70-centimeter band

From Wikipedia, the free encyclopedia
  (Redirected from 70 centimeters)
Jump to: navigation, search

The 70-centimeter amateur radio band is the 420 to 450 MHz 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.

History[edit]

Band allocation[edit]

The band's allocation varies regionally. In the United States,[1] Australia and Trinidad and Tobago[2] 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,[3]which runs just south of the Canada–US border from Washington state to Maine, and east of Line C, which runs from northeast to southeast Alaska.

Propagation characteristics[edit]

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.[4] 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[citation needed], 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[edit]

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).[1] 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.[citation needed]

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.

Use for the radio control hobbies[edit]

In some countries, particularly Germany and Switzerland, a portion of the 70 cm band overlaps with a secondary frequency allocation for the operation of Radio control models. In Germany, 33 frequencies are available for RC use,[5] and in Switzerland, ten frequencies are available.[6] These frequencies fall within the LPD433 band used by short range devices in Europe.

In North America, licensed amateurs may conduct RC operations in the 70 cm band, but unlike similar operations in the 6-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.[7] Canadian radio amateurs may use any amateur frequency above 30 MHz for the control of RC models.[8]

See also[edit]

References[edit]

  1. ^ a b "§ 97.301 Authorized frequency bands" (PDF). The Federal Communications Commission. 1 October 2008. p. 2. Retrieved 15 November 2009. 
  2. ^ "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.
  3. ^ "Archived copy". Archived from the original on 15 December 2009. Retrieved 13 September 2009. 
  4. ^ Taylor, Clayborne D.; et al. "13 - Measurement of RF Propagation into Concrete Structures over the Frequency Range 100 MHz to 3 GHz". Wireless Personal Communications : Advances in Coverage and Capacity (PDF). pp. 13–14. ISBN 978-1-4613-7861-7. Archived from the original (PDF) on December 27, 2014. measured attenuation above 200 MHz appears to increase with an increase in frequency 
  5. ^ "United Kingdom Radio Control Council: International Frequencies - Germany". UKRCC. UKRCC. Retrieved 13 July 2013. 
  6. ^ "United Kingdom Radio Control Council: International Frequencies - Switzerland". UKRCC. UKRCC. Retrieved 13 July 2013. 
  7. ^ "§97.215 Telecommand of model craft". Federal Communications Commission. Retrieved 26 May 2016. 
  8. ^ "Frequencies for Radio Control of Models". Industry Canada. Retrieved 26 May 2016. 

External links[edit]