Today we will talk about the lower shortwave bands. These bands are below the the shortwave range on most home radios. This part of the spectrum was sometimes covered by the marine band on radios that had it. This range contains the important 80m amateur band.
This is one of the busiest amateur bands and it has existed since the very beginning of radio amateur bands. This band is used during the day as a short range band. In The Netherlands you can hear mainly German stations during the day, but also Dutch and Belgian stations. Most communication between amateurs within The Netherlands (if not on VHF or higher) is in this band. This band (even during the day) offers a longer range than VHF and it is very suitable to communicate between distant places in The Netherlands.
During the day you can only hear nearby European stations on this band, during the night this band is a real DX band, especially in winter.
The first 100kHz of the band (3500-3600kHz) is mainly used for morse or other forms of telegraphy. The rest is mainly used for telephony, almost always in LSB.
In America this band runs till 4000kHz. It is possible to hear American stations between 3800 and 4000kHz in Europe during the night, but European broadcast stations in the 75m band may pose a problem.
Radio amateurs have to pass an exam (radio theory, electronics, antennas, regulations) and then they can obtain a license. Until comparatively recently a morse code test was also required before you could use any shortwave bands, but in most countries this requirement has been abolished. Amateurs have a call sign, e.g. PE1MYU, which has to be spelled out in the NATO spelling alphabet, like Papa, Echo, One, Mike, Yankee, Uniform. The first letters (and sometimes digits) indicate the country in which the station is registered. In The Netherlands, the prefixes are PA through PH plus PI for special-purpose stations. On qrz.com you can look up amateurs by their callsign.
In 1985 I could borrow an R107 communications receiver from a friend, the first radio with a true BFO. It was big and heavy, it dated from World War II and it covered the range from 1.2 to 18MHz in three bands. Above 10MHz it was very insensitive. but it could receive the 80m ham band very well.
The worldwide radio amateur organization is the IARU. The Netherlands has two associations of radio amateurs: VERON and VRZA. The VERON is the oldest one and they used to have the call sign PA0AA. In the early 1980s this had to be renamed PI4AA. It used to transmit a news bulletin each week on 80m. This has since been reduced to one bulletin every four weeks on Thursday evenings on 80m. In the 1980s they transmitted from a paint factory in Sassenheim, The Netherlands, today they transmit from Eindhoven. The other organization transmits every Saturday morning on 80m using the call sign PI4VRZ/A from Kootwijk.
The 80m band is the first band on which reasonably efficient antennas can realistically be constructed by radio amateurs. A quarter wave vertical is 20m tall and a half wave dipole (horizontally polarized) is 40m wide, which is still way too big for the average suburb garden in The Netherlands. You can use a shorter antenna and put a coil in series to make it resonate on the desired wavelength again, but this is less efficient. If you must, you can use almost any length of wire as an antenna, but this is not always efficient. For a transmitter it is also important to match the antenna impedance to that of the antenna. If the mismatch is severe, it is even possible to damage the transmitter. Antenna tuners are available to match almost any length of wire to the 50 Ohm output of a transmitter.
A second amateur band is about to emerge in this part of the spectrum, now called the 60m amateur band. Several countries, like the USA and the UK allocate a few fixed channels in the range 5260-5410kHz to radio amateurs. They have to use USB. Some Scandinavian countries have an allocation of a complete band.
There are two broadcast bands in this part of the spectrum. The 75m band is only allocated in Europe and it is the lowest non-tropical broadcast band. It is the lowest band that is intended for international broadcasting. Not many stations are active on it. because it has a limited range during the day and not all shortwave broadcast receivers can tune to it. One station in the band is the DRM transmission from Deutsche Welle.
The other broadcast band is a tropical band. At night it is comparatively easy to hear stations on this band in The Netherlands. Note that the exact frequency of 5MHz is reserved for time stations.
|75m||3900-4000kHz||Shortwave broadcast band|
|60m||4750-5060kHz||Tropical broadcast band|
Of course this part of the spectrum is also used by utility stations. An important group of users are aircraft. When they are flying over densely populated areas. they cam make use of the VHF air band. When they are flying over the ocean, they are out of reach of VHF. Then they use shortwave frequencies to communicate. They can keep in touch with ground stations all the time, they can report their position and they can report technical problems. They use SSB (USB) telephony.
There are special stations to broadcast weather reports to aircraft, the so-called VOLMET stations. These report the weather conditions on all airports in a fixed format. They report place, time, wind direction (in degrees) and speed (in knots), clouds, temperature and dew point and air pressure (called QNH). In The Netherlands it is easy to hear Shannon Volmet on 5505kHz and Royal Airforce Volmet on 5450kHz. These stations are transmitting all the time, as opposed to other air traffic communications.
The reason why shortwave is suitable for long-distance communication is the fact that these waves can be reflected by layers in ionized gas in the upper atmosphere. At these altitudes the air pressure is very much lower than at sea level, completely insufficient to breathe. Here on earth we would call it vacuum. Solar radiation (mainly X-rays and ultraviolet) ionize gas molecules, which means that electrons are split off these gas molecules, so there are positively charged gas molecules (ions) and negatively charged electrons. Such layers of charged particles can sometimes reflect radio waves. There are several layers of ionized gas in the upper atmosphere (we call them collectively the ionosphere).
|D layer||60-90km||Day time||Absorbs lower frequencies|
|E layer||90-120km||Day time||Sporadic E can reflect VHF|
|F1 layer||200km||Day time||Lower part of F layer|
|F2layer||300km||Day time||Upper part of F layer|
|F layer||250km||Night time||Single layer at night|
By far the most important layer for shortwave propagation is the F layer (F2 during the day). This is the upper layer of the ionosphere. When you send a radio wave straight up, it is reflected back to earth if it is below the critical frequency and passes through the ionosphere into space if it is above the critical frequency. The critical frequency varies between 5 and 10MHz. Signals in the 80m band that are transmitted straight up are usually reflected back to earth, so they can be spread across a large area (hundreds of kilometers wide). This is a short distance as far as shortwave goes.
If you send the radio waves to the atmosphere at an oblique angle, they are reflected back at frequencies above the critical frequency. When they are reflected, they reach earth at a much greater distance (say 2000km or more). If you use even higher frequencies, the signal will pass through the ionosphere again, so there will be a maximum usable frequency (MUF) to reach a certain destination. During the day this MUF will be much higher (20-30MHz) than during the night (10-12MHz).
The D layer absorbs low frequencies (especially mediumwave) and keeps radio waves from reaching the F layer. In the lower shortwave bands, radio waves beamed straight up will still reach the F layer (making it possible to reach nearby European destinations on 80m), but waves beamed at a shallow angle get completely absorbed. Frequencies above 10MHz are not much absorbed by the D layer, making them suitable for daytime communication, while frequencies below 10MHz are suitable for night time communication.
There are three categories of shortwave receivers.
Shortwave receivers can also be categorized along the frequency ranges they cover:
Before the late 1970s, general coverage receivers were either way to expensive for amateurs to buy (real communications receivers) or they were cost compromised radios that performed rather poorly. You had to live with poor image rejection, poor selectivity, poor frequency stability or inaccurate dial readout or all of the above. Dedicated amateur band receivers offered much better performance at a slightly higher price. but they could not receive the broadcast bands, at least not all of them. In the later 1970s, radios like the Yaesu FRG-7 offered acceptable performance, while still covering the entire shortwave spectrum. From 1980 all serious amateur receivers are general coverage receivers. Nowadays it is possible to buy a general coverage broadcast receiver with SSB capability (so you can also listen to amateurs) for less than 100 euros.
See you next time when we discuss three of the busiest shortwave broadcast bands.
I added several diagrams. The Netherlands has a 60m amateur band from 5350 to 5450kHz, but the internationally agreed allocation is just 15kHz wide, from 5351.5 to 5366.5kHz. It is likely that The Netherlands will eventually adopt the internationally agreed allocation.