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You may be familiar with the concept that the entire spectrum is laid out in bands. Within these bands, there are sub
bands and sub-bands may exist within these.
There is in fact no point at which a step change occurs between HF, VHF, UHF
and microwaves.
There is, however, a gradual change in character in radio transmissions as you
move through the spectrum.
HF or High Frequency (to give it its common but
confusing description) is everything below 30MHz. This is considered a major watershed
although in operation, there is no distinct boundary. Very often, equipment is
designed to work either above or below this frequency, but it is really only
scanners and a few amateur receivers which cover both. This region in the
overall spectrum can achieve some considerable distances, but its coverage is
greatly affected by the time if day, time of year, and the time in the cycle of
the 11 year sunspot activity. It is not generally used for short range use with
the exception of older generation cordless phones, and CB radio.
VHF - Very High frequency is considered to be between 30 and 300 MHz
UHF - Ultra high frequency is considered as lying between 300MHz and 3000MHz (3GHz)
Above that are the microwave regions and although they
are used, there are virtually no analogue voice comms to be found here so don't
feel that you are missing out if your scanner doesn't cover from DC to light.
In fact, nearly all local two way radio occurs between 68 - 470MHz.
There are no commercial, H.O. or military analogue speech comms above
470Mhz. The only thing you may find is the occasional natter in the
23cms and 13cms amateur bands but you'll need a narrow band high gain
antenna for each of these bands to have a realistic chance of hearing
both sides of a conversation.
Apart from that, there are a few low power wideband FM devices in the
860MHz region. These include licence exempt domestic devices such as
cordless headphones and speakers. Also at the top of the TV band, spare
channels are sometimes used under JFMG licence for SAB (Services
Ancillary to Broadcasting) but these tend to be again low power short
range transmissions from talkback devices and radio mics.
Lets start with PBR. This is private business radio,
it's a class of licence which is issued to the vast majority of civilian
business users.
PBR splits into four sections, VHF Low, VHF Mid, VHF High, Band
III and UHF1 and UHF2.
Most scanners cover all the PBR bands, although some might not have Band III
VHF Low band - 68-87.5MHz
This is the lowest PBR band and occupies a space between a JFMG band at the top of Band I, and the Band II FM broadcast band. At this band, range is quite good, especially in a rural environment. It's not so good in urban areas and not well suited to handportable operation. Because of its low frequency, it is susceptible to interference from distant users under lift conditions.
If you look at a taxi cab, you may see an antenna which is about a metre long with no loading coil at the bottom. This is a low band quarter wave. Cab firms like low band because it is cheaper to hire frequencies in this part of the spectrum and it is well suited to mobile working.
VHF Mid Band and High Band -
The next chunk of PBR band allocation used to be considered as two bands, Mid and High bands, but their usage and properties are similar.
This is more popular by users and works well with wide area repeaters which communicate with vehicular mobile users. It's not as affected by enhanced radio conditions conditions as low band but this can still be a factor from time to time.
It's again widely used by vehicular users, but you will also find some handheld users. So, lots of cabbies once again, and skip firms feature commonly here, but also sectors such as shopwatch and pubwatch schemes. You'll also find security and industrial users working through on-site repeaters using handhelds.
So you can see, VHF High/mid is used for but short range and long range comms. For the wider area stuff, repeaters are sited at remote hilltop locations, for the on site applications, a repeater is located on a building and covers an area generally considered to be within a 3km radius.
Band III
This is split into three sub-bands. Sub-bands 1,2 and 3. This band is almost exclusively used for vehicular mobiles. There's no real technical reason for this, but licences were made available usually for wide area trunked systems, and these generally assume a vehicular installation with its higher power and more efficient antenna.
There are two very large systems which are almost nationwide in this band. NRN or National Rail Network system has coverage of most of the railroads in England and Wales.
Fleetcom operate a large network of trunked repeaters which are linked to provide a radiotelephone service throughout England and Wales.
UHF 1 & UHF 2
As with VHF High and Low bands, UHF 1 & UHF 2 are considered to be two separate PBR bands, but in fact, they run shoulder to shoulder, and their technical characteristics are very much the same.
UHF is the most crowded of all the PBR bands. The reason for this stems from the fact that it has an advantage over all the other bands that, because of its higher frequency, it works best in an urban environment. As built up areas are where most radios are used, it is the most popular band for users.
The range with UHF is in fact less than the other bands, but that is part of the reason for its success. With business radio, you have a certain area to cover, and you want your comms to cover that area consistently. UHF is far less affected by freak radio conditions than the lower frequencies so when enhanced conditions occur in the lower bands and users start to overlap each other, UHF carries on pretty much as normal.
There's another clear advantage to the urban user too. At the higher frequencies that UHF offers, signals start to bounce off structures such as buildings, so although direct line of sight may not be guaranteed, penetration into every nook and cranny of our towns and cities is more of a certainty.
UHF1 is less used than UHF2. For some reason, Ofcom prefer to bunch everything up into UHF with the exception of the big cities like London and Manchester. Where UHF1 is not generally used in your area, you will still find services such as PMR446 and UK General in use. |
