Today we discuss the upper part of the shortwave spectrum that is generally usable during the solar minimum. These bands are less popular than those discussed last week. While many home radios had the 16m band included, the 13m band is far less common: it is only found on radios with multiple shortwave ranges.
|16m||17450-17900kHz||Shortwave broadcast band|
|15m||18900-19020kHz||Shortwave broadcast band|
|13m||21450-21850kHz||Shortwave broadcast band|
The 15m broadcast band was allocated in 1999 and it is not widely used. The band was created when shortwave broadcasting was already on the decline.
The 13m broadcast band is used by some broadcasters for long-distance communication. Many simple radios do not have this band and during solar minima it has only short opening hours.
This part of the spectrum contains two amateur bands.
|17m||18068-18168kHz||Amateur band||15m||21000-21450kHz||Amateur band|
The 17m amateur band is a WARC band (allocated during WARC'79) and it is a contest-free band. As opposed to 30m, SSB is allowed here. During contests the 20m band can be very crowded and this band is then ideal for the non-contest DX work. It has shorted opening hours than 20m, but it can be used as a reliable daytime band at all times.
The 15m amateur band was allocated to amateurs after World War II, to make up for the loss of most of the 160m band, so it is much less old than the 10m, 20m, 40m and 80m bands, but of course much older than the WARC bands. This band allows for very long distance communication with low power and modest antennas. This is the highest amateur band that provides some opening hours nearly every day. It is the lowest band that is sometimes used for satellite communication.
During a solar minimum these bands are usable for long distance communication during at least some hours each day. During solar maxima, opening hours are much longer. Then the 16m broadcast band and 17m amateur band can be used for long distance communication well into the night.
The previous sunspot cycle (number 23) started in 1996 and its peak was in 2000. So before the start of the cycle (early 1996) we had a solar minimum. During 2007 and 2008 the number of sunspots reached a very deep minimum, so there was very little propagation on the higher shortwave bands. The current solar cycle (number 24) started in December 2008, but the number of sunspots was increasing very slowly and there were long periods with no sunspots at all. People were speculating that we would not have any significant solar maximum and we would enter a period of very low solar activity. As we write in 2010, the upper shortwave bands do come to life again. The maximum of this cycle is expected to occur in 2013.
Before the mid-1950s it would be infeasible to construct a receiver like this, as frequency stability would have been very poor. But in the late 1940s the South African engineer Trevor Wadley invented the Wadley loop, which was used by the Racal communications receivers of the 1950s and 1960s. In this circuit there is a variable frequency oscillator (VFO) that mixes the desired receiving frequency to obtain the high IF (it runs from 40 to 70MHz) to obtain the 40MHz IF, but the same oscillator is used (by mixing it with harmonics from a 1MHz crystal oscillator and then filtering the desired mixing product) to mix the first IF down to the second IF (a range from 2-3MHz). The trick is that same oscillator is used in both conversions and any small deviations cancel each other out. So even though this oscillator is continuously variable, it acts like a band switch that selects the desired megahertz band. A conventional superheterodyne circuit mixes the 2-3MHz second IF to the third IF (100kHz in the Racal receivers, 455kHz in most others). It contains the tuning knob to tune within a megahertz band.
In the 1960s a RACAL RA17 cost around 20000 guilders (9000 euros). Corrected for inflation this would be around 45000 Euros (60k dollars) today! Remember that when you think an ICOM R9500 is way overpriced. In the late 1980s a surplus example could be had for the equivalent of 300 euros or less (still close to 500 eurose if corrected for inflation). I have owned a Racal RA17L from 1989 to 1992. It weighed more than 30kg and contained more than 20 tubes. Its main tuning dial was a piece of photographic film of 1,5m long, so excellent accuracy was achieved. When each individual radio was manufactured, the markings wer put on the film to match the tuning mechanism exactly. Even though it had crystal filters for CW, the 3kHz filter (used for SSB) was rather poor. A separate SSB unit could be purchased for this receiver, but that was much rarer than the receiver itself.
In 1973, the Barlow Wadley XCR-30 was manufactured in South Africa. It was a transistorized portable radio with the Wadley Loop circuit inside. It did not perform as well as the Racal receivers, but it was affordable to amateurs. In the late 1970s the Japanese firm Yaesu made the famous FRG-7 receiver, which used the same principle and had better performance. Several other Japanese radios existed that used the Wadley loop as well. In the early 1980s the Kenwood R1000 and Yaesu FRG-7700 were the most popular general coverage receivers for amateurs. They employed PLL synthesizers (for the megahertz multiples) combined with a VFO (to tune around in a band 1 megahertz wide) and they did not use the Wadley loop. This way the signal had to be mixed two times (once to a high first IF, once down to 455kHz), instead of three times in the Wadley loop.
A Phase Locked Loop or PLL is a circuit that adjusts the frequency of a voltage controlled oscillator (VCO) to a reference signal. By putting frequency dividers (which are in fact digital counters) in the loop, it becomes possible to generate any desired exact multiples or fractions of the reference signal. By making the division factor adjustable, we create a PLL synthesizer. It can generate any exact multiple of a certain frequency within a certain range. In the late 1970s these circuits became practical and affordable. It took a few more years before these circuits were good enough to be put into a high performance shortwave receiver. First a PLL was used to generate just the megahertz multiples to select any of 30 1MHz wide bands (and this had to be combined with a VFO), later it would replace the VFO entirely.
By using a PLL synthesizer it was no longer necessary to use the drift cancellation trick of the Wadley Loop to obtain good frequency stability in a shortwave receiver. Plus theat it was excellently suitable for microprocessor control. Frequency memories (presets) were no longer an expensive luxury item, but came almost for free in a microprocessor controlled receiver. From the mid 1980s all amateur receivers used a PLL synthesizer. They were produced by Yaesu, Kenwood, ICOM, JRC, AOR (all Japanese), Drake (USA) and Lowe (UK). My Lowe HF-225 was introduced in 1989 and produced until 1995. It uses a PLL synthesizer with 1kHz steps and the mixing oscillator between the first and second IF is a VXO (a crystal oscillator that can be varied in a small range by a variable voltage). The VXO generates the steps between the 1kHz multiples (128 steps). This is all controlled by the same tuning knob, so it is invisible (and almost inaudible) to the user who gets the impression of continuous tuning.
Around 1980 Sony introduced the ICF-2001 shortwave receiver, the first general coverage broadcast receiver with a high fist IF and a PLL synthesizer. It had no tuning knob, but a keypad instead. You could enter the frequency of the station directly on the keypad. It could receive SSB, but tuning was awkward, using up and down buttons and the fine tuning knob. Its performance was impressive for the time and that made up for its unwieldy battery consumption. It set the standard for digitally tuned shortwave broadcast receivers and many such radios were made since 1980.
In 1985 Sony introduced the ICF-2001D (called ICF-2010 in the USA). It had a real tuning knob and could also receive the air band. It was the first consumer radio that had a synchronous detector. Looking back, this model performed better than most of its successors. Since then, innovation in this area has been stagnating.
My Sony ICF-SW7600GR is the latest portable shortwave radio made by Sony. It has synchronous detection, but reportedly not as good as that of the ICF-2001D. It does not have a real tuning knob, but it performs very well as far as portables go. However, it Sony really cares to be the market leader in this segment, it should have had a version with DAB and DRM by now. There are many Chinese made radios with only marginally inferior performance to the ICF-SW7600GR, but for less than half the price.
Sangean (Taiwan) is still producing portable shortwave radios and recently they have introduced a model with DSP (Digital Signal Processing). The IF signal is digitized and further filtering and demodulation is performed digitally. Although this is innovative, they should have taken the next step and add DRM (and not remove SSB).
I'd like to complain a lot about the bad shortwave reception conditions in urban areas today and this of course justified. But in the 1980s it was not ideal either. At that time there existed no EMC regulations in most of Europe. Television sets were able to cover large sections of the shortwave spectrum with interference and the early home computers were heavy electromagnetic polluters, even by today's standards. The difference between cheap and expensive light dimmers was not their ability to dim light and not their reliability (if anything the expensive light dimmer could break more easily, because it contained more parts). The difference was that the cheap light dimmer contained no RF filtering and made anything below 10MHz unlistenable in the whole block.
In the 1980s there were more strong shortwave broadcast stations and they would overcome the man-made noise anyway. Most broadcasts that you hear today, are not directed to Europe. A home radio would not receive SSB transmissions anyway, so you would not miss them either. Serious shortwave listening without a serious outdoor antenna has never been a pleasure.
One interference source that has disappeared is the Russian Woodpecker, an over-the-horizon radar system used by the Russians in the 1980s. It was like radar, but it used very powerful pulses on shortwave frequencies. These pulses (and their echoes) would be reflected by the ionosphere, so the Russians could have a look at aircraft and (supposedly) nuclear missiles far beyond the horizon. These pulses were very wide band and very powerful, so they made large chunks of the shortwave spectrum unlistenable. People with good antennas in rural areas were also affected. Over-the-horizon radar systems still exist, but they are far less antisocial than the Russian Woodpecker was.
It is possible to send frequencies over existing wires that are much higher than the frequencies those wires were originally designed for. For example it is possible to send high speed digital signals over existing (analog) telephone lines. This is called ADSL. ADSL signals are relatively unproblematic for radio reception. It is also possible to use the electrical wiring inside a house to transmit high speed network data. The problem is that these frequencies lie in the shortwave spectrum (in some cases even the VHF spectrum) and that the power lines are not intended to carry such signals. They are unshielded and they work as reasonably efficient transmitting antennas on some bands. If powerline communication becomes more widespread in Europe, reception possibilities for shortwave (or even VHF) signals will rapidly deteriorate. Add to this the interference generated by switched mode power supplies and plasma televisions and the situation for shortwave listeners and radio amateurs does not look good at all.
See you next time when we discuss the top end of the shortwave spectrum.
I added several diagrams. I corrected the item about Wadley loop receivers. The R1000 and FRG7700 are not Wadley loop receivers.