When towing a vehicle, a UHF CB Radio is a must have and here are some tips for use.
What is UHF?
UHF CB or citizen band radio is a two-way radio system that uses the 476.4250–477.4125MHz, radio spectrum for short-distance communications. It is divided into 80 channels for various uses.
The service is for public access and available to everyone but not all channels can be used by anyone for just any reason – there are significant penalties for misuse of channels.
Advantage & Disadvantage of use
UHF’s distinct advantage over mobile phones is that it can work anywhere and requires little to no infrastructure to be in place. At the user end, all that is required is a basic radio set.
The key disadvantage is that it operates on a line-of-sight basis, and therefore has very short reach. Under normal conditions, you can expect a good signal over a distance of 5 to 8km; in a high position (such as a hill), this can be increased to up to 25km.
The upside is that you’re always communicating with those who are in your immediate vicinity.
Legally restricted channels
The following channels are legislated as a part of the ACMA UHF CB Class Licence.
Channel 5 and 35: are the designated emergency channels and are not to be used except in an emergency.
Channel 11: is the ‘call channel’ and is only to be used for initiating calls with another person, you should quickly organise another vacant channel to continue your discussion on.
Channel 22 and 23: are only to be used for telemetry and telecommand, packet data and voice transmission are not allowed.
Channel 61, 62 and 63: are reserved for future allocation and transmission on these channels is not allowed.
Each of the 80 UHF channels has the following accepted use:
Channels 1-8 and 41-48: Duplex channels (output).
Channels 31-38 and 71-78: Duplex channels (input).
Channels 5 and 35: Duplex channels strictly used for emergency communications.
Channels 9, 12-17, 19-21 24-28, 30, 39, 49-60, 64-70, 79 and 80: General chat channels, simplex use.
Channel 10: 4WD Clubs or Convoys and National Parks.
Channel 11: Call Channel used for locating friends – a general meeting point for when communications are lost or beginning, before moving to another channel.
Channel 18: Caravanners and Campers Convoy Channel.
Channel 40: Australia Wide road safety channel used primarily by truckies and oversized load pilot vehicles.
Channels 22 and 23 (25kHz): Telemetry & Telecommand used for automated data communications only.
Channel 29: Road safety channel Pacific Hwy, Pacific Mwy (NSW & QLD).
Channels 61-63: Reserved for future use.
The ‘duplex’ function of the UHF system helps increase the range of UHF radios using repeater stations set in ideal locations, such as hills. Any transmissions sent on non-duplex channels are sent in simplex mode, or directly between radio sets without the use of a repeater.
- It is important to remember that channels 5 and 35 are strictly for emergency communications, as emergency services monitor channel 5 for requests for help.
- Once communication is established, it’s accepted that both parties continue on another channel to free the channel up. If they’re taking place over a short distance, these ‘one on one’ conversations can continue on any of the general-use channels.
- It’s important to understand that all communications on every channel are public. Anyone within range of you or a repeater that you’re using can hear you and join in.
- UHF radio is a great way of staying in touch with your convoy or just to see who’s about.
- Most importantly, it is a vital link to the outside world when things go wrong.
The part of a transmitting or receiving system which is designed to radiate or to receive electromagnetic waves”. An antenna can also be viewed as a transitional structure (transducer) between free-space and a transmission line (such as a coaxial line). An important property of an antenna is the ability to focus and shape the radiated power in space e.g.: it enhances the power in some wanted directions and suppresses the power in other directions.
A citizens band (CB) radio antenna is a device designed to do two things: It captures radio-frequency signals that are then converted to electrical signals by the receiver, and it takes electrical signals from the transmitter and converts them into radio-frequency signals. This second function is where tuning comes into play, because an antenna has to radiate radio-frequency signals, something that’s done best when the length of the antenna precisely matches the wavelength of the transmitted radio frequency.The CB portion of the spectrum begins at 25.01 megahertz, so a full wavelength antenna would be a bit more than 39.34 feet long. That’s obviously a little long to attach to your bumper, so people tend to use antennas that are a fraction of the wavelength: 1/2, 5/8, 1/4 and 1/8 are all common wavelengths for antennas. In the case of CB, the 1/4 antenna at just under 10 feet long is the common “whip” that you may see on cars and trucks.
The trouble is that there are 40 channels on modern CB transceivers, each corresponding to a different frequency. It’s not practical to have a separate antenna for each frequency, so antenna designers have to compromise, usually picking a frequency in the middle of the spread and choosing the antenna length to correspond.
When a compromise like this is made, you have to see whether it’s a good compromise. This is done by measuring the Standing Wave Ratio (SWR) of the antenna and cable between the antenna and tuning the antenna until the SWR is acceptable.
Every antenna and every antenna feed-line have a characteristic impedance, or opposition to electrical current. In an ideal situation, the impedances of line and antenna match perfectly, and 100 percent of the electrical energy sent to the antenna is converted to radio energy and radiated into the atmosphere. In a less than ideal case, when the impedances aren’t perfectly matched, some of the electrical energy sent to the antenna won’t be converted to radio energy, but will be reflected back down the feed-line. The energy reflecting back from the antenna causes standing waves of electrical energy in the feed-line. (An example of standing waves outside the electronics world is found in river rapids. When water passes around and between boulders it may form a wave that doesn’t go up or down the river, it just stays in one place. That is a standing wave of water.) The ratio of highest voltage on the line to lowest is the standing wave ratio. In the perfectly matched system, the SWR is 1:1.
To tune an antenna, use an SWR meter attached between the transmitter and antenna feed-line. Depending on the meter, you can either use a button on the meter to generate a signal on the various channels, or key the microphone on the CB transceiver to generate a signal while you look at the SWR reading. In general, if the SWR never goes above 1.5:1, you’re in good shape. If the SWR does go above 1.5:1, then watch the meter on different frequencies to see the trend develop: The SWR will be greater either on the higher channels or the lower. If the SWR is greater on the lower channels, then try gradually lengthening the antenna by moving it in the base. If the SWR is greater on the higher channels, try shortening the antenna.
Do be aware that the electrical ground of the antenna, the structure around the antenna and any other antennas near the CB antenna can all affect the antenna’s impedance and the SWR. There are enough variables that tuning an antenna blends art and science, but your equipment and radio contacts will all be grateful if you take the time to tune up.