70 centimeters
Encyclopedia
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.
, 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.
, 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.
" 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 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 diplexer
s small and simple, making it easy to cross-band repeat
signals between the two bands with a single dual-band radio.
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
band is a portion of the UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
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.
Band allocation
The band allocation vary regionally. In the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
and Trinidad and Tobago
Trinidad and Tobago
Trinidad and Tobago officially the Republic of Trinidad and Tobago is an archipelagic state in the southern Caribbean, lying just off the coast of northeastern Venezuela and south of Grenada in the Lesser Antilles...
the band ranges from 420 to 450 MHz with some geographical limitations; in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, the band is only 430–450 MHz; in the UK
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, 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.
Propagation characteristics
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 floorNoise floor
In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where the noise is defined as any signal other than the one being monitored....
, 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 duckRubber Ducky antenna
The Rubber Ducky antenna is an electrically short monopole antenna which functions somewhat like a base-loaded whip antenna and is sealed in a rubber or plastic jacket to protect the antenna...
" 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
Amateur television
Amateur television is the transmission of Broadcast quality video and audio over the wide range of frequencies of allocated for Radio amateur use. ATV is used for non-commercial experimentation, pleasure and public service events...
, wideband digital modes, and point-to-point linking, which may not 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 diplexer
Diplexer
A diplexer is a passive device that implements frequency domain multiplexing. Two ports are multiplexed onto a third port . The signals on ports L and H occupy disjoint frequency bands...
s small and simple, making it easy to cross-band repeat
Radio repeater
A radio repeater is a combination of a radio receiver and a radio transmitter that receives a weak or low-level signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. This article refers to professional, commercial, and...
signals between the two bands with a single dual-band radio.