Column 6: From 49m to 31m (5.8-10MHz, 2010-10-20)

Today we get straight into the frequency range that most people know as shortwave. Some people will remember shortwave mainly as that band on the radio that was too crowded with strange noises, others will remember it when trying to receive radio stations from the home land when they were on holiday abroad.

Band diagram 5.8-10MHz

Broadcast receivers

Many ordinary home radios from the 1930s to the 1970s had a single shortwave range. This almost always started at the 49m band and it extended to whatever band the designers of these radios still thought useful. I have seen radios with just the 49m band, with 49m and 41m, with 49-31m, with 49-25m. 49-19m and 49-16m. Radios had analog dials with the broadcast bands marked. On some older radios, the broadcast bands were even marked with station names, but it was about impossible to find out which station belonged where in the broadcast band. Having a single 10MHz wide shortwave range made the shortwave band appear very crowded, especially the broadcast bands. These broadcast bands were like islands chock full of stations amidst a sea of strange noises. Finding the desired station was like finding a needle in a haystack. This was less of a problem on radios that offered the 49m band only.

The home radios of that period were not very suitable for shortwave reception for the following reasons:

But sensitivity was adequate, as was dynamic range (the ability to cope with strong signals). Synthesizer noise was absent, because those radios did not have a PLL synthesizer.

Some radios had two or more shortwave ranges, each one covering several shortwave broadcast bands. Depending on the model, these ranges were or were not contiguous. A radio could have the ranges 5.5-7.5MHz (49+41m), 9.1-12.1MHz (31+25m) and 15-18MHz (19+16m). Then there were radios that covered the shortwave broadcast bands only, one dial per shortwave broadcast band. Early models with the broadcast bands only were very well built and performed better than the average home radios. During the 1990s and 2000s however, cheap analog world receivers have become cheaper and lower quality. The old Grundig Satellit radios from the 1970s had two sets of shortwave ranges: one set of 8 ranges covered the entire spectrum from 5.5 to 30MHz, then there were ranges for the eight broadcast bands: 49m. 41m, 31m, 25m, 19m, 16m, 13m and 11m.

Some radios with two or more shortwave ranges miss important broadcast bands or miss part of a broadcast band. Fairly recently (1990s) there were (PLL synthesized) world receivers on the market that had the shortwave ranges 2300-7300kHz and 9500-21850kHz, Officially the 41m band has had an upper limit of 7300kHz and the 31m band has had a lower limit of 9500kHz. But even when those limits were still officially in force, reality was different and you missed out important stations. As these radios had a PLL synthesizer, they could not be simply adjusted.

To improve image rejection, one can construct a radio with a higher intermediate frequency. But if the IF is too high, it becomes harder to get good selectivity. This is possible with crystal filters, but these are expensive. Therefore many radios had two intermediate frequencies: one high IF to obtain good image rejection and one lower intermediate frequency to obtain good selectivity. A second local oscillator (at a fixed frequency) mixed the signal from the high intermediate frequency to the low intermediate frequency. Intermediate frequencies of 2MHz, 9MHz and 10.7MHz were sometimes used. See the block diagram of a double-conversion superheterodyne receiver. Block diagram of double conversion superhet

Before digital frequency readout became commonplace, some radios had a combination of bandspread dials and crystal calibrators. One would first calibrate the bandspread dial to a fixed frequency in or near a specific (broadcast) band, then one could have accurate frequency readout on the bandspread dial. For example: the crystal calibrator generates all harmonics of 500kHz. When one wants to tune the 31m band, tune the main tuning dial to 9500kHz and the bandspread dial to 0. If the crystal calibrator is engaged, one can tune to the harmonic at 9500kHz. Once the calibration is done, one can tune the bandspread dial between (-250kHz and +250kHz), tuning the range from 9250 to 9750kHz.

At least this is how the Panasonic DR22 (called RF2200 in the USA) worked. It had six shortwave bands, each 4MHz wide, spanning the range from 4 to 28MHz. At the extreme ends, most of the 80m band and part of the 10m amateur band could be tuned. The main dial was almost perfectly linear (one revolution of the tuning knob changed the frequency by the same amount, throughout the shortwave range). The bandspread dial was connected to the same tuning knob (but it could be decoupled to calibrate it). Using the crystal calibrator, frequency readout was fairly accurate. I owned this radio in the 1990s. It had a BFO, but it was not adjustable, it was optimized for USB. Stability was not great for SSB, but the first owner used it to decode RTTY. It also had great reception on mediumwave and FM.

International broadcasting

The shortwave bands between 49m and 11m (plus 75m) are intended for international broadcasting. International broadcasting on the shortwave bands started in the 1920s and The Netherlands was one of the first countries to do it. These days, shortwave stations targeting Europe and the USA are on the way back. From 1930 to 2000 these bands were chock full of stations around the clock, whenever or wherever you tuned to these bands. At least since the 1970s, shortwave broadcasters use frequencies that are a multiple of 5kHz, using a audio bandwidth of around 3kHz. Adjacent channels run into each other, but often one sideband of a station is relatively undisturbed,
49m 5730-6300kHz Shortwave broadcast band
41m 7200-7600kHz Shortwave broadcast band
31m 9250-9990kHz Shortwave broadcast band

During the day these bands are mainly used for short range (up to 1000km on 49m) to medium range (1000 to 2000km on 31m) broadcasting. As there are far fewer European stations than ten years ago, these bands are not as busy as they were. At night these bands are still very crowded.

The 49m band is primarily used for short range broadcasting. Many shortwave broadcasters rent transmitters in foreign countries. Radio Netherlands is using transmitters in Germany. But China Radio International also rents European shortwave stations to transmit some of its broadcasting from Europe, close to the target area.

The 41m band borders to the 40m amateur band. It used to run from 7100 to 7300kHz, but it has moved up a bit. ITU Region 2 (America) dit not originally have this band, but mowadays it is used there, above 7300kHz. The range between 6900kHz and 7000kHz could also be used on non-interference base, but very few stations use it.

The 31m band is is a very interesting band on which stations from many countries can be heard, especially during the evening and night. When listening in Europe, this is less dominated by European stations than the 49m and 41m bands.

The indicated limits of the bands are not very hard. There are official broadcast bands, some sections where broadcasting is allowed on a non-interference base and some sections where it is not allowed, but occurs nevertheless. An amateur operating just a few kilohertz outside the amateur band will be caught, he will get a fine and may lose his license. When a government allows broadcasters to transmit outside the broadcast bands, very little can be done about it. Sometimes it helps if you complain.

International broadcasts can be divided into those that target citizens of their own country abroad (usually in the language of the originating country) and those that target foreign citizens. Depending on which side you are, these contain either objective information or harmful propaganda. Broadcasts in a single language for a single audience typically last for 30 or 60 minutes. Transmission schedules and frequencies change often, typically twice per year. An invaluable resource is the World Radio and Television Handbook That said, some shortwave stations do have frequencies that they have been using for decades, e.g. Radio Netherlands on 5955kHz and Deutsche Welle on 6075kHz.

During the Cold War, the Voice of America and Radio Free Europe beamed powerful signals into what was then still called the USSR. Te Russians used local jamming stations to make receiving these broadcasts almost impossible. Sometimes these transmitters broadcast irritating noises, but in some cases they just happened to be transmissions of Radio Moscow. In the West, Radio Moscow could be heard without interference. At times it seemed that every other shortwave frequency was in use by Radio Moscow. Nowadays China Radio International tends to dominate the broadcast bands, but not nearly as badly as Radio Moscow did in the 1980s. Albania is a small European country, today the poorest country by far, back then the most totalitarian country by far. Possession of shortwave receivers was prohibited (it was not in the Soviet Union), but Radio Tirana spread its propaganda shamelessly all over Europe and also on frequencies it was not supposed to use (like the 40m amateur band).

DRM (Digital Radio Mondiale) was already mentioned in my column about the mediumwave band. It is a digital transmission method for the longwave, mediumwave and shortwave bands. In Europe, an English language programme from BBC and Deutsche Welle is currently transmitted in DRM, almost around the clock. At the moment, very few DRM receivers are commercially available, so it reamins to be seen if this will ever become a success. The lowest part of the 49m band (below 5900kHz) has a relatively high percentage of DRM stations.

Amateur band

The 40m band is a busy amateur band, especially during the evening and night. As it borders to a busy broadcast band (with stations that have 10 to 1000 times more power), receivers for this band are required to have a good dynamic range. On this band, a skip zone can often be observed. A station may be inaudible at a distance of 500km, but it may be heard loud and clear at a distance of 1000km. At a distance of 500km the signal only exists high up in the ionosphere, where it is reflected, so it is not available at ground level.
40m 7000-7200kHz Amateur band
In America this band runs till 7300kHz. They did not have the 41m broadcast band. In Europe this band ran only till 7100kHz, but in 2009 it was extended.

Radio amateurs use a lot of specialized jargon (or slang) on the bands. They use a lot of abbreviations, even more when they use morse telegraphy. If you overhear a conversation between amateurs, it may very well be the case that you hear only one party. Even though most conversations are simplex (use only one frequency), it all depends on the position of all participants relative to your position (and the quality of your antenna) whether you can hear them. A conversation often starts with CQ, which means a general call, a call to any amateur who happens to hear that station. All parties will mention their own call sign after the call sign of the other party (if applicable). Signal reports are often exchanged, like "you are five by nine", which means Readability 5 (perfectly intelligible) and signal strength 9 (plenty of signal strength). They often make use of the Q-code, a set of three-letter codes all starting with the letter Q. Basic understanding of the Q-code is a required part of the amateur exam. The following types of conversations can be heard:

Other users

Of course this section of the spectrum also has its fair share of maritime and aeronautical bands, but this time we will discuss some other users.

During the Cold War, both the West and the East had spies in each other's territories. Secret messages had to be sent to these spies and they were sent on shortwave by powerful AM modulated transmitters. This way, the spies needed only an innocent looking cheap world receiver to hear them. Everyone else could hear those transmissions as well, but only the spies could decrypt the messages, basically using nothing more than paper and pencil. The stations transmitted endless sequences of groups of five digits. Most numbers stations that could be heard in The Netherlands broadcast in German. Espionage (and the use of numbers stations) has continued after the Cold War.

The station WWV in the USA transmits time and other information (such as ionospheric conditions) on several shortwave frequencies, 10MHz being the most important one (but also on 2.5, 5, 15 and 20MHz). This is a combination of second pulses, time codes and voice information. Due to the variable delay of ionospheric propagation, the received time from this station is less accurate than that of LF stations.

See you next time when we discuss the three other busy shortwave bands.

Update 2013-07-10

Shortwave broadcasting directed to Europe has declined rapidly over the past few years. Only a few European national broadcasters are still transmitting powerful signals directed to Europe. These countries include Spain and Romania, but I guess these will fall victim of budget cuts sooner rather than later. Radio Netherlands and Deutsche Welle only transmit towards certain countries where shortwave is still a major source of uncensored news.

Oddly enough, BBC World Service has ceased analog transmissions directed to Europe, but still transmits on DRM. Compared to other regions of the world and compared to the extremely reduced analog shortwave transmissions, there are still a surprising number of DRM transmissions directed to Europe. Those transmissions are probably for an audience that can be counted on two hands. The number of standalone consumer DRM receiver models that are officially on the market today is exactly one, the Chengdu NewStar Electronic DR111.

There is still a Dutch station, KBC Radio, regularly transmitting on 6095kHz, probably the only official Dutch language station on shortwave.

Update 2015-05-10

I added several diagrams. There are fewer European DRM transmissions than just a few years ago. I think it is officially declared dead in Europe. India may adopt the system though. Recently another DRM receiver was introduced. the Avion Electronics DR-1401, apparently only available in India.

Romania is still active on shortwave, Spain is no longer. KBC Radio has very little shortwave activity.