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CB ACCESSORIES


SWR Meter

The most important accessory that you should own is an SWR Meter:

SWR Meter
A typical CB SWR Meter with the necessary PL259 to PL259 Patch Lead
as supplied by
Truck King

See this page for more information.


Patch Cable

A patch cable is essential for connecting an SWR meter or a 'low pass' (TVI) filter between the CB radio and the antenna. They can be bought as a ready made item, or made up out of separate PL259 plugs and a suitable length of 50 ohm coaxial cable. Cable will generally be good quality RG58/u or the higher quality Mini RG8

PL259 to PL259 Patch Lead
PL259 to PL259 Patch Lead
as supplied by Knights CB

TVI (BCI) Filter

An essential accessory for any home-base CB radio installation.

All radio transmitters, including CB radios, will transmit harmonic or spurious signals that are not on the desired frequency. For example a radio transmitting on 27.8 MHz could also transmit on harmonically related frequencies 55.6 MHz (2nd harmonic) 83.4 MHz (3rd harmonic), 111.2 (4th harmonic), 139.0 MHz (5th harmonic) etc etc. These unwanted signals have the potential to cause great interference to other radio services.

Most modern CB radio transmitters suppress these unwanted emissions to very low levels, but to keep TV and other interference to the absolute minimum it is wise, if not essential, to install an external TVI filter.

Such a filter will only allow signals below a certain frequency to pass, usually those frequencies below 30 MHz. Frequencies above 30 MHz will be greatly attenuated thereby reducing the possibility of interference to radio services on those higher frequencies.
RM Low Pass TVI Filter
RM Low Pass TVI Filter
as supplied by Knights CB


Extension Speaker

Generally a mobile CB radio will be used for home-base use. Unfortunately the loudspeaker on a mobile CB radio is on the bottom of the case, so the sound will be muffled.

The ideal solution is to use a separate external loudspeaker. Most CB radios have a loudspeaker socket on the back panel, so connection is very easy.

Extension loudspeakers are also very useful for obtaining the best and clearest sound in cars, vans and trucks.

The loudspeaker shown below also features a mute switch and a useful filter switch that cuts down the high frequency output from the speaker. It is really a tone control, but it makes noisy or sharp sounding signals easier on the ear.

Extension Loudspeaker
Mini Extension Speaker with Filter and Mute
as supplied by Knights CB



Spare Fuses

It's always as well to keep a number of spare fuses handy.

Fuses are available in different amp ratings and different sizes e.g. 20mm and 32mm. It is very important to use the correct rating of fuse as well as the correct size. The most common fuse used in standard CB rigs is a 5 amp fuse that is 32mm in length.
Fuses
A box of fuses as supplied by Knights CB

Power Supply Unit

A power Supply Unit (PSU) is often referred to as a 'power pack' and allows a mobile CB or other accessory that requires a 12 to 14 volt supply to be powered from 240 volt mains electricity.

Never use a 12 volt car battery to power your CB radio! This is a very dangerous practice due to the hazardous and corrosive chemicals that are contained in car batteries and the risks of dangerous fumes being released. A car battery is definitely NOT something that you want in a home environment.

Sharmans 22 amp power supply
A large high current power supply
from Sharman Multicom




Headphones

A pair of headphones can be used for private listening and can also be extremely useful to assist with the audibility of weak DX signals.



Desk Mic

Although most people prefer to use the normal hand-held fist microphone that is supplied with most CB radios there is also the option of using a Desk Microphone - a free standing mic that stands on the table or desk, or one of many other alternative hand-held microphones that are available.
JCD 201M Desktop Microphone
The microphone shown above is the JCD 201M, a sophisticated desktop microphone with a built in speech compressor which can really help audibility when signals are difficult, especially with SSB. Available from Knights Electrocom



Audio Filter

There are some audio filters available that can help reduce the noise on weak 'DX' signals. These may range from a simple tone control, as shown on the extension loudspeaker above, to sophisticated (and expensive) DSP (digital sound processing) filtering such as the popular NES10-2 mkII made by BHI Instrumentation.



Antenna Matchers or Antenna Tuning Units

MFJ 945E Antenna Tuning Unit
MFJ 945E Antenna Tuning Unit

Antenna matchers or antenna tuning units (ATU's) are not commonly used in the average CB stations, but can be helpful when trying to operate an antenna that is not, or cannot be matched correctly to the transmitter (CB radio). An ATU is connected between the radio and the antenna with a good quality patch lead and is left in line permanently.

To be efficient an antenna must be tuned to the frequency, or a limited range of frequencies, that will be used by the transmitter. In the case of CB radio this is 26.9 to 27.9 MHz. If correctly tuned the antenna will present the necessary 50 ohm impedance to the transmitter. If the antenna is not tuned properly the impedance will be incorrect the antenna will not receive as well as it should and some of the power supplied by the transmitter will be reflected back towards the transmitter. This problem will be shown by the SWR meter as a high VSWR (voltage standing wave ratio) reading. If the VSWR is too high, approaching 3.0 and over, damage can be caused to the power transistors in the transmitter.

If the antenna cannot be tuned for acceptably low VSWR (2.0 and below), or the range of frequencies required ("bandwidth") is too wide to obtain an acceptable VSWR from the lowest frequency to the highest frequency, an ATU can help.

A badly matched antenna may present an impedance of many hundred ohms, and ATU can be adjusted to compensate for this and present the radio with the 50 ohms that it required. This will avoid the risk of causing any damage to the transmitter, however it will not make a poor or badly tuned antenna into a good antenna - the antenna itself will still be inefficient, but the ATU will help make the best of a difficult situation. An ATU may be considered as an essential piece of equipment by radio amateurs who need to transmit across a very wide range of frequencies.

ATU's suitable for transmitting will often have a meter on the front that will indicate power and VSWR. The controls that allow the adjustments to be made take the form of a knob that selects an appropriate amount of inductance, from an internal coil, for the band being used and two further knobs that allow the capacitance on the antenna side and transmitter side of the matching circuit to be finely adjusted until the required low VSWR to be obtained .

MFJ 945E Antenna Tuning Unit - rear panel
Rear panel of the MFJ 945E Antenna Tuning Unit


Linear Amplifier

A Linear Amplifier, sometimes referred to in CB language as a 'boot', 'shoes', or 'help', is a strictly illegal device that can be connected in between the CB radio and the antenna. It will amplify the transmitted output power to unauthorised and illegal levels. Some of the most popular linear amplifiers tend to produce an output of 25 to 40 Watts, though larger models are available.

Although it is completely illegal to use a linear amplifier, if one is used, it must have effective interference filtering installed. Don't even think about sing a linear amplifier without the necessary TVI filters.

For effective filtering two TVI (low pass) filters must be installed - one between the CB and the amplifier and one between the amplifier and the antenna. This is to ensure that any spurious emissions from the CB radio transmitter, that would be amplified by the linear amplifier, are filtered out and kept to a minimum, otherwise interference will be caused to other services and perhaps to your, or your neighbour's television and radio reception.

You have been warned - Linear amplifiers are illegal and have the potential of causing a great deal of interference. So if you must use one, make sure that it is VERY well filtered!

RM KL-60 linear amplifier
RM KL-60 linear amplifier
as supplied by Truck King





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Mike Smith - www.MDS975.co.uk © 2003 - 2013