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UHF Explained

 

 

Introduction

Whether you are planning a trip into the Australian outback, towing a caravan on back top or a weekend 4WD adventurer, then a UHF Radio is an essential communications tool to have. UHF (477MHz) radios have nearly replaced the older 27MHz CBs, mainly due to their considerable price drop over the past decade and the extended range offer by repeaters located throughout Australia.

 

 

UHF CBs use Frequency Modulation (FM) to transmit/receive. Since UHF signals generally travel in a straight line, the terrain impact how well the signal is transmitted or received. For example, UHF CB radios will perform poorly in hilly or forested areas and perform well in flat open countryside. UHF CBs can also access repeater stations that re-transmit the signals on another channel which can provide much further coverage (see Repeaters for more information).

 

40 Channel v 80 Channel Radios

The recent restructured on the 477MHz UHF CB band by the ACMA and now utilises 80 individual 12.5kHz wide channels in the Ultra High Frequency (UHF) range, which covers 476.425MHz to 477.4125MHz. If you own a 40 channel system, please read the special section below "New Rules" to clarify what you need to do

 

Repeaters

UHF repeaters are special transmitting/receiving stations that are usually located in high areas to allow extended coverage. These stations, which are usually owned by businesses, farmers and clubs, allow UHF users to use them to re-transmit their signal. It works when you press your microphone button with the “Duplex or Repeater” button selected also. You must transmit between channels 1 to 8 (and now you also have access to 41 to 48) because the “Duplex or Repeater” selection will add 30 channels and will now transmit to channels 31 to 38 (and now also 71 - 78) on the repeater. The repeater, which previously collected the signal on 31 to 38 (or 71 - 78), then switches to its output channel 1 to 8 (and 41 - 48) to transmit simultaneously to say another radio user on the other side of the hill.

 

UHF Radio Channel Allocation

Below you will find a list of UHF Radio channel allocations. There are many channels that have been established by law including the Emergency channel 5 and the data transmission channels 22 and 23. The new 80 channel system does not change the allocation use of these channels.

 

Channels

Channels Used For

1 - 8 &

41 - 48

These channels, which are established by law, can be used when sending a signal to a repeater which will help increase the communication distance

 

5

This channel, which is established by law, can be used by anyone but only in an emergency situation

9

Used for conversations

10

Used by 4WD enthusiasts, clubs, convoys and in national parks

11

Calling channel. This channel, which is established by law, is used to call or locate another station. Parties will then switch to another channel to continue with their conversation

12 - 17

Used for conversations

18

Holiday maker’s communication channel (e.g. when in a convoy)

19 - 21

Used for conversations

22 - 23

These two channels are used for data transmissions and is established by law. Voice transmissions are not allowed on these two channels

24 - 30

Used for conversations. Note Channel 29 is specifically as a road channel.

29

For Pacific Hwy (NSW) and Bruce Hwy (Qld) communications which are mainly used by truck drivers and other highway users

31 - 38

These channels, which are established by law, are received by a repeater and re-transmitted on channels 1 to 8 to help increase the communication distance

35

Can be used in case of Emergencies also

39

Used for conversations

40

Highway Communications which are mainly used by truck drivers and other highway users

41 - 48

New channels (use same as 1-8) are established by law, can be used when sending a signal to a repeater which will help increase the communication distance

49 to 60

New channels can be used for conversations

61 - 63

Currently reserved for future expansion

64 - 70

New channels can be used for conversations

71 - 78

New channels, which are established by law, are received by a repeater and re-transmitted on new channels 41 to 48 to help increase the communication distance

79 - 80

New channels can be used for conversations

 

Summary:

·         Channels 1 to 8 and 41 - 48 are repeater channels. Press the duplex button on your radio to use any available repeaters.

·         Channels 5 & 35 - emergency use only

·         Channels 22 & 23 - data only

·         Channels 31 to 38 and 71 - 78 - are repeater inputs, do not use these channels for simplex transmissions as you can interfere with conversations on channels 1 to 8 and 41 to 48.

 

Telemetry Systems

This technology is used for the transmission of data. By utilising telemetry systems via UHF signals, farmers can remotely control the operation and function of various types of equipment such as electric fences, water pumps and dam levels from many kilometres away.

 

Purchasing a UHF CB Radio

When purchasing a UHF CB radio, there are a number of things to consider:

1.   Where are you going to mount the UHF radio?
This will guide you to what type of UHF radio you are going to purchase (i.e. standard DIN size, slim models or models with remote microphone)

2.   Where are you going to mount the UHF antenna?
The mounting location of your antenna will help you decide what type of antenna you are going to install. (see UHF Antennas for more information)

 

UHF Antennas

 

Why is the antenna important?

The antenna is a device for transmitting and/or receiving signals – the eyes and ears of your UHF radio. A good antenna is designed to be “in-tune” to the signal you are seeking – your radio “sees” the signal and you hear a clear conversation. Use a poor antenna or mount the antenna is a bad location, will greatly reduce your range and clarity.

 

Antenna radiation patterns

Mobile antenna should radiate in a symmetrical pattern 360° around the antenna. However depending on where you mount your antenna, will affect the way the antenna radiates. The best location for an antenna is in the middle of you roof however this is generally not practical. Antenna are normally mounted the guard or bull bar. This these cases, it is essential that you A) use a ground independent antenna (see Ground Independent Antenna for more information), B) mount the antenna away for vertical metal section like windscreen pillars etc. and C) mount the antenna as high as possible. Following these suggestions will help minimise any affect on the radiation pattern.

 

Antenna gains

As gain is increased, the radiation gets compressed into a thinner pattern and reaches out further to the side. The more gain an antenna has the thinner the pattern becomes and the further the signal can travel or reach. However a thinner radiation pattern may prevent strong signal reception when driving through hills.

 

So in summary:

·         Low gain antennas (2-4 dBi) are good for hilly terrain but lack range in flat terrain.

·         Medium gain antennas (5-7 dBi) are good all round antenna which works well in hilly and flat terrain.

·         High gain antennas (8+ dBi) are good in flat terrain but poor in hilly terrain.  

 

A final word on antenna gain & patterns

An antenna must physically meet a certain requirement to deliver it’s claimed gain. The laws of physics cannot be defeated and without these characteristics there is simply no way to increase antenna gain. Various antenna manufactures use different references when declaring their gain figures. In the absence of a defined refer4ence, some claims made in catalogues and on retail packaging by some manufacturers are just plain wrong.

 

When selecting an antenna be sure to make a couple of basic checks:

 

1.  Look for the stated gain reference, we would suggest dBi, dBd or dBq (gain over a ¼ wave). We can’t promise that this means the manufacturer has actually tested the gain, but it certainly indicates that they know they need a reference.

2.  Compare a couple of antenna that are hanging next to each other in a store, or check the physical length of the antenna in a catalogue. If one is claiming 6.5 dBi and is 900mm long and another is claiming 9dBi at a similar length, then consider how can this be true.

Antenna Installation

As discussed previously most UHF antennas require a ground plane, which means they should be mounted on top of a metal surface. The best spot in this regard would be the centre of your roof. Unfortunately for some people, this may not be a great idea because it could require drilling through the roof for installation. Travellers also prefer to utilise their roof rack space for carrying supplies. If the roof is not on the cards, then other good locations to mount an antennal can include: the front guard or on the bull bar, which is probably the most common.

 

Co-Axial Cable

In all types of UHF antennas, feed line loss can be a constant threat in regards to performance. Bad feed lines can swallow up half or more of your transmitting power and therefore degrading reception. Loss can be determined by length and quality of the feed lines, so the use of thicker co-axial cables over thinner ones is a much better option. Some common cable specifications, each being 50ohm impedance and the loss figures on a 30m run are listed below:

 

·         RG-58U series including the RG-58C has a loss factor of 13.5dB over 30m

·         RG-213 has a loss factor of 5dB over 30m

 

Handheld UHF Radios

Handheld models are also popular with obvious advantages of portable, the high advances in electronic technology and their increased affordability. These CBs range from as little as $40 to over $400 and provide as much functionality as the base station CBs. There are some top end models that have a range of up to 12km with an impressive 5 watts of TX output power. As well as being lightweight, ruggedly built with waterproof construction, these CBs are no toy. These units can be very handy for travellers in a whole range of situations.

 

Some handheld radios are sold in sets of 2 handheld units you can lend one unit to another vehicle/person to enable UHF communication in an instant such as in the following scenarios; passenger leaves vehicle to check the track ahead and can use handheld unit to communicate back to the vehicle with information search and rescue situations involving 2 people (or as many handheld units are available) spreading out in different directions used in conjunction with an installed UHF system in a vehicle is an effective and sensible capability you can coordinate if planning a remote trip one party may wish to stay behind at the campsite whilst another member goes boating, fishing, bushwalking etc.

 

With handhelds UHFs you have an extra level of safety and peace of mind when heading off on solo excursions away from the vehicle, or campsite for minimal cost.

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