There is no single mileage for a 27 MHz CB. At the legal 4 watts the same set can cover a few streets in town, work across open ground, and occasionally be heard in another country. Antenna height, antenna length, the match, coax loss and terrain decide it, not the radio.
British 27 MHz CB is licence exempt at 4 watts on AM and FM across the 40 UK and EU channels. Regulation 9 of the Wireless Telegraphy (Exemption) Regulations 2021 exempts equipment complying with Ofcom’s interface requirement IR 2027.2, for AM, SSB and DSB use in the Citizens’ Band Radio Service, provided it causes no undue interference. Our UK CB legality guide covers that. Here we deal with distance.
What is the average range of a CB radio, in miles?
There is no average worth quoting. The same radio at the same 4 watts can work one street or, when the band is open, be heard hundreds of miles away on the same afternoon. Mileage claims on boxes and in forum posts are not measurements. What can be shown honestly is what manufacturers publish: heights, gain in dBd, power ratings, SWR bandwidth, cable loss. Every figure below is one of those, from our own listings, or a rule of thumb stated as such, ranked by the difference it makes: antenna height, antenna length, the match, coax loss, terrain.
Antenna height does more for range than anything you can buy
At 27 MHz the ground wave depends on how far the antenna can see, so height is the change that most reliably moves range. The Sirio Titanium 800 mobile whip stands 790 mm. Base verticals are in another class: 5750 mm for the Sirio GPS27, 6700 mm for the Sirio 827, about 8.5 metres for a Sirio Vector 4000. Then add the mast, and clear the ridge line.
How far can a CB base station reach?
Further than a vehicle, and mostly through height rather than gain. The gain ladder across our home base CB antennas is narrow: the GPS27 at 0 dBd, the GPE27 at 1.2 dBd, the 827 at 1.5 dBd, the Vector 4000 at 2 dBd. Where radials are impractical, the Prosistel PST-1011J J-pole is specified as not requiring a ground plane.
Does CB antenna length matter? What the specifications show
Yes, though the tables show it indirectly. Manufacturers publish bandwidth and power handling rather than range, and across the Sirio Twin Log family both climb with height:
- Twin Log 3, 945 mm: 600 kHz bandwidth at SWR 2, 300 watts CW continuous
- Twin Log 4, 1250 mm: 1.1 MHz at SWR 2, 500 watts CW continuous
- Twin Log 5, 1475 mm: 1.5 MHz at SWR 2, 1000 watts CW continuous
Mobile ratings follow suit: the 790 mm Titanium 800 is rated at 15 watts CW continuous, the 1965 mm Performer 5000 at 1500 watts. Why a short whip gives ground is ordinary aerial practice rather than a table entry: it is brought to resonance with a loading coil, and that loading costs efficiency. PNI say it plainly in their copy for the ML 100, that the longer the antenna, the greater the range. Take it as the general rule it is: a long whip on a poor mount still loses to a shorter one fitted properly.
How far can a CB radio reach from a vehicle? Mount and ground plane
Mobile range is limited by the vehicle before it is limited by the aerial, because a whip works against the body and the mount is part of the antenna. The centre of the roof is the best position most drivers have, giving the largest and most symmetrical metal surface. A wing or bumper mount sits lower, against a lopsided ground plane, so the pattern leans. Our magnetic mounts run from the single 150 mm magnet of the BA150, through the 170 mm PNI 170, to the Trimag-S triple mount at 30 by 30 cm. Magnets need steel, so on fibreglass a magnetic base has nothing to grip and nothing to work against, and a bracket or through-hole mount is the answer. Our vehicle antenna guide covers body types.
SWR: range you lose before the signal leaves the vehicle
An antenna that is not matched reflects power back down the coax instead of radiating it, and on a marginal contact you hear the difference. Tune the whip where it will live, doors shut. Our SWR guide covers the method, and the meter must cover the band: the Moonraker SWR-125 is specified at 26 to 30 MHz.
Coax: the range that disappears in the cable
Every metre takes a share. These figures come from our coax cable listings:
| Cable | Diameter | Braid | Loss per 10 m at 100 MHz |
|---|---|---|---|
| RG58 military spec | 6 mm | 89% | 1.5 dB |
| MINI8 | 7 mm | 88% | 1.2 dB |
| RG213 military spec | 9 mm | 98% | 0.7 dB |
Those are 100 MHz figures and we have not converted them. Coaxial loss rises with frequency, so real loss at 27 MHz is lower, but the ranking is the usable part. It matters most on a base station, where a 20 or 30 metre run makes RG213 worth its stiffness. On a vehicle the 4 metre RG58 tail is rarely the limit. A bad PL259 is.
Terrain: cities, hills and the Forth valley
A hill between two stations costs you heavily at 27 MHz, because what gets through arrives by diffraction over the top. Standing on that same hill turns an ordinary mobile into a station everyone hears. In town street lighting, LED signs and vehicle chargers raise the noise floor, so what shortens is what you can copy.
What CB radio has the longest range?
On AM and FM every UK licence exempt set is held to the same 4 watts, so the radio is not where range lives. Doubling power adds only 3 dB anyway, and the exemption sets the ceiling. What a better set gives you is receive quality: a noise blanker and ANL filter that work, a squelch that opens on a weak signal. That is how we choose the CB radios we stock.
SSB and the 10 metre band: where the extra reach lives
Single sideband transmits one sideband and no carrier, so the power goes into the intelligence. As a rule of thumb SSB holds up where AM and FM have faded, though the advantage narrows when local noise is the limit. Our all mode SSB sets quote higher sideband figures: the Superstar SS 3900 is rated at 4 watts AM and FM and 12 watts SSB. The detailed limits sit in IR 2027.2, not in the regulations, so check the current Ofcom document before relying on anything beyond the 4 watt AM and FM position. The 28 to 29.7 MHz 10 metre band is amateur spectrum and needs a licence.
Skip, DX and why a CB sometimes reaches another country
When the ionosphere is sufficiently ionised, 27 MHz signals refract back to earth hundreds or thousands of miles away instead of passing through. That is skip, and it follows the solar cycle, the season and the hour. It is a condition of the band, not something you can buy. Sirio quote a low radiation angle for DX on the GPS27 and GPE27, because a shallow angle gives the longest hop.
How to increase CB radio range: the order of work
- Height. Up, and clear of obstructions.
- Mount and ground plane. Roof centre, clean metal, largest base that fits.
- SWR. Tune the whip in its final position, across the band.
- Coax. Check connectors, then upgrade a long base station run.
- The radio. Last, for receive quality rather than output power.
Frequently asked questions
What is the average range of a CB radio?
There is no meaningful average. At the legal 4 watts a well fitted mobile works locally over open ground, a base station with a high antenna reaches further, and when the band opens the same 4 watts can be heard hundreds of miles away.
Does CB antenna length matter?
Yes. Across the Sirio Twin Log family the quoted SWR bandwidth rises from 600 kHz at 945 mm to 1.5 MHz at 1475 mm. A short whip gives ground because the loading coil that brings it to resonance costs efficiency.
How can I increase my CB radio range legally?
Raise the antenna, improve the mount and ground plane, tune the SWR, then check the coax and connectors. Transmit power is fixed at 4 watts on AM and FM, so there is nothing to gain by chasing it.
Does a magnetic mount CB antenna need to be grounded?
It does not bolt to earth, but it does need the vehicle body. The magnet couples to the roof through its protective pad, and that steel panel acts as the ground plane the whip works against. On fibreglass a magnet has nothing to hold, so use a bracket or through-hole mount.
What coax cable is best for a CB base station?
For the long runs a base station needs, RG213 is the sensible choice. Our listings quote 0.7 dB loss per 10 metres at 100 MHz for RG213, against 1.2 dB for MINI8 and 1.5 dB for RG58. At 27 MHz the loss is lower, but the ranking holds.
Build a setup that actually reaches
Most range problems we see in the shop are fitting problems, not buying problems. Work through the mobile CB antennas, magnetic mounts, coax cable and SWR meters in that order. We are in Larbert, Falkirk, with UK wide courier delivery.