Amateur radio high bands
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In the hobby of Amateur radio, participants use radios to communicate. The radio frequencies that Amateur radio users can use are grouped into sections of frequencies in the radio spectrum known as Amateur radio frequency allocations or simply, bands. The Amateur radio high bands are those groups of frequencies above 30 MHz allocated by international agreement for use by amateur radio operators. They are categorized as either VHF, UHF or microwave. The bands are generally named according to the approximate wavelength of the frequencies in the band.
While many radio amateurs use VHF or UHF frequencies primarily for local communications, other amateurs build sophisticated systems to communicate over as wide a range as possible.
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[edit] The bands
The Amateur radio high bands are:
Very High Frequency (VHF) (30 to 300 MHz)
- 6 meters: 50 to 54 MHz
- 4 meters: 70-70.5 MHz (Some ITU Region 1)
- 2 meters: 144 to 148 MHz
- 1.25 meters: 220 to 225 MHz, with a limited allocation at 219 to 220 MHz (ITU Region 2 Only). Some countries use 222-225 only.
Ultra High Frequency (UHF) (300 MHz to 3 GHz)
- 70 cm: 420-450 MHz
- 33 cm: 902-928 MHz (ITU Region 2)
- 23 cm: 1240-1300 MHz
- 13 cm: 2300-2310 and 2390-2450 GHz)
Super High Frequency (SHF) (3 to 30 GHz)
- 9 cm 3.3 - 3.5 GHz
- 5 cm 5.65 - 5.925 GHz
- 3 cm 10.0 - 10.5 GHz
- 1.2 centimetres (24.00 - 24.25 GHz)
Extremely High Frequency (EHF) (30 to 300 GHz)
- 6 mm 47.0 - 47.2 GHz
- 4 mm 75.5 - 81.0 GHz
- 2.5 mm 119.98 - 120.02 GHz
- 2 mm 142 - 149 GHz
- 1 mm 241 - 250 GHz
- An Amateur allocation from 76-77 GHz has been suspended till the FCC can determine that interference will not be caused to vehicle radar systems
Bands above 30 MHz are known as VHF; above 300 MHz as UHF; and all above 2 GHz as microwave.
[edit] Characteristics of Radio Wave Propagation in the High Bands
Despite the common misconception of 'line of sight' a VHF signal transmitted from a walkie-talkie (or as hams call it a Handi-talkie or HT for short) will typically travel about 5-10 km depending on terrain, and with a low power home station and a simple antenna to around 50 km. With a large antenna system like a long yagi, and higher power (typically 100 or more watts) contacts of around 1000 km are common. Such operators seek to exploit the limits of the frequencies' usual characteristics looking to learn and experiment with radio technology. They also seek to take advantage of "band openings" where due to various natural occurrences, radio emissions can travel well over their normal characteristics. There are numerous causes for these band openings and many hams listen for hours to take advantage of their rare manifestations, which may be of fleeting duration.
Some openings are caused by intense excitement of the upper atmosphere, known as the ionosphere. Other band openings are caused by a weather phenomenon known as an inversion layer, where cold air traps hot air beneath it, which forces the radio emission to travel over long weather layers. Radio signals can travel hundreds or even thousands of kilometres due to these weather layers.
[edit] Repeaters
To help extend the range of stations, signals can be received by a station at a high location and retransmitted automatically. Such a device is simply called a Repeater. Repeaters are usually located on the top of a mountain, a tall building, or they may also be installed on satellites to enable communication over a very large area.
Some repeaters may operate in a crossband configuration, for example, listening on the 70 cm band and retransmitting on the 2 m.
Similarly, some repeaters are linked, effectively forming a large repeater that listens on several frequencies and retransmits on several frequencies.
Linked repeaters may create links between bands, also. If a 2m repeater were linked to a 70cm repeater, it would grant some limited access to users of 2m radios to the 70cm band, and vice-versa.
In addition to creating expanded band or frequency coverage, linked repeaters can also create expanded geographic coverage, by linking two or more repeaters into a network with great physical separation between them. Such networks might make for an easy contact, for instance, between Albany, New York and Buffalo, New York (and many points in between) despite the fact that they are separated by a great distance.
Repeaters are also sometimes permitted in the 10 m HF band. These repeaters can often hear signals from great distances, and be heard from great distances. It is not uncommon, for instance, to hear traffic from New England or the Pacific Northwest on the W5DFW repeater, locted in the Dallas/Fort Worth area in Texas.
A 10m repeater, linked to a repeater on another band, say, for instance, 2m, can be used by Technican class operators to gain limited access to HF.
[edit] Amateur Television
Amateur television (ATV) is the hobby of transporting broadcast-quality video and audio over radio waves allocated for amateur radio. It also includes the study of building of such transmitters and receivers and the propagation between these two.
In NTSC countries, ATV operation requires the ability to use a 6 MHz wide channel. All bands at VHF or lower are less than 6 MHz wide in their entireties, hence ATV operation is confined to UHF and up. Bandwidth requirements will vary from this for PAL and SECAM transmissions.
ATV operation in the 70 cm band is particularly popular, because the signals can be received on any cable-ready television. Operation in the 33 cm and 23 cm bands is easily augmented by the availability of various varieties of consumer-grade wireless video devices that exist and operate in unlicenced frequencies coincident to these bands.
ATV operation may be enhanced by using specially-equipped repeaters.
[edit] EME (Moonbounce)
Many Amateur radio operators communicate over long distances by bouncing their signals off the moon. This EME (Earth-Moon-Earth) operation, as it is known allows communications between any two places on Earth which can see the Moon at the same time.
