst listening to community station Kane FM at a distance of approximately 13 km from the transmitter, that french station France Inter on the same frequency of 103.7 MHz, became stronger. So much so, that the radio in the car decoded the RDS of France Inter and switched between 103.7 and 99.6 MHz where a second transmitter could be received.

The transmitter of France Inter on 103.7 MHz uses a massive power of 400 kW from a site in Lille (approximately 260 km away). It is actually one of the highest powered FM transmitters in the whole of Europe. The France Inter transmitter on 99.6 MHz uses a transmitter power of 50 kW from a site in Caen (approximately 225 km away). If you consider the free space path loss from these sites, and their transmitter power, you can roughly calculate the expected signal strength that each transmitter would produce.

Radio Station	Frequency	Power	Distance	 Path LossÂ	 Field StrengthÂ
Kane FM	103.7	25 W	13 km	95 dB	56 dBuV/m
France Inter (Lille)	103.7	400 kW	260 km	121 dB	72 dBuV/m
France Inter (Caen)	99.6	50 kW	225 km	120 dB	64 dBuV/m

Assuming free space loss, therefore, the signals from France would be 8 to 16 dB higher than those from the nearby Kane FM transmitter.

However, this is nothing like reality: free space path loss gives a result which would represent the strongest possible signal that could be received. Of course none of the signals would be propagating in this way, as there would be innumerable obstacles along the way that would deviate wildly from 'free space' and the signals would come nowhere near these values, especially those that have travelled 200 km or more from France. Note that various studies have indicated that for reception in a car, a signal strength around 40 dBuV/m is needed suggesting that the additional path loss caused by propagation effects would have been in the order of 16 to 32 dB, which seems reasonable.

FM receivers have an effect called the 'capture ratio'. In general, if two signals are on the same frequency, and one is more than around 3 dB stronger than the other, then the strongest signal will win-out and the weaker one will disappear. So at least it is clear that the signal from France Inter on 103.7 was a few dB stronger from that of Kane FM.

None of this is groundbreaking nor even necessarily that interesting, but it does suggest that the actual path loss obtained during reception by tropospheric ducting can be relatively low. And for radio stations in areas prone to tropospheric ducting (predictions of which can be found on Willam Hepburn's excellent web-site), using a few extra dB of transmitter power may be necessary to ensure reliable reception. Stations, for example, on either side of the Arabian Gulf often use tens of kiloWatts of transmitter power even to provide coverage of just one city, as problems with ducting of signals from the other side of the Gulf are an almost daily occurrence.



Of course, the converse is that using more power then causes increased interference to those on the other end of the duct. Which in turn requires them to turn up their power to combat the problem. Which makes the problem worse. And so forth...

For many years, Wireless Waffle has enjoyed the occasional bout of FM DXing. That is to say that when the propagation conditions have permitted, time has been enjoyably spent tuning up and down the FM band to see what can be heard.

The logbook of this anomalous reception has been sitting waiting to be published for ages and now, with a quiet weekend with not much else to do has presented itself, it's finally morphed into a web-page of it's own.

Wireless Waffle is therefore proud to announce the FM DX Logbook. If this means nothing to you, then don't take a look. Even if it does, just one look is all it takes, as someone wiser and more lyrical once suggested.

Back in December when we discussed the launch of Radio Caroline on 648 kHz and suggested that they were using wider bandwidth AM, Mike left a comment concerning Stereo AM. Stereo AM makes a great article for a post, so here we go!

AM broadcasting in the medium-wave (MW) band is traditionally mono only. Back in the early 1980's four competing technologies were running trials of AM stereo, and in 1988 a solution proposed by Motorola called Compatible-Quadrature Amplitude Modulation (C-QUAM) which had gained significant popularity, was chosen as the de-facto standard in the USA (it was formally adopted in 1993). The word 'compatible' refers to the fact that one of the primary criteria for the solution was that an 'old fashioned' mono AM receiver would not notice any difference when tuned to an AM-stereo signal, that is to say that it would continue to receive a mono signal as if nothing had changed. Thus the stereo signal would remain compatible with mono receivers (note that exactly the same is true of FM stereo).

The C-QUAM solution used the properties of AM and of FM to do something novel. In an AM waveform (as shown on the right), the information (in this case audio) is imposed onto the transmission frequency (the 'carrier wave') by changing the amount of power transmitted, that is to say, changing the amplitude of the transmission and hence amplitude modulation (AM). At the receiver, only the amplitude of the signal is used to recover the data.

For an FM system, the amplitude or power of the transmitter remains the same and only the frequency is varied, hence frequency modulation (FM). In reality, the amplitude of the signal at the receiver will often vary, especially if the receiver is mobile, but as long as the signal that arrives at the receiver is strong enough to overcome any background noise, the receiver will still be able to detect what frequency was being transmitted and recover the original data or audio.

What Motorola did was to use both FM and AM at the same time. It is not as easy to see what is going on in the C-QUAM waveform shown on the right, but the principle is fairly straightforward. If you feed this signal into an AM receiver, it will only detect the changes in amplitude and will ignore the changes in frequency completely. If you feed this signal into an FM receiver, the opposite happens and the AM part of the signal is ignored and only the information modulated on the FM part of the signal is decoded. Hence you can send two sets of information at the same time, one using AM and the other FM.

Compatibility is maintained by sending the Left and Right signals added together on the AM element of the transmission, and the 'stereo difference' (Left minus Right) on the FM part. Thus a mono AM receiver will see no change, but a stereo FM receiver will be able to add the mono and difference signals together to recover the original stereo signal. This is the same principle that applies for stereo FM broadcasting, though the method of transmitting the sum and difference signals are different.

The C-QUAM system is actually a little more complex than the description I have given above, for example, a pilot signal is added into the FM part of the signal to alert receivers that the transmission they are receiving is in C-QUAM, but the logic is in principle the same.

There are lots of videos of C-QUAM AM stereo on YouTube, but the one below is perhaps one of the best illustrations of what it sounds like on-air. It's actually fairly impressive.



So why is it that you've probably never heard of Stereo AM before? Here are some thoughts:

• Though a number of stations (particularly in the USA, Canada, Japan and Australia) did adopt the standard, there may have been insufficient stations on-air for consumers to properly experience the difference.
• The cost of receivers was always higher than mono ones as they were far more complex. This was exacerbated for a while as Motorola held the patents for the system and would have charged manufacturers a handsome fee to adopt it (they were forced to remove such payments in 1993 in the US when the FCC formally adopted the system).
• The quality of transmission of AM stereo, though better than AM mono, was not competitive with FM stereo which, by the time the C-QUAM standard had been adopted, were far more widespread than when the original AM stereo trials began.
• Maybe, and this is just supposition, the types of station on AM were in themselves less popular than the growing FM market, or possibly in some cases the programme material (e.g. talk) was not well suited to stereo.

For whatever reason, AM stereo has largely taken a back-seat in the broadcasting world. By 2000 there were around 300 AM stereo stations on-air in the USA, today there are around 90 and a handful in other countries around the world. The most prominent AM stereo signal in Europe (France Bleu on 864 kHz) was sadly turned off at the end of 2015.

Wireless Waffle previously sang the praises of the boat trips to visit the Ross Revenge, the home of former radio pirate, Radio Caroline. We also noted that they had been awarded a licence to operate a 1 kiloWatt transmitter on 648 kHz in the Suffolk and north Essex area.

Well, it seems the engineering bods on the east coast have managed to get it together, and recently test transmissions on 648 kHz were spotted by a number of listeners, such as a DXer in Humberside who posted the video below on YouTube.



Though you may not be able to interpret the waterfall display shown on the video, what you see is the Radio Caroline signal in the middle. The two bright lines either side represent radio stations on the adjacent frequencies (639 and 657 kHz respectively). Normally, for AM broadcasting, each station would be allowed to occupy half of the bandwidth between its assigned frequency and the adjacent channels, meaining that it would extend +/- 4.5 kHz either side of its centre frequency. It is this limitation they gives medium and long wave broadcasting their characteristic 'muddy' sound, as the limitation in spectrum also restricts the amount of audio bandwidth that can be transmitted.

It's therefore notable that the Radio Caroline transmission on 648 kHz extends far closer to the adjacent frequencies than 4.5 kHz. It appears closer to +/- 6.5 kHz wide (or maybe even more). This would allow the station to transmit a wider audio bandwidth and thus sound a little 'brighter' on-air. Such derogations from the norm are not unusual as the medium wave band has become emptier, as there is more space for stations to spread out and sound better.

As an example, the three audio clips below have been filtered with different bandwidths. Just click on the relevant button to hear the difference (note that this doesn't work in all browsers.

Audio Bandwidth	Play
15 kHz, stereo (FM Stereo)
6.5 kHz, mono (extended bandwidth AM)
4.5 kHz, mono (standard bandwidth AM)

Given that of the neighbouring frequencies, the nearest stations on 639 kHz are in the Czech Republic and Spain (previously crowned the queen of medium-wave broadcasting) and on 657 kHz in Spain (again) and North Wales, it seems unlikely that the additional bandwidth being used by Radio Caroline will give any problems and we are sure that listeners will enjoy the cleaner, brighter sound that they will have on-air.