LAST UPDATED: 5^th March 2021

   BROADCAST RADIO

This page will be devoted to broadcast radio, and in particular Short Wave broadcasting, although there will be details on LW and  MW.

It is unfortunate, and a little sad that shortwave broadcasting is on the decline and one day will probably cease to be used for that purpose, but that is unlikely to happen completely for a considerable time to come. Hopefully I can generate some interest in SW radio and get some of you to try it, maybe for the first time.

I have always had an interest in shortwave broadcasting, as far back as I can remember. Both my mother and father held ham licenses so I grew up around radio so it was natural that it wold interest me, mind you I didn't realise that that interest would stay with me and eventually lead to employment!

Anyway, onto Shortwave broadcasting. I am not going to give a long lecture about SW broadcasting, rather i would like to give some information to start you off (BTW I recommend purchasing a guide to broadcasting, such as the extremely comprehensive and accurate WRTH (World Radio TV Handbook), this will give you all the details you will need about broadcasting stations, from broadcasting schedules to transmitter sites, address and contact details plus equipment reviews and more).

Shortwave radio enables broadcasters to send a signal into whichever part of the world they wish to target. Shortwave radio has a great advantage over other distribution methods in certain areas - some may not have internet access, a computer or even a phone line. In some countries there may only be one radio for the whole village, and invariably this has a shortwave band. On this radio, the villagers can hear broadcasts in their own language from stations 1000's of miles away.

Although SW broadcasting is in decline, mainly due to cost, well over 200 languages are still transmitted each week by international broadcasters. If you add in the domestic (that is stations broadcasting into their own country) the total goes up to probably 300 or more languages and dialects.

Here are the SW Broadcast and Amateur radio bands:

Shortwave broadcasting uses a standard of 5 kHz separation between channels for analog broadcasting and 10kHz for DRM (Digital) broadcasting. However, some broadcasters do not stick to this convention and operate between ‘channels’. This is particularly true of clandestine/covert stations and some of the domestic broadcasters in the tropical bands. The 120, 90, 75 and 60m bands are known as the ‘Tropical Bands’ because the majority of broadcasting on these bands emanates from the Tropical regions of the world (Indonesia/Australasia, Latin/South America and Africa)
 

The table below shows the broadcasting and Amateur radio allocations, together with the ‘Meter Band’ name that it is usually known by. This is not an exact ‘wavelength to frequency’ equivalent however! Broadcasting bands are marked in Orange type. In between the broadcast and amateur radio allocations are vast expanses of the spectrum that is filled with all manner of point-to-point communication links, ship to shore phone patches, HF aviation links and military, embassy and commercial  communications, plus a whole lot more. The military and embassy traffic tends to be encrypted for security reasons. There are a lot of weird and wonderful data modes etc. and sometimes these will be heard in broadcasting segments, either legally or otherwise. There are also a lot of signals that are of unknown origin or purpose (i.e. the ‘numbers’ stations - are they relaying instructions to spies in the field? One can only speculate on that because there is little hope of anyone owning up to it)!

DRM (DIGITAL RADIO MONDIALE)

DRM has been around for a long time but the uptake by broadcasters has been slow - this has not been helped by the appalling lack of affordable receivers on the market. Only recently has there been any that are useable by the general public, and these are expensive (upwards of £200 for what equates to a portable transistor radio!) and their performance is not great. So what is all the fuss with DRM and what is it? In a nutshell, DRM is digital broadcast radio that is used on LW, MW and SW. FM has been given over to another form of digital broadcasting, DAB and DAB+. These systems are not compatible and require differing hardware to work.

DRM has it’s own set of problems - it requires a good strong, stable signal in order to be decoded by the receiver. If the signal drops in strength below a threshold, the receiver does not decode anything and you are left with nothing - no hiss or or speech, just a blank, empty channel. When the signal returns to it’s former strength, the audio is restored as the receiver is able to decode it once again. Occasionally, the sound will become garbled, this is usually due to a brief dip in signal strength or a burst of noise. The level of signal required for a full solid decode is much greater than is required for an analogue AM broadcast, where the signal may fade down but is still perfectly audible. We are all used to this as it is the nature of the ionosphere to be constantly changing and hence the propagation conditions from any given path will also change constantly. DRM does not cope well with this kind of change. Now having said that, when you can get a good decode, the audio quality is very good indeed - no background hiss, no fading and no interference. Is that worth the extra expense and hassle? In my opinion, no it isn’t. Add to that the extra bandwidth a DRM signal takes (they use a full 10kHz of bandwidth) where as an analogue SW broadcast fits inside a 5kHz channel with room to spare, so DRM is less spectrum friendly than it’s analogue counterpart, taking up at least 2 analogue channels. On MW the situation is just as bad, where the channels are spaced at 9kHz (or 10kHz if you are in the Americas). For the experimenters among us there are a few ways of listening to DRM without shelling out a small fortune. There are quite a few radios that lend themselves to fairly simple modifications to enable them to receive DRM, details of which can be found on the internet. There are also small kits available to fit into some radio’s so they can decode DRM. I am using a different method to decode DRM as my receiver is not easy to modify, and the Icom does not lend itself to modification. The receiver I use for DRM is the Racal RA1792, which has a choice of filters available, the widest of which is 16kHz, which is too wide for DRM, BUT if you put the radio into CW mode and adjust the BFO offset to -6kHz, this effectively reduces the bandwith of the filter to the 10kHz required by DRM. The audio from the Racal is then fed into the soundcard on my pc. Using a program called “Dream” I am able to decode the DRM signal. I can also get an up to date broadcast schedule as well as view a technical analysis  of the signal. Dream will also pickup a station identification, often the ident is received but the signal is not strong enough for the audio to be demodulated. Below are some idents I have seen/heard using Dream and the Racal and, later, the SDR-IQ receiver together with the Wellbrook loop. DRM is in decline now, with few stations transmitting it. This is down to a lack of available receivers. Other than us technical types using PC’s, there are only a couple of receivers/radio’s available to the general public and their performance is not great. It is a shame, the methods used to generate and receive DRM were outdated even before it was being used.

Below is a screen grab from my SDR-IQ receiver, showing a strong DRM signal near the centre of the screen (on the bottom waterfall) as you can see it takes up a solid 10kHz block of spectrum. Any signal in that 10kHz will not be audible and will be swamped by the digital hash. By contrast, to the right you can see a strong analogue signal, which appears to be quite wide, but in reality it will only be tyhe voice peaks that ‘splatter’ on to the next channel. To the lkeft of the DRM signal, every 5kHz, carriers can be seen. Each of these carriers is a signal, some will be resolvable with audio, others will be too weak. To the far left, there is another strong analogue signal, but you can also see that the carrier 5kHz up is barely affected by it. If that were a DRM signal, the carrier would be totally squashed. ALso the DRM signal needs to be very strong in order to decode successfully, whereas the analogue signals can be very weak and still be usable. I will, in the near future include a recording of a SW DRM signal and it’s decoded audio so that this can be compared to an analogue signal from the same band.

Below is another screen grab from my SDR-IQ receiver, this time showing a selection of analogue signals of varying strengths. It is easy to see the stronger ones and also when a station signs on/off, or if a station is off frequency. The bottom waterfall has a ‘view’ of 100kHz of spectrum. The top waterfall is zoomed in on the area of interest.