The 70-centimeter amateur radio band is a portion of the UHF radio spectrum. This band is used by amateurs engaged in emergency communications where portable and mobile radios are frequently used. Many such radios have dual-band capabilities, operating on both the 70-centimeter and 2-meter bands.
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The band's allocation varies regionally. In the United States, Australia and Trinidad and Tobago the band ranges from 420 to 450 MHz with some geographical limitations; in Canada, the band is only 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, which runs just south of the Canadian border from Washington state to Maine, and east of Line C, which runs from northeast to southeast Alaska.
70-centimeter propagation characteristics lie midway between 2-meter and 33-centimeter (~900 MHz) bands. As frequency increases, building penetration improves. However, smaller obstacles may also block or reflect the signal. Higher frequencies also present a lower noise floor, making it easier to overcome both natural and artificial interference, especially prevalent in urban environments.
Atmospheric thermal ducting is often more intense at UHF, because shorter wavelengths have much greater refraction angles than longer ones. However, a much stronger thermal inversion is often required than is needed for ducting in the 2-meter band.
Comparison of the 2-meter and 70-centimeter bands
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. 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). This allows for many more channels, accommodating fast 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.
A problem found with all UHF and higher frequencies is the prevalence of multipath 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 make diplexers small and simple, making it easy to cross-band repeat signals between the two bands with a single dual-band radio.
- "§ 97.301 Authorized frequency bands" (PDF). The Federal Communications Commission. 1 October 2008. p. 2. Retrieved 15 November 2009.
- "Trinidad and Tobago Frequency Allocation Table (9 kHz to 1000 GHz)" (PDF). The Telecommunications Authority of Trinidad and Tobago. 16 October 2009. p. 154. Retrieved 15 November 2009.. See Footnote TT34.
- Spectrum Requirements for the Amateur and Amateur-satellite Services, Oct., 2009
- DX-Sherlock's real-time 70cm propagation maps
- DX-Sherlock's real-time VHF&up propagation ticker
|International amateur radio frequency allocations|
|Range||Band||ITU Region 1||ITU Region 2||ITU Region 3|
|LF||2200 m||135.7 kHz - 137.8 kHz|
|MF||600 m||472 kHz - 479 kHz|
|160 m||1.810 MHz - 1.850 MHz||1.800 MHz - 2.000 MHz||1.800 MHz - 2.000 MHz|
|HF||80 / 75 m||3.500 MHz - 3.800 MHz||3.500 MHz - 4.000 MHz||3.500 MHz - 3.900 MHz|
|60 m1||5.250 MHz - 5.450 MHz|
|40 m||7.000 MHz - 7.200 MHz||7.000 MHz - 7.300 MHz||7.000 MHz - 7.200 MHz|
|30 m2||10.100 MHz - 10.150 MHz|
|20 m||14.000 MHz - 14.350 MHz|
|17 m2||18.068 MHz - 18.168 MHz|
|15 m||21.000 MHz - 21.450 MHz|
|12 m2||24.890 MHz - 24.990 MHz|
|10 m||28.000 MHz - 29.700 MHz|
|VHF||6 m||50.000 MHz - 52.000 MHz1||50.000 MHz - 54.000 MHz||50.000 MHz - 54.000 MHz|
|4 m1||70.000 MHz - 70.500 MHz|
|2 m||144.000 MHz - 146.000 MHz||144.000 MHz - 148.000 MHz||144.000 MHz - 148.000 MHz|
|1.25 m||222.000 MHz - 225.000 MHz|
|UHF||70 cm||430.000 MHz - 440.000 MHz||420.000 MHz - 450.000 MHz3||420.000 MHz - 450.000 MHz3|
|33 cm||902.000 MHz - 928.000 MHz|
|23 cm||1.240 GHz - 1.300 GHz|
|13 cm||2.300 GHz - 2.450 GHz|
|SHF||9 cm||3.400 GHz - 3.475 GHz3||3.300 GHz - 3.500 GHz||3.300 GHz - 3.500 GHz|
|5 cm||5.650 GHz - 5.850 GHz||5.650 GHz - 5.925 GHz||5.650 GHz - 5.850 GHz|
|3 cm||10.000 GHz - 10.500 GHz|
|1.2 cm||24.000 GHz - 24.250 GHz|
|EHF||6 mm||47.000 GHz - 47.200 GHz|
|4 mm3||75.500 GHz1 - 81.500 GHz||76.000 GHz - 81.500 GHz||76.000 GHz - 81.500 GHz|
|2.5 mm||122.250 GHz - 123.000 GHz|
|2 mm||134.000 GHz - 141.000 GHz|
|1 mm||241.000 GHz - 250.000 GHz|
|THF||Sub-mm||Some administrations have authorized spectrum for amateur use in this region.|
1 This is not mentioned in the ITU's Table of Frequency Allocations, but individual administrations may make allocations under Article 4.4 of the ITU Radio Regulations. See the appropriate Wiki page for further information.
|See also: Radio spectrum · Electromagnetic spectrum|