70-centimeter band

70-centimeter band

70 centimeters is a common amateur radio (ham) band in the UHF spectrum. The exact amateur frequency allocations varies regionally. In the US and Australia 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.

This band is popular with amateurs engaged in emergency communications where portable and mobile radios are frequently used. Often, hams have "dual-band" radios, capable of operation on both 70 centimeters and 2 meters.

Propagation Characteristics

70cm propagation characteristics lie midway between 2m and 33cm (900 MHz) bands. As the frequency increases, penetration through windows and other openings in buildings becomes easier because the openings are larger relative to a wavelength. It also means that smaller obstacles block the signal, or can reflect it. The higher frequency also results in a lower noise floor overcoming both natural and man made noises which are prevalent in urban environments.

Atmospheric thermal ducting is often found to be more intense on the UHF band because the smaller wavelength allows for refraction angles much greater than those allowed by lower frequencies however a much stronger thermal inversion is often required than that needed for ducting in the 2 meter band.

2m vs. 70cm

Propagation considerations often take a back seat to channel availability or economic concerns in system planning. One practical concern when comparing 70cm to 2m is that a quarter-wave antenna is much less unwieldy at 70cm than it is at 2m. Portable antennas for 2m are generally continuously loaded coil spring or "rubber duck" types while on 70cm they can be full quarter-wave. The difference can be as much as 8 dB. The primary advantage of 70cm 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 level repeaters which can then concentrate all of their power and receive sensitivity at the horizon.

The US 70cm amateur band also provides 30 MHz of spectrum, versus 4 MHz on 2m. This allows many more channels, accommodating fast scan television, wideband digital modes and point-to-point linking, which are not permitted on 2m and lower frequencies.

A problem found with all UHF or higher frequencies is the prevalence of multipath signals. The reflective properties of the 70cm 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 signal and reflected signal(s) causing cancellations as the direct and reflected signals are combined in the receiving antenna. This can cause "picket fencing" or rapid fluctuations in signal strength to be experienced by stations in motion. The problem is much less severe with modern FM systems because the limiter circuit in the receiver 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.

70cm is very close to the third harmonic of 2m, which allows sufficiently broad band 2m antennas to also be used for 70cm. Antennas specifically designed to work on both bands are common. Also, 2m is far enough away from 70cm in the spectrum to make duplexers small, requiring just a few components, so it is easy to cross band repeat signals between the two bands with a single dual band dual receive radio.

Links to 70 cm propagation sites

* [http://www.vhfdx.net/spots/map.php?Frec=430 DX-Sherlock's real-time 70cm propagation maps]
* [http://www.vhfdx.net/spots/map.php?Frec=TIC DX-Sherlock's real-time VHF&up propagation ticker]


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