27MHz CB and Freeband

Citizens’ Band, or CB for short, is an allocation of 80 channels (in the UK) for “short range” communications. Legalised in the UK in 1981 after sets imported from the US were used illegally for many years, the original UK allocation introduced when CB was legalised was deliberately made incompatible with the US system. Whereas the US use the range 26.965 – 27.405MHz with amplitude modulation (AM), the UK system uses 27.60125 – 27.99125 with frequency modulation (FM), though a further 40 channels, which are legal in Europe as well as the UK are on the same allocation as the US, though originally using using FM only, since the 27th June 2014, AM and SSB are permitted as modes of transmission on these frequencies, the UK-specific frequencies remain FM only.


Freebanding is an activity that uses channels not authorised for use for CB, which fall between the legal channels, using “export” radios, which are usually aimed at Amateurs as radios for the 10-meter band, or modified legal CB equipment that supports the correct transmission mode, and transmission usually uses SSB (Single sideband) as opposed to AM or FM. Due to the 11-year sunspot cycle contacts over long distances can be made, even on a legal CB radio as well as an “export” radio, do note that freebanding is illegal and if you do this you do it at your own risk.


The radios used for CB and freebanding generally fall into 3 types

  1. Home base – as the name implies it is for use at home, and generally are bulky, no new homebase radios are on the market at this time
  2. Mobile – these are very common, and can be used both in a vehicle and at home, for home use a power supply to convert the mains electricity to 13.8 volts is needed, an external speaker may also be needed for some radios as the speaker is usually on the bottom of the radio and the sound would be muffled otherwise
  3. Handheld – These are self-contained and run off batteries, these usually have two issues, one being the supplied antenna drastically decreases range, the other being the battery life. They can, however, also be used as homebase and mobile radios, as the current range of handheld CBs on the market have car kits available, these include connections for a car cigarette lighter socket and a normal antenna. Though for this use you’d need to buy a speaker-microphone and for home use you’d also need an adaptor for the cigarette lighter plug fitted to the cable. You can also use a handheld as a motorcycle CB, you may need to fit a cigarette lighter socket to your motorcycle first, but using a motorcycle headset for PMR446 that use the two-pin “Standard” connector you have a better alternative to PMR446, especially if you fit a standard antenna, though you will need an artificial/electronic ground plane as well.

Radio controls

A basic radio will always have channel change controls (either up and down buttons or a rotary control), a squelch control to cut background noise, a volume control, and a channel display. A “TX” indicator is also provided so you know when the radio is transmitting.
Other controls found on CB radios include:

  • S-Meter: This can either be analogue or digital, this doubles as a power meter when transmitting and on some radios a SWR meter, but a seperate SWR meter should be used to make sure, see Antennas section
  • AM/FM/USB/LSB switch: Found on multi-standard radios, this switches between AM, FM, upper side band, and lower side band where allowed, on radios set up for use in the UK, pressing this will switch between CEPT and UK, and on the most recent version of the President Grant 2 this changes the mode between FM, AM, USB, and LSB when the EU channels are selected, although see below.
  • UK/EU switch: Found on older PR27/97 radios, this button changes between the UK and EU bands, these radios are FM only
  • 9/19 or EMG switch: This switch allows quick access to channel 9 and 19. Channel 9 is by convention designated emergency use only but this is not written down in law in the UK so people use it as a normal channel. 19 is usually mobile calling, although heavily abused in the UK on the UK only band. On some radios, “EMG” will be shown on the display.
  • DW switch: This is a dual-watch facility, allowing you to monitor two channels at once, useful if you want to, for example, keep an eye on 19 and a channel you regularly use as well
  • LC or LCR switch: This switch is for the last channel recall facility, will switch between the current channel and the last channel you used
  • SC or Scan switch: This scans the entire block of 40 channels you are using, the scan will stop when a signal is detected and restart when the channel clears
  • ME or M* switch(es): This allows you to store frequently used channels in the radio, how many spaces the radio will have will depend on the radio itself
  • Menu switch: This is present on some radios for changing some advanced settings
  • LCD display: Shows various operating parameters on screen, these include the channel number, bandplan, frequency, power output, transmit indication, receive indication, battery state on handheld radios, S-meter (see above) and whether the radio is set to transmit using AM, FM, USB or LSB.
  • Lock button: Locks the keys on the radio, on some radios this is incorporated into the microphone to lock those buttons
  • Backlight button: Turns the display backlight on and off, useful on handhelds to save the batteries
  • VOX button: found on some radios such as the Team RoadCom, allows you to speak into the microphone without pressing the PTT key
  • SWR meter controls: found on radios that have a built-in analogue SWR meter, they work exactly the same as a standard SWR meter except built into the radio, useful for making periodical checks of the antenna system, but a personal recommendation is that you should use a seperate SWR meter connected in-line, see below
  • Mic Gain: adjusts the microphone sensitivity
  • Local/DX switch or RF gain control: adjusts the the receive sensitivity of the radio, ideal if you only want to make local contacts


With 27MHz antennas, bigger really is better, base antennas generally can be up to 18ft long, but shorter antennas are available for use when a larger antenna would not be practical, such as in mobile setups. Antennas designed for mobile use can be used as base antennas however they MUST have a groundplane, this is a piece of metal on which the antenna sits, in mobile installations this is usually the metal bodywork of the vehicle.

When installing antennas, care must be taken to ensure that the VSWR (Voltage Standing Wave Ratio) is as low as possible, this is to prevent damage to the transmitter, which in this case is a CB radio. This is measured with a SWR meter, this connects between the antenna and the radio, the readings should then be taken as follows:

  1. Connect the meter between the Antenna and radio. Set your radio to it’s LOWEST channel, on a legal 80 channel radio this is EU/CE1 (you may have to switch your radio to EU/CE then change the channel to 1)
  2. Ensure the channel is clear, select “FWD” on the meter, press transmit, the needle will move to the opposite side of the meter, turn the control labelled “CAL” until the needle is resting on “set” if needed, select “REF” while still transmitting, the needle will drop and give you a reading. Repeat this for the middle channel (UK1 on UK legal 80 channel radios) and the last channel (UK40 on UK legal 80 channel radios). The meter has a red section above 3, if the needle stops anywhere in this area with the switch set to “REF”, STOP TRANSMITTING. A SWR of 3 or more could ultimately damage the transmitter, check your feedline (antenna cable), antenna, and groundplane, and make any adjustments and check again. Aim for about 1.5:1 across the band
  3. When the system is all set up, remove the meter, this is to reduce interference to other services

I would advise that you perform the SWR checks and adjustment on AM or FM, SSB power is dictated by the audio going in, if no audio there is no power, the SWR meter may not work as expected in USB or LSB

If on the off chance you simply cannot get an acceptable SWR reading,double-check everything, and test your coaxial cable to ensure there is no continuity between the outer and inner parts of the PL259 plug, ensure it is disconnected from both ends first, unless you’re using a magmount in which case remove the antenna, if it reads continuity between the outer and inner parts of your plug then you may have to replace your cable or mount, if not there may be something else upsetting it, so check absolutely everything, write a checklist of all possible issues and tick them off if it isn’t them once you have tested it. If you get to the end of your list that you have made and still cannot drop the SWR to a safe acceptable manner, an antenna tuner or matcher may be able to help you make the most of a bad situation.

What an antenna matcher does is presents as near to 50 Ohm load to the CB radio so as not to blow the finals, it doesn’t change anything about the antenna or feedline, that will behave the same no matter what. To use this device you will still need your SWR meter, there are two knobs to look at, and the idea is to keep turning each knob until the SWR is as low as possible on the frequency you want to use

Other accessories

There are many CB accessories available, these are as follows:

  • Desk microphones – Some people use these instead of the normal hand microphone supplied with the radio, these are in most cases preamplified and incorporate features such as a locking PTT switch for those longer overs, a roger bleep (see seperate heading below), tone controls, and a VU meter, when using such microphones, care must be taken to not cause overmodulation, as this will cause distortion and possible bleedover
  • BCI filter – This device sits permanently inline between the CB and the antenna, it’s purpose is to reduce interference, it does this by attenuating anything above the CB and 10-meter allocations, use one if you are causing interference
  • Linear amplifier – This device sits permanently in line with between the CB and antenna, it is used for increasing the transmit power above the 4 watts ERP (12 watts PEP on SSB) the radio can put out, however they are illegal to use, if you choose to use one, use 2 BCI filters, one between the CB and the amplifier, and one between the amplifier and the antenna, this will keep the interference down. It is also important to know if the antenna you are using can handle the power being fed to it or you risk damaging it. Remember, linear amplifiers are illegal in the UK on 27MHz, if you choose to use one you do so at your own risk.
  • Extension speaker – This is in it’s simplest form, a loudspeaker in an enclosure and can be made at home, there are also some with tone controls to reduce the harshness of some signals, this is used either when your CB is used in a homebase installation, or if used in a vehicle, in particular if you install your radio into a DIN mount (see below)
  • Roger bleep – This is a small circuit which sends a beep when you let go of the PTT button, although designed for use with AM and SSB transmissions, they are also used with FM transmissions despite the end of transmission being marked with a “crunch” or the static if the squelch control is open, in it’s simplest form it is just a beep, others are two or 3-tone beeps, some radios have this on board as well.
  • DIN mount – this is used to install a CB radio into a car stereo sized space, this in turn makes the installation look much tidier that it otherwise would be, there are CB radios on the market that are car stereo sized which mount without the need of a DIN mount, and there are a lot of radios now that have a front-facing speaker to nearly eliminate the need for an extension speaker
  • Dummy load – takes the place of an antenna and is used to test the CB radio to ensure it is working, the simplest type contains a special light bulb that lights up when transmitting, this sort is normally rated at 5 watts, so suitable for a CB radio, there are others which are rated up to 1kW, but you’d not need these unless testing amplifiers. Always make sure there is a load connected, be it an antenna or dummy load, devices such as switches (see below) or meters do not count as a load
  • Antenna switch – This device, connected permanently, allows you to connect two antennas to a radio or two radios to an antenna, if use to connect two radios to one Antenna it is important to make sure that the radio you use is the one with the load, or damage to the radio will occur, the switch alone is not a load, so take care.
  • External s-meter – This can be connected to your CB radio via a 2.5mm connector in the back of the radio next to the extension speaker socket, it does more or less what the meter on the radio does but gives a better indication (often LCD based s-meters are not particularly good), not all CB radio retailers sell these, shop around