The most important accessory that you should own is an 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.
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
PL259 to PL259 Patch Lead
as supplied by Knights
TVI (BCI) Filter
An essential accessory for any home-base CB radio
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
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.
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
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.
Mini Extension Speaker with
Filter and Mute
as supplied by Knights CB
It's always as well to keep a number of spare fuses
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.
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
A large high current power
from Sharman Multicom
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.
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
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
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
made by BHI Instrumentation.
Matchers or Antenna Tuning Units
MFJ 945E Antenna Tuning Unit
Antenna matchers or antenna tuning units (ATU's) are not commonly used
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
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 .
Rear panel of the MFJ 945E
Antenna Tuning Unit
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
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
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
as supplied by Truck
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