ABOUT UHF CB

Duplex, Simplex & repeaters....What do these mean?

Simplex - One way communication only. There is only one transmission and no reply expected.

Half-Duplex - Two way communications, however only one transmission at a time.

Full-duplex - Two way simultaneous communication. Both stations can talk and receive at the same time. Such as Telephones.

On UHF CB when repeaters are used it's termed duplex, and when repeaters aren't used it's termed simplex. So they actually both represent half-duplex transmissions on CB but are used only used to differentiate between repeater usage and non-repeater usage.

A repeater is a special type of radio, it receives a signal on one channel, but gives it out on another channel. That is why you need 'Duplex' on 477MHz C.B. So you can access the repeaters and make use of them. The C.B will display channel 1, because that is what you are receiving, but if you are using 'Duplex', then you are transmitting on 31 and receiving on 1. That is because the repeater receives signals on channel 31 and re-tranmsits the signals on channel 1. The repeaters are also allowed to be more powerful than just a standard CB, because the purpose of the repeater is to extend communicaion range. Here is a diagram:

Limit on power.

UHF CB radios are generally short range radios, and the Australian Government decided to limit the power of the CB radios so people couldn't talk too far and so I suppose there wouldn't be over crowding of the band due to a lot of high powered stations talking. The UHF radios are limited to no more than 5W coming out of the back of the radio.

The signal from a CB transmitter goes through coaxial cable to an aerial. Maybe not just one aerial, perhaps 2 or 3 aerials. Not all at once of course, one aerial at a time. Perhaps aerials with different polarisations, radiation patterns, and gains.

Aerials have different names, for instance:

Yagi Beam or just "Yagi" or "Beam", Dipole, Helical Whip or just Whip and some others.

The length of the antenna for 477MHz CB is extremely critical, the wavelength of 477MHz is around 62cm. For this reason many 477MHz "whips" come out of the factory pre-tuned. Quarter wave ground planes are the most basic whips. They are constructed of stainless steel or wire, for more flexibility. At 15cm length they are relativley small. Quarter-wave antennae have a naturally high angle of radiation (explained later) and are best used in hilly country and with repeaters, as they are ideal for carrying the mobile signal from a low point such as your vehicle up to a repeater on the top of a mountain or city building.

The best mount position for a quarter wave is in the middle of any groundplane surface that can offer at least the radius of the whip and is unobstructed, such as the middle of the car's metal roof.

Centre-loaded helical whips are the next step up, with gain figures varying from 3dB to 4.5dB. This will give you a relatively good "flat" signal component for direct car-to-car or simplex range, as wall as a good angle for accessing repeaters.

Dipoles are the next step up, from a fairly middle-of-the-road 3dB to centre-loaded whips with a high gain figure of some 6dB.

The higher gain causes a flatter radiation angle resulting in more ground-plane aspect for your signal. This longer direct travel is achieved over flat terrain, and the range can be quite stunning in the country-however, a mobile in hilly areas would find themselves at a distinct disadvantage with a high-gain whip, especially where repeater access is needed.

Base stations.

Here, the choice is much simpler. Almost all 477MHz base antennae other than directional beams or "Yagi" arrays are of "co-linear" design. These vertical omni-directional antennae are usually manufactured from fibreglass and aluminium, and the main variant outside the construction and materials used is their gain figure. The most common figures are 6dB, 9dB and 12dB. Which one is right for you? It depends almost entirely on the surrounding terrain. The general rule is that the flatter the land around your station, the less the gain required.

Yagi Antennas.

Much like the Yagi-uda beam, this aerial gives extra range, either to talk through a repeater or stretch your simplex talk-distance. They are just a lot smaller because the wavelength is a lot smaller. Beam antennae effectively harness the limited output of 477MHz CB stations and also cut down on interference from other stations that may be on the same channel although not along a direct path between the two stations. Due to the fact that they concentrate signals in one direction only when horizontally polarized they enhance the range in that direction only and signals coming into the back of the aerial are attenuated.

Cable is your radio's lifeline through which the RF flows.

There are 4 common cable specifications, each of 50ohm impediance (the resistance of the cable formed by some factors.), and their loss figures are rated on a 30m run.

The thinnest cable, and the one most suitable for mobile installations, is RG-58U or the superior RG-58C/U. RG-58 has a loss of 13.5dB over 30 metres at UHF CB frequencies, so even a 15m run there's a fair bit of loss. RG-213 is the most commonly used for base stations and represents an improvement of massive proportions over the RG-58 family. It has a loss factor of only 5dB over 30m, although after adding a few in-line connectors and other things, the figure will come to a good 6dB.

That is all I have about UHF CB radio. I hope you enjoyed and gained something from this information. Thankyou for reading!