Column 13: Higher VHF bands (108-174MHz, 2010-12-08)

Today we talk about the portion of the VHF spectrum between the FM broadcast band and the high VHF TV band. It contains the air band and the 2 meter amateur band, as well as various landmobile bands. Compared to the low VHF band, band plans for this part of the spectrum are less variable across the world.

Air band

The band between 108 and 137MHz is the air traffic control band. The part between 108 and 118MHz is mainly used for beacons and some airport services, while the part between 118 and 137MHz is used for voice communication with aircraft. This band was allocated shortly after World War II and is used truly worldwide.

Each larger airport has a control tower that uses one or more specific frequencies, so pilots know on which frequency they must communicate when they approach a certain airport. Large airports may use separate frequencies for each runway or separate frequencies for departure and approach. The distress frequency is 121.500MHz.

Many airports have ATIS (Automatic Terminal Information Service). This is a voice transmission that continuously transmits weather information and other relevant information of the airport. The format of the weather information is not unlike VOLMET. Some frequencies are allocated to hot air balloons, gliders and other specialized users.

AM modulation is used on the air band, unlike most other voice communication in the VHF band. One reason is that this band was established quite long ago and on a worldwide scale. Changing the modulation mode would require the participation of all countries. Another reason for AM is that weak stations that transmit on the same channel as a strong station have a greater chance of detection than with FM. So at least the traffic controller can hear that there might be another aircraft.

When the air band was allocated, the channel spacing was 25kHz. This is very wide for voice grade AM (6kHz would do). At that time however, frequencies of transmitters and receivers could not be set very accurately. By setting the filter bandwidth in the receiver wide enough, the station could be heard even if it was 10kHz off channel. With the increase of air traffic, the air band has become very crowded. A move to 8.33kHz channel spacing (25/3) is underway. Those intermediate channels are not widely used at the moment, but large aircraft in Europe are already required to have radio equipment capable of using the extra channels. Modern VHF receivers and scanners have the 8.33kHz step size, but older equipment does not have it. Some scanners can be set to a 5kHz step size, which allows them to be tuned close enough to these extra channels.

There are many aviation enthusiasts who regularly listen to the VHF air band, even in countries where it is not officially allowed. In The Netherlands it is one of the few things you can still hear on a scanner. Transmissions from aircraft flying at a high altitude (10km) can reach hundreds of kilometers, just by line of sight communication. Therefore, even if you live far away from an airport, you should be able to hear the planes approaching a distant airport. Hearing the tower requires you to be closer to an airport (within 30km, or further away if you have a good antenna). Hearing the planes on the ground requires you to be closer still.

Space communication

The 137MHz band is used by the NOAA polar weather satellites. They transmit pictures using an analog system. The signal is FM modulated and to receive it properly, one needs a bandwidth of around 30kHz. Too narrow and the picture gets distorted, to wide and sensitivity is insufficient.

The Russian space station MIR used the frequency 143.625MHz in the late 1980s and early 1990s. It could be received with handheld scanners or amateur transceivers. All communication was in Russian, so I could not understand it.

Australia used to have a TV channel between 137 and 144MHz. In France, TV band III used to start at 162MHz and apparently in Morocco it still does.

Radio amateurs

In the 1930s the 2.5m amateur band was allocated between 112 and 116MHz. Amateur bands were allocated starting at 160m and for every half wavelength another band was allocated: 80m, 40m, 20m, 10m, 5m, 2.5m, 1.25m and 62.5cm (now 70cm). After World War II, the band was relocated to 144-148MHz (ITU Region 2) and 144-146MHz (other regions). From then on, it became known as the 2 meter band.

The lower part of this band is mainly used for CW and SSB long-distance communication. Using high-gain antennas (horizontally polarized Yagi-Uda antennas), it is possible to achieve a range of 300km or more, even with weak propagation conditions.

In this band sporadic E propagation is still very well possible and tropospheric propagation is fairly common. From The Netherlands, it is possible to reach every part of Europe eventually. In most countries, shortwave is now accessible without a morse code test and many amateurs who were only licensed to use 2m and up, are now allowed to use shortwave bands as well. Therefore, 2m is much less popular than it was 20 years ago.

Some parts of this band are primarily used for FM communication. The frequencies 145.200-145.5875MHz (12.5kHz spacing) are used for simplex FM communication in Europe. The band 145.0-145.2MHz is used for repeater inputs and the band 145.6-145.8MHz is used for repeater outputs. Repeaters are relay stations, primarily intended to give handheld and mobile transceivers an extended range. The repeater receives everything on one channel (say 145.100MHz) and retransmits it 600kHz higher (145.700kHz). The handheld transceiver must then be programmed to transmit 600kHz lower (say 145.100kHz) than it receives (145.700kHz). Using a repeater, two stations can communicate with each other if they can both reach the repeater, even if they cannot reach each other directly. The 2m band is the busiest amateur band for local communication, but it is much less busy than in the 1990s.

The band between 145,8 and 146.0 is reserved for satellite communication. Signals will nearly always pass through the ionospheric layers, hence they can reach a satellite. As opposed to shortwave, this will always work in VHF bands. Yagi antennas are used for the more serious satellite communication. Not only must they be rotated in the horizontal plane, but their elevation (the angle with respect to the horizon) has to be adjustable as well. Amateur satellites are not geostationary, so they move with respect to stationary station on Earth. They can be reached for relatively short periods and the antenna has be be moved continuously to maintain contact with the satellite.

Some amateurs specialize in having their signal reflected by the moon. Moon-bouncing (as this is called) used to require huge power (1kW or more) and high-gain antennas, but nowadays (thanks to increased receiver sensitivity) it is possible using moderate power (50-100W). Meteor scatter is also very well possible in this band. F-layer propagation on the other hand, does not occur at these higher frequencies.

In the 1990s radio amateur used to experiment with packet radio. They used radio links to transmit packets over a network. On 2m a data rate of 1200bps was mainly used. Even though packet radio often used the Internet Protocol, it was not allowed to carry Internet traffic over these links. Packet radio still exists in UHF bands.

The D-Star system is a more modern system for digital communication using amateur radio. It allows digital voice communication, as well as data communication. Repeaters and linking between repeaters are possible. However, the speech codec used in this system is proprietary. Not only is it protected by patents, but by trade secrets as well. You can buy a chip that performs speech coding and decoding, but you may not know how the coding algorithm works. This runs contrary to the spirit of amateur radio.

Marine VHF

The marine VHF band is used for communication with and among ships in the coastal waters and in inland waters (rivers and lakes). is allocated worldwide and FM modulation is used. It consists of two frequency ranges. For duplex channels, the lower range is used by the ship and the higher range is used by the shore station. Duplex channels have an offset of 4.6MHz.
Channels Frequencies. Use
1-28,60-88 156.025-157.425MHz Ship stations, simplex channels
1-28,60-88 160.625-162.025MHz Shore stations for duplex.
Originally the channels had a spacing of 50kHz. Channel 1 is 156.050MHz (160.650MHz), channel 2 is 156.100MHz (160.700MHz). This way channels 1-28 are allocated. Later the spacing was reduced to 25kHz and the extra channels were numbered 60 (156.025MHz and 160.625MHz) through 88 (167.425MHz and 162.025). The distress channel is channel 16 (156.800MHz).

In the Dutch and many European inland waters, channel 10 (156.500MHz) is used as a calling channel and also for urgent warnings. Ships are required to monitor this channel when they are not using the transceiver for other purposes. Transceivers used on ships in the Dutch and many European inland waters are required to have ATIS (Automatic Transmitter Identification System), which transmits a digital code, which identifies the ship, with each transmission. This is the same acronym (ATIS) as the system we have just seen in the air band, but it is a different system.

If you do not live close to a busy river or the coast, you will hear very little on these channels. Coastal stations direct their signals toward the sea, so reception inland is very difficult.

Landmobile communication

The frequency band between 146 and 174MHz is one of the busiest bands for landmobile communication. It is used for this purpose in nearly all countries in the world. In Germany it was sometimes called the 2m band (not to be confused with the 2m amateur band). On many cheap toy radios this part of the spectrum was called Police Band. In many countries, this was the primary band for police communications. In The Netherlands however the main police frequencies were in the low VHF and the UHF bands. Some police services were allocated in this band though. This was the main band for fire department and ambulance frequencies (167-169MHz), but nowadays all police, fire and ambulance communication in The Netherlands is digital and encrypted, so it is off-limits for scanner listeners. Some fire departments occasionally use analog walkie-talkies, as the digital ones do not work reliably inside buildings. You can still hear many taxi companies in this band.

In the 1980s, The Netherlands (along with Germany) operated the first mobile telephone network. By that time mobile telephones were mounted in cars, as they were bulky and required too much power to be portable. The network operated on frequencies between 148 and 149MHz (car) and between 153 and 154MHz (base station). This system already featured automatic dialing, but no seamless switchover between cells. Around 1990 this system was still in use, mainly by boat owners. In 1995 the system was closed down.

This car telephone system is an example of a trunking network. The system automatically selects a free channel, so it is not predictable in advance on which channel a particular call will be made. This makes it harder for scanner listeners to pick out the desired communication, but it can still be heard once the correct channel is found. In some countries, police communications moved to trunking networks as well.

From 2006 those bands were used for church radio ("Kerktelefoon" in Dutch) in The Netherlands. People who are not able to go to church, can still hear the mass via the radio. In the past, telephone lines were used for this purpose, but when the telephone company was privatized (and subscribers had to pay the real cost), such lines were no longer cost effective. The intention was that subscribers would buy or rent a dedicated receiver that could only pick up the signal from their own church, but cheap secondhand scanners work just as well. At that time, analog police communication was abolished, so a lot of used scanners became available.

In The Netherlands, a paging system is used by fire departments and ambulance services, called P2000. It was introduced together with C2000 (the digital and encrypted voice communication network) and it operates on 169.500MHz. It is not encrypted and suitable pagers are available to the general public. It is still possible for hobbyists to receive these paging messages.

In the USA, the NOAA operates special weather channels, intended to be received by the general public. These are 7 channels, from 162.400 to 162.550MHz.

See you next time when we discuss VHF TV band III and the military air band.