The70-centimeter or440 MHz band is a portion of theUHF radio spectrum internationally allocated toamateur radio andamateur satellite use. TheITU amateur radio allocation is from 430 to 440 MHz; however, some countries, such as the United States, allocate hams 420 to 450 MHz. Depending on the country the band is shared with other radio services (in United States with governmentradar systems such asPAVE PAWS).[1]
70 centimeters is a popular ham band due to the ready availability of equipment in both new and used markets. Most amateurs operating on 70 cm use either equipment purpose-built for ham radio, or commercial equipment designed for nearbyland mobile frequencies. Amateurs usually use the band forFM or digital voice communications throughrepeaters (useful for emergency communications), as well narrow band modes (analog and digital) for long-distance communications (called "DX", includingMoon bounce). The band is also popular for Amateur Satellite Service. Due to itssize, it's the lowest frequency ham band which can supportamateur television transmissions.
The band's allocation varies regionally. In the United States[2] andTrinidad and Tobago,[3] the band ranges from 420 to 450MHz with some geographical limitations. InCanada andAustralia, the band is 430–450 MHz. In the UK, amateurs are allocated 430–440 MHz. By international treaty between the US and Canada, operation in the portion of the band from 420 to 430 MHz is prohibited north of Line A,[4] which runs just south of the Canada–US border fromWashington state toMaine,[5] and east of Line C, which runs from northeast to southeastAlaska.
70-centimeter propagation characteristics lie midway between 2-meter and 33-centimeter (~900 MHz) bands. Above 200 MHz, as frequency increases, building penetration is reduced.[6] Smaller obstacles may also block or reflect the signal. However, higher frequencies also present a lowernoise floor, making it easier to overcome both natural and artificial interference, especially prevalent in urban environments.
Propagation considerations often take a back seat to channel availability or economic concerns in system planning. One practical concern when comparing the 70-centimeter band to the 2-meter band is that a quarter-wavelength antenna is much less unwieldy at 70 centimeters than it is at 2 meters. Portable antennas for 2 meters are generally continuously loaded coil spring or "rubber duck" types, while on 70 centimeters they can be a full quarter wavelength. The difference can be as much as 8 dB[citation needed]. The primary advantage of 70 centimeters is that base station antennas of very significant gain (up to 11 dB or so) are practical while 6 dB is about the practical limit on 2m. The extra 5 dB of receive and transmit gain are often critical for long-range communication, particularly for high-power repeaters which can then concentrate all of their power and receive sensitivity at the horizon.
The 70-centimeter amateur band also provides a wider spectrum than the 2-meter band (in the U.S., this is 30 MHz of spectrum, compared to only 4 MHz on the 2-meter band).[2] This allows for many more channels, accommodatingfast scan television, wideband digital modes, and point-to-point linking, which may not be permitted on 2-meter and lower frequencies, depending on local regulations.[citation needed]
A problem found with all UHF and higher frequencies is the prevalence ofmultipath signals. The reflective properties of the 70-centimeter band allow signals to be reflected by dense and solid material such as cement or rock. This creates a slight time delay between the primary and reflected signals, causing cancellations as direct and reflected signals are combined in the receiving antenna. This can cause receiving stations to experience rapid fluctuations in signal strength, or "picket fencing", when they are in motion. The problem is much less severe with modern FM systems because the receiver's limiter circuitry compensates for variations in received signal strength over a very wide amplitude range. In properly engineered systems, multipath can also be reduced by assuring that the transmitter uses only the minimum necessary power, allowing the reflected signals to be lower than the receiver's detection threshold.
70 centimeters is very close to the third harmonic of 2 meters, which allows sufficiently broadband 2–meter antennas to be used for 70 centimeters. Antennas specifically designed to work on both bands are common. Also, 2 meters is far enough away from 70 centimeters to makediplexers small and simple, making it easy to cross-bandrepeat signals between the two bands with a single dual-band radio.
In some countries, particularlyGermany (until the end of 2008)[7] andSwitzerland, a portion of the 70 cm band overlaps with a secondary frequency allocation for the operation ofRadio control models. In Germany, 33 frequencies were available for RC use,[8] and inSwitzerland, ten frequencies are available.[9] These frequencies fall within theLPD433 band used byshort range devices in Europe.
In North America, licensed amateurs may conduct RC operations in the 70 cm band, but unlike similar operations in the6-meter band, no specific frequencies have been set aside for RC use. American radio amateurs may use a maximum of one watt of radiated RF power, on any ham frequency authorized for data emissions, to control RC models.[10] Canadian radio amateurs may use any amateur frequency above 30 MHz for the control of RC models.[11]
Plus or minus 5 MHz is a common repeater frequency offset in the 70 cm band.
The band 420-450 MHz is used extensively by the military agencies for land-based, shipborne, and airborne radar systems to perform important national security functions.
FCC Part 97.3a, subsection (30):Line A. Begins atAberdeen, WA, running by great circle arc to the intersection of 48° N, 120° W, thence along parallel 48° N, to the intersection of 95° W, thence by great circle arc through the southernmost point ofDuluth, MN, thence by great circle arc to 45° N, 85° W, thence southward along meridian 85° W, to its intersection with parallel 41° N, thence along parallel 41° N, to its intersection with meridian 82° W, thence by great circle arc through the southernmost point ofBangor, ME, thence by great circle arc through the southernmost point ofSearsport, ME, at which point it terminates.
measured attenuation above 200 MHz appears to increase with an increase in frequency