Monday, February 06, 2006

 

Frequency and "Bands".

For personal communication, we primarily use only two frequency bands. The spectrum of radio frequencies is split into various bands which is somewhat indicative of there performance qualities. For instance the group of frequencies from 3 to 30 million cycles per second (3-30MHz or megahertz) is called the HF or High Frequency Band, 30-300MHz = Very High Frequency Band (VHF), 300-3000MHz = Ultra High Frequency (UHF) band. But the word band has other uses, like the CB band or the 20 meter ham band (both in the HF band). In general, band just means a group of frequencies, so the use of the word band can get very confusing depending on its context.

The major performance quality difference has to do with range. Frequencies in the HF band will “bend” around the earths curvature thereby providing world wide coverage. Frequencies in the VHF, UHF and higher frequency bands do not “bend” so are limited to what is called “line of sight” ranges. More on “Line of Sight” later.

Mostly the performance qualities are subtle. The VHF band has a bit longer propagation with the same power level when compared to UHF. But UHF bounces better in buildings and urban areas like cities. It has to do with the length of the radio wave. Shorter wavelengths (higher frequencies) bounce better than longer ones.

The actual frequencies we use are 144-148MHz in the VHF band and 420-460MHz, in the UHF band. Why two bands? Well, they have different performance qualities and it allows us to do a trick called “full Duplex” or duplex for short. That will be the next topic.


# posted by Ron @ 7:20 PM
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