Column 7: From 25m to 19m (10-16MHz, 2010-10-27)

Today we discuss another very popular part of the shortwave spectrum. Many old home radios cover this part of the shortwave spectrum, as well as last week's part. The bands in this part of the spectrum provide good propagation during daylight hours and they are usable during all seasons and throughout the sunspot cycle.

Band diagram 10-16MHz

Amateur bands

This part of the spectrum contains two amateur bands.
30m 10100-10150kHz Amateur band
20m 14000-14350kHz Amateur band

On the 1979 World Administrative Radio Conference of the ITU, three new amateur bands were allocated: 30m, 17m and 12m. These bands are still known as the WARC bands. When these bands were still new, amateur transceivers were not yet capable of using these bands. Even today, these bands are used quite a bit less than the "regular" amateur bands. This is partly because these bands are comparatively narrow, partly because most amateurs do not construct antennas for these bands and partly because contests are not allowed on these bands. The restriction of contests is mainly a convention of amateur associations, rather than a prohibition by law.

The 30m band is only 50kHz wide and therefore only morse telegraphy and narrow band digital modes (like PSK31) are allowed. This is the only shortware amateur band in which contests are prohibited by the license conditions in The Netherlands, on the other two WARC bands the license conditions do not prohibit contents, yet it is simply not done to organize them. This band has both daytime and night time propagation characteristics, which makes it a very interesting band.

The 20m ham band has existed since the 1920s and it is one of the busiest amateur bands. This band offers good DX opportunities during the daytime, regardless of season, regardless of solar activity. This band is not very suitable for shorter distances (500-1000km). As with other amateur bands, CW is concentrated in the lower part of the band (up to 14099kHz), telephony is concentrated in the higher part of the band (above 14101kHz).

The NCDXF beacons form a network of propagation beacons. They help radio amateurs find out which parts of the world are reachable by radio on the 20m band (and on higher shortwave bands). 18 beacons on 18 locations all over the world each transmit for exactly 10 seconds on 14100kHz according to a fixed schedule, which is repeated every three minutes.

Before the 1970s many amateurs still built their own equipment. A popular transceiver design used an intermediate frequency (IF) of 9MHz and a variable frequency oscillator (VFO) between 5.0 and 5.5MHz. With these you could either get the band 3.5-4.0MHz (80m) or the band 14.0-14.5MHz (20m). You only had to change the filters on the receiver input and transmitter output. When you built the transceiver for one band, you could get the other band almost for free. In fact you chose the IF such that the image frequency of one band was the other amateur band. But this clever design had the consequence that the dial of the 80m band ran backwards with respect to the dial of the 20m band and if you transmitted or received USB on one band, you would transmit or receive LSB on the other band. This is the main reason why today the low amateur bands use LSB and the higher bands use USB.

A clear advantage of this band over 40 or 80m (let alone 160m) is that efficient antennas have a size suitable for the average suburban garden in The Netherlands. A quarter wave vertical is only 5m tall and a half wave dipole is 10m wide. An Inverted V construction can be made to fit into almost any garden. People with more space can already build effective directional antennas, such as Yagi-Uda antennas. Rotatable three element antennas for this band are not uncommon.

Broadcast bands

This section of the spectrum has three busy broadcast bands.
25m 11500-12100kHz Shortwave broadcast band
22m 13570-13870kHz Shortwave broadcast band
19m 15000-15800kHz Shortwave broadcast band

The 25m band is mainly a daytime band, but in the summer and during solar maxima, it is also usable during the night. In Europe this is typically the first band on which you can hear many stations from outside Europe during the day.

The 22m broadcast band was created by WARC'79 (the same conference that created the WARC amateur bands), but it was only effectively allocated around 1989. Nevertheless, broadcasters started to use it a few years earlier. No tube radios have this band marked on the dial, but if their shortwave range covers 19m. 25m and everything in between, they can of course tune to this band. This band is missing on older broadcast receivers with incomplete shortwave coverage.

The 19m band offers reliable long distance communication during the day, even in winter and during solar minima.


Signals above 10MHz are not significantly absorbed by the D layer and they can reach the F2 (during the day) or F (during the night) layer of the isonosphere practically all the time. Therefore propagation is determined by how well the F(2) layer can reflect these signals. This depends on the angle at which the wave hits the ionosphere and on the critical frequency. Above the critical frequency, anything sent straight up will pass through the F(2) layer without being reflected, The lower the angle, the higher the frequency at which the signal can still be reflected and the longer the distance reached. On these bands it is often the case that a long distance can easily be reached, but a shorter distance cannot be reached. This is called skip zone. To reach really long distances (more than 5000km), the signal must be reflected to the ionosphere, then back to Earth, then to the ionosphere again and then back to Earth. This is called two hops and of course a signal can travel with more than two hops.

The higher the critical frequency, the higher the maximum usable frequency at any angle. The lowest angle is reached when the signal is radiated parallel to the earth's surface. If even at this angle, a signal is not reflected, then this frequency is unusable for long distance communication. The maximum usable frequency depends on the level of ionization of the F(2) layer and this in turn depends on solar radiation. The following are the main periodic changes:

Propagation is a lot more complex than mentioned in this article and it cannot be predicted exactly. Sometimes lower layers do have influence. This makes it sometimes frustrating for professional users and fascinating for radio amateurs.

If you want to reach a certain destination at a certain time, you need to pick a frequency that is suitable for the job. Of course you must pick a frequency that you are allowed to use. This applies to all users of the shortwave spectrum, not just to radio amateurs. Therefore all users have many bands throughout the shortwave spectrum. There are bands for marine communications, for air communication, for fixed stations and for mobile stations, just as there are many amateur and broadcast bands. Of course radio amateurs can take advantage of the unexpected; they can choose to communicate with stations at whatever distant locations for which a favorable propagation path exists. Professional users on the other hand, have a need to reach their intended target (and not somebody else many thousands of kilometers away).

Other things to hear

The Intermediate frequency of most broadcast FM radios and many other VHF receivers is 10.7MHz. A shortwave radio can be tuned to this frequency and one can hear the station a nearby FM radio is tuned to. Even though the sound is severely distorted, it can be used to diagnose a receiver (verify that the IF signal is present).

Part of the radio spectrum is reserved for Industrial, scientific and medical applications (ISM bands). The original intention is that these frequencies can be used for applications of radio frequency energy other than communication. for instance heating things up. The most well-known such application is probably the microwave oven on 2450MHz. Because these bands are potentially full of interference, they are not used for professional communications, but all kinds of short range or low power communications are often tolerated in these bands, for which no frequencies are reserved. Therefore we got Wifi and Bluetooth in the 2450MHz band.

There is also a very narrow ISM band around 13.56MHz and today this is mainly used by RFID chips. While this frequency could have been used for world wide shortwave communication, RFID is the exact opposite. Communication distances are measured in centimeters (sometimes in meters). Antenna sizes inside the RFID tag are also measured in centimeters and hence are extremely short compared to the wavelength. Communication is therefore achieved more by inductive and capacitive coupling than by radio waves in free space. It is definitely near field communication. The chips have to run on whatever little power they can extract from the signal picked up by their antennas. They have to use that power to transmit the response as well.


In the beginning of radio, all messages were transmitted with morse telegraphy. This is comparatively slow and it requires skilled operators, both for transmitting and for reading. Teletypes are electric typewriters that transmit over a line each key typed on them and that can type onto paper each code received from the line. If two teletypes are connected to each other by a wire (e.g. a telephone line). everything typed on one machine will be printed on the other machine. Teletypes have existed since the early 20th century. Even though teletype machines were completely electromechanical (no electronics inside), the signalling is remarkably similar to RS232, albeit with only five bits per character and with much higher voltages. Even today, serial ports on computers can be configured to transmit with five bits per character and at the same speed (but not the same voltages) as teletypes.

Using a teletype is of course much easier to learn than morse telegraphy. If you can type on a typewriter (even with two fingers) you can do it. The use of RTTY (Radio Teletype) started in the early 1930s. The signalling (two voltage levels) had to be modulated onto radio waves and Frequency Shift Keying was used: the high voltage level got a slightly higher frequency than the low voltage level. At the receiver the signal was detected using a BFO, so the higher frequency has converted into a higher audio tone than the lower frequency. Those audio tones were then converted back to voltages. Radio amateurs almost always use these audio tones at the transmitter as well, They feed those audio tones to the input of an SSB transmitter. The modulated signal is the same in both cases.

In the early 1980s RTTY was still widely used by press agencies, but teletype machines could be purchased on the surplus market by amateurs. Amateurs were allowed to use it on the amateur bands. You were supposed to receive amateur transmissions only. You were not supposed to receive press agencies (and if you did accidentally, you had to keep it secret). Newspapers paid a lot of money for a subscription to those services. Most other RTTY transmissions (e.g. from the military) were encrypted anyway. So an old friend of mine accidentally received a press bulletin from an Egyptian press egency. The date was October 6th 1981, the day that President Sadat was assassinated. It took two hours before this news was on broadcast radio. Had he leaked the news during those two hours, he would have been in big trouble. It was a really strange feeling, knowing that some earth-shattering event had happend and not being able to tell anybody.

Today RTTY is still used by amateurs (and by some weather stations). A computer with a sound card generates and decodes the audio tones in software and an SSB transceiver does the rest. However, if you use a computer anyway, there are more efficient (in terms of bandwith and resistance to interference) ways to transmit text messages. One such method is PSK31, phase shift keying with a bit rate of 31 bits per second. Although RTTY uses a bit rate of 50 (or 45) bits per second, PSK31 uses the bits more effiiently, so both systems can transmit text at comparable speed, but PSK31 does it in a fraction of the transmitter bandwith. And it costs nothing extra. The software is free and you use the same hardware as for RTTY.

See you next time when we discuss the higher shortwave bands.

Update 2016-05-10

I added a band diagram.