What is VHF?
Above 30 MHz
Above 30 MHz, the waves propagate in a rather different way than described so far. Only in the E-layer, for the 50, 70 and 144 MHz bands, can some reflections occur. For the bands above 30 MHz there is another phenomenon called tropospheric propagation. Above 1 GHz, and therefore also for microwaves, the waves propagate in straight lines with few reflections from the atmosphere; interesting reflections can however be obtained from rain clouds, meteor trails or aircraft.
Phone and telegraphy
The single sideband (USB), as well as telegraphy (CW), are used for long-distance contacts. In addition to line-of-sight propagation, the VHF-UHF-SHF bands allow for experimentation with specific propagation modes.
Frequency modulation (FM)
Frequency modulation (Narrow Band Frequency Modulation) is used to achieve radio quality. The range is limited to a short distance (5 to 50 km). NBFM is mainly used for local communications and for communications via portable and mobile stations, as well as for traffic via relay stations.
Repeater stations, also known as relays, are installed on top of buildings, towers or hills. They are used to relay the signals of portable and mobile stations, thus increasing their range. The difference between the user's receive frequency and the user's transmit frequency is -600 kHz for the 144-146 MHz band and +1.6 MHz or -7.6 MHz for the 430-440 MHz band. Relays are activated by sending a CTCSS tone over the voice. Some relays must be activated by a tone at 1750 Hz for 3 seconds. The repeater is identified by giving its callsign in Morse code. In Belgium there are 17 VHF and 20 UHF relays. Almost all of them belong to sections of the UBA.
Computers are becoming more and more important, also for radio amateurs. Radio amateurs are therefore also experimenting with digital transmissions for data, images and digital voice.
In the meantime, other digital modulation techniques have been developed to transmit digital voice (DV), images or data: DSTAR, C4FM, DMR are examples. Each system offers not only the possibility to establish direct contacts or via a repeater (relay), but also to interconnect stations and relays via an RF backbone or the internet, thus making it possible to reach a correspondent at the other end of the world.
Packet-Radio is a digital transmission system based on the AX.25 protocol (similar to X.25). It allows transmissions with error detection and recovery. Keyboard-to-keyboard contacts and computer-to-host transfers can be made. Servers (Bulletin Board System) can be used, which are connected to other BBSs and route messages around the world. DX-Clusters are used by radio amateurs interested in DX (see HF bands) to signal the presence of rare stations, etc. The Packet Radio network consists of nodes that transmit information between remote users, between BBSs or between DX-Clusters. Packet Radio combines two technologies: radio and computers.
The APRS system uses the AX.25 protocol to transmit its position and some information to the APRS network where you can follow the radio amateurs on a map. The AX.25 protocol is also used by many amateur radio satellites to transmit telemetry (satellite status).
High-speed Amateur Multimedia Network uses similar technologies as WiFi, but in the amateur bands. This makes it possible to transmit digital data, images and voice at very high speeds and to create MGM (Machine Generated Mode) mesh networks (MESH).
With the advent of Digital Signal Processing (DSP) techniques and the fact that computers are becoming more and more powerful, it is very easy to create exotic modulation modes, optimised for transmitting information with a minimum of energy over long distances and in the presence of noise.
Examples include PSK31, JT65, FT441, FT8, FT9, WISPR.
Amateur television (ATV)
ATV or "Amateur Television" also attracts many followers. Colour television images are exchanged using the 625-line system. The bands used are 70 cm (amplitude modulation) and 23 cm, 13 cm and 3 cm (frequency modulation). The DATV (Digital ATV) system uses modulations similar to the DVB-S system to send images. There are even repeaters specially designed to relay ATV and DATV signals.
Types of VHF and UHF propagation
The propagation modes are different from those used in HF. In VHF-UHF a distinction is made between :
- Straight line propagation, i.e. visual, limited to the horizon line of the antennas. This is the propagation mode used, locally, in FM mode, for direct or relay links.
- Tropo: tropospheric propagation, which occurs under high barometric pressure. In such circumstances, an inversion layer (of temperature and refractive index) can form, at an altitude varying from 100 to 2500 metres, a "conduit" allowing links of 300 to 1200 km.
- Sporadique E: Sporadic E propagation, which occurs when this ionised layer of the atmosphere appears during the summer months. Reflections from this layer allow contacts up to 2000 km, but these propagation conditions are very short (< 1 minute). The phenomenon is only usable for the 50 MHz and 144 MHz bands.
- FAI: There are a number of strange phenomena, such as "Field Aligned Irregularities". Radio amateurs try to find and exploit these anomalies to make contacts over long distances.
- Meteor scatter: When meteorites enter the atmosphere, they produce heat and ionisation. This ionisation produces propagation phenomena. The openings are very short and vary from 1 second ("ping") to about 20 seconds ("burst"). There are 14 meteor showers suitable for these experiments, the most important being the Perseids (around 12 August), Geminids (around 14 December) and Quadrantids (around 3 January).
- Rain scatter: When radio waves hit a rain cloud, part of the signal is reflected. This phenomenon makes it possible to make connections beyond the field of view, especially in the millimetre bands.
- EME: Another remarkable activity is the reflection on the moon (Earth-Moon-Earth). The radio waves are very strongly attenuated (about 262dB over 70 cm). This is why high power ratings of 250 to 1000 Watts (possibly a special licence must be applied for from BIPT!), high-gain antennas or antenna arrays (26dB or more) and a very low-noise preamplifier should be used.
Amateur radio satellites
Radio amateurs build their own satellites and have them put into orbit. These satellites are equipped with transponders that retransmit the received signals (uplink) on another frequency (downlink). The frequencies used are in the 10 m, 2 m, 70 cm, 23 cm, 13 cm and 3 cm bands. Some satellites are in low (polar) orbits. At each pass, the station's antennas must be constantly reoriented towards the satellite (tracking) using antenna motors in azimuth and elevation. Other satellites are in very elliptical orbits and allow a long operating time.
Contacts with the International Space Station (ISS) are frequent. Our famous astronauts Dirk Frimout (ON1AFD) and Frank Dewinne (ON1DWN) are also radio amateurs. They have been contacted several times, during their revolutions around the earth, by Belgian and foreign radio amateurs (notably in the framework of ARISS).
Microwaves offer a vast field of experimentation where self-building dominates to a very large extent, since very little commercial equipment is available. This difficulty results in the small number of radio amateurs who devote themselves to this aspect of the hobby. However, µ-waves are an exciting field where one can experiment with striplines, waveguides, varactors, Gunn diodes, dielectric resonators, parabolic antennas, loops, etc...
Beacons are used to monitor and study the particular propagation phenomena described above.
What equipment do you need? The answer to this question depends on what you want to do. Many beginner radio amateurs (ON3) start with a simple handheld device (walkie-talkie) with which they can contact local relays.
For a new laptop you will need to spend around €250. If you buy a second-hand one, half the price should be sufficient. If your job involves a lot of driving, you may want to consider a 2m or dual-band "mobile station". Then you'll need some courage to drill through the roof of your car (for the antenna!). To help you do this, remember that you are wearing out the seats, the engine, the tyres, etc... So why not make a small hole for your exciting hobby? A mobile transmitter costs €400 or more when new. A used one in good condition will cost you half that.
If, on the other hand, you want to do DX, tropo, E-sporadics, meteorscatter, work with satellites, do EME, you will need a 2-metre transceiver and a 70 cm "all mode" transceiver, allowing you to do FM, SSB and CW, or else a "dual band" and "all mode" transceiver (new price from 1000 €). The antennas will be yagis with the highest possible gain, mounted on masts to be well clear of surrounding obstacles.
In the upper bands, there is little ready-made commercial equipment. Here DIY is almost compulsory, although commercial modules and realignment surpluses can be found in the amateur bands.
There are also SDR (Software Defined Radio) solutions. Instead of building new equipment for each modulation type, a simple generic SDR module and software on a PC or tablet takes care of everything. To change the modulation type, the computer is simply reprogrammed with new software. For reception, a "DVB-T" type USB stick typically covers the 60 MHz - 2.1 GHz range and costs only 20€. For transmission there are also SDRs.