CD250 Band 20 + Band 3 Explained: How the 17dBm Dual-Band Booster Covers EE, O2, Vodafone and Three 4G Across 250m2 in the UK
The Boost4U CD250 repeats two UK 4G layers at once: Band 20 at 800 MHz carries O2, Vodafone and Three 4G, while Band 3 at 1800 MHz carries EE 4G plus 2G voice and SMS; it delivers at least 60 dB uplink gain and 65 dB downlink gain on both, and sets itself apart from the smaller siblings with a >=17 dBm maximum output that fills a 250 m2 medium home, small office, cafe or pub for up to 25 simultaneous users. This deep-dive explains what each band does, which UK networks and MVNOs ride which layer, the exact uplink and downlink frequency pairs, how the >=17 dBm output compares with the >=10 dBm output of the CD200 and CD100, why a dual-band chassis beats the single-band C250, how VoLTE and 2G calls travel, what Ofcom licence-exempt status and the UKCA, CE and RoHS marks mean, and the honest limits: no 3G Band 1 at 2100 MHz, no 5G, and coverage rated only to 250 m2.
Answer first: the CD250 is a dual-band FDD LTE repeater that tunes Band 20 (800 MHz) for O2, Vodafone, Three and their MVNOs, and Band 3 (1800 MHz) for EE, ASDA, BT, Orange, T-Mobile, The People's Operator and Virgin 4G plus circuit-switched 2G voice and SMS. Its >=60 dB uplink and >=65 dB downlink gain lift weak donor cells, while its >=17 dBm output power is the hardware step above the >=10 dBm units that pushes the amplified field across 250 m2 and supports 25 sessions. It stays Ofcom licence-exempt and carries UKCA, CE and RoHS marks, but it does not repeat 3G Band 1 at 2100 MHz, does not carry any 5G band, and should not be stretched beyond 250 m2.
Band 20 (800 MHz): The Long-Range Layer for O2, Vodafone and Three
Band 20 occupies the 800 MHz spectrum that Ofcom reallocated to 4G LTE after the analogue TV switch-off. Its low wavelength travels farther from the macro mast, bends around terrain and penetrates brick and foliage more cheaply than higher bands, which is why O2, Vodafone and Three deployed Band 20 as the rural and deep-building 4G layer. Inside a UK home or business, Band 20 is the band that reaches the garden room, the back office and the upper floor where the higher-capacity band fades.
Uplink and downlink frequency pairs for Band 20
| Band 20 link | Direction | Frequency range | Users on this layer |
|---|---|---|---|
| Uplink | Phone to tower | 832 to 862 MHz | O2, Vodafone, Three and their MVNOs |
| Downlink | Tower to phone | 791 to 821 MHz | O2, Vodafone, Three and their MVNOs |
A repeater works on a paired FDD block: the phone transmits on one block and receives on another, and the amplifier must repeat both directions without mixing them. The CD250 repeats the 832 to 862 MHz uplink and the 791 to 821 MHz downlink together so a phone that sends a request to the Band 20 tower hears the reply amplified indoors.
Why 800 MHz reaches the garden room but carries less capacity
Lower frequency propagates farther and passes walls with less loss, but each MHz block carries fewer simultaneous data sessions than the higher bands. Band 20 is therefore the coverage band: it keeps a phone connected at the edge of the property, while EE's Band 3 carries the capacity where many devices gather. A rural Devon cottage, a Yorkshire home or a Welsh office on O2, Vodafone or Three relies on Band 20 being repeated indoors; the CD250 does that on the donor side before the >=17 dBm output spreads it inside.
Band 3 (1800 MHz): The EE Capacity Layer and 2G Voice Path
Band 3 sits at 1800 MHz, the original GSM cellular band that EE and its family use for both 4G LTE capacity and legacy circuit-switched 2G voice and SMS. Its shorter wavelength carries more data capacity per block but attenuates faster through brick, double-glazing and floor slabs, which is why it reads weaker deep inside a building even in a well-covered city.
Uplink and downlink frequency pairs for Band 3
| Band 3 link | Direction | Frequency range | Users on this layer |
|---|---|---|---|
| Uplink | Phone to tower | 1710 to 1785 MHz | EE family 4G plus 2G voice |
| Downlink | Tower to phone | 1805 to 1880 MHz | EE family 4G plus 2G voice |
The CD250 repeats the 1710 to 1785 MHz uplink and the 1805 to 1880 MHz downlink so EE-family phones get both LTE data and the 2G GSM voice and SMS path on the same layer. An older feature phone, an elderly relative's handset, or a phone in an area where VoLTE has not yet been enabled still completes a normal call over the repeated 1800 MHz GSM layer rather than dropping to no service.
Why EE customers need this band rather than Band 20
EE does not rely on Band 20 for its domestic 4G in the same way; an EE SIM placed on a Band 20-only single-band unit would see no EE layer indoors. The CD250's dual-band chassis keeps Band 3 live alongside Band 20 so one box serves an EE household and an O2 visitor at the same address, which is the practical reason dual-band beats buying a single-band unit for a mixed-network home or business.
UK Network and MVNO Mapping: Which SIM Works on Which CD250 Band
The table below maps every SIM named on the CD250 product page to its host network and the band the booster repeats for it. Before ordering, check your SIM's host carrier rather than the shop brand, because the MVNO brand name itself does not appear on the tower.
| SIM brand | Host network | CD250 band repeated | Services indoors |
|---|---|---|---|
| O2 | O2 | Band 20 (800 MHz) | 4G LTE data |
| Vodafone | Vodafone | Band 20 (800 MHz) | 4G LTE data |
| Three | Three | Band 20 (800 MHz) | 4G LTE data |
| Giffgaff | O2-hosted Band 20 | Band 20 (800 MHz) | 4G LTE data |
| Tesco Mobile | O2 | Band 20 (800 MHz) | 4G LTE data |
| Lycamobile | O2 | Band 20 (800 MHz) | 4G LTE data |
| Sky Mobile | O2 | Band 20 (800 MHz) | 4G LTE data |
| iD Mobile | Three | Band 20 (800 MHz) | 4G LTE data |
| Smarty | Three | Band 20 (800 MHz) | 4G LTE data |
| FreedomPop | Three | Band 20 (800 MHz) | 4G LTE data |
| Plusnet | EE | Band 3 (1800 MHz) | 4G LTE plus 2G voice and SMS |
| EE | EE | Band 3 (1800 MHz) | 4G LTE plus 2G voice and SMS |
| ASDA Mobile | EE | Band 3 (1800 MHz) | 4G LTE plus 2G voice and SMS |
| BT Mobile | EE | Band 3 (1800 MHz) | 4G LTE plus 2G voice and SMS |
| Orange, T-Mobile, The People's Operator, Virgin | EE | Band 3 (1800 MHz) | 4G LTE plus 2G voice and SMS |
Two households on different hosts therefore benefit from different halves of the CD250: the O2 household sees Band 20 light up, the EE household sees Band 3 light up, and a shared home where one person uses Giffgaff and another uses BT Mobile gets both bands boosted at once, which is the practical reason a dual-band chassis beats two single-band boxes across a 250 m2 property.
Gain Explained: >=60 dB Uplink and >=65 dB Downlink on Both Bands
Gain, measured in decibels, is the factor by which the amplifier boosts the weak donor signal before re-radiating it. The CD250's >=60 dB uplink gain and >=65 dB downlink gain are asymmetric: a tower transmits with high power while a phone transmits at low power from a battery, so the uplink path loses more energy on the way back and the amplifier still only needs to restore the faint phone signal to a level the tower can decode. The 5 dB gap between the two figures is normal for a home repeater and not a defect.
How the 65 dB downlink margin buys walls and floor loss
Every indoor surface costs signal: a plasterboard partition costs a few dB, an external brick wall costs 10 to 20 dB, a store or kitchen wall with appliances costs more, and a ceiling slab between two floors adds its own loss, with double glazing adding still more. The >=65 dB downlink margin absorbs that cumulative loss so the phone still shows bars in the upstairs bedroom, the back office and the garden room, where a smaller unit's thinner margin runs dry.
Noise figure <=5 dB and why it matters
A perfect amplifier adds no noise; every real amplifier adds some, measured as the noise figure. The CD250's noise figure of no more than 5 dB keeps that added noise low enough that the amplified signal still decodes cleanly rather than arriving louder but garbled. A high noise figure would raise the indoor noise floor, cause the phone to select a louder but noisier cell, and reduce the usable uplink; a <=5 dB figure is the standard home-repeater quality level.
Gain is not the same as output power
Gain describes amplification factor; output power, covered next, describes the absolute ceiling the repeater may transmit at from its indoor antenna. A unit can have high gain yet still keep output low, because the automatic gain control raises gain only until a stable indoor signal appears and then backs off rather than running at maximum all the time. The CD250 reaches farther through structure not by over-amplifying, but by combining steady gain with a higher output budget.
Why >=17 dBm Output Is the CD250 Key Difference: vs the CD200's >=10 dBm
Maximum output power, listed as >=17 dBm on the CD250 spec, is the regulated ceiling the repeater may radiate from its indoor antenna. The CD100 and CD200 both sit at >=10 dBm despite different gain stages; the CD250 raises that output to >=17 dBm while keeping the same >=60/65 dB gain as the CD200. That 7 dB step represents roughly five times the radiated power delivered to the indoor whip after cable and wall losses.
| Attribute | CD100 | CD200 | CD250 | C250 |
|---|---|---|---|---|
| Tuned bands | Band 20 800 + Band 3 1800 | Band 20 800 + Band 3 1800 | Band 20 800 + Band 3 1800 | Band 20, 800 MHz only |
| Uplink / downlink gain | >=45 / >=50 dB | >=60 / >=65 dB | >=60 / >=65 dB | About 55 / 60 dB |
| Maximum output | >=10 dBm | >=10 dBm | >=17 dBm | About 10 dBm |
| Rated coverage | 100 m2 | 200 m2 | 250 m2 | 250 m2 |
| Simultaneous users | 10 | 20 | 25 | About 25 |
| 4G networks | EE, O2, Vodafone, Three | EE, O2, Vodafone, Three | EE, O2, Vodafone, Three | O2, Vodafone, Three |
What 17 dBm of radiated power means in practice
Output power sets how loud the indoor field reaches past partitions, a solid bar counter, a shop display or a fridge bank. The CD200's >=10 dBm fills 200 m2; the CD250's >=17 dBm keeps the same good listening and raises the final loudness, pushing the field an extra 50 m2 across the larger footprint and from 20 to 25 sessions. The extra output buys reach through structure, not a louder distortion, because the <=5 dB noise figure and automatic gain control keep the field clean.
More amplification does not mean more transmitted watts
More gain inside the box does not force the indoor donor farther past the regulations; the automatic gain control balances the two so a 65 dB stage still ends at the controlled >=17 dBm ceiling rather than blowing past it. A weak donor needs a better antenna position or the optional high-gain Yagi, not a higher output that the unit already holds in reserve.
Ofcom Licence-Exempt Status, UKCA, CE and RoHS Explained
Four marks sit on the CD250: Ofcom licence-exempt status, UKCA for the UK market, CE for European conformity, and RoHS for restricted hazardous substances. Each answers a different regulatory question, and none changes the tuned bands or the >=17 dBm output figure.
Ofcom licence-exempt
Ofcom's permitted-class rules allow a low-power indoor repeater on licensed mobile bands without the owner holding a spectrum licence. The CD250 meets the output and isolation conditions of that class, so no application, fee, annual renewal or registration number is required, provided the unit is used as supplied, kept separated from its own donor antenna, and not modified to raise output. The higher >=17 dBm figure sits inside this class because the automatic gain control caps the radiated field; the owner still must use the unit as supplied and keep separation.
UKCA, CE and RoHS
UKCA confirms the product meets the applicable UK safety, electromagnetic compatibility and radio-equipment requirements for sale in Great Britain after the CE transition, the UK-market conformity declaration appearing on units supplied to UK customers. CE confirms conformity with the European Union Radio Equipment Directive and related health and safety standards, which is why the same kit ships across the EU delivery list including Germany, France, Italy, Spain, the Netherlands, Belgium, Austria, Sweden, Denmark, Finland, Ireland, Portugal, Poland and the Czech Republic as well as the UK. RoHS restricts lead, mercury, cadmium and other hazardous substances in electronics, so the chassis, cable and adapter comply with waste-electrical rules rather than containing banned materials. None of these marks alters how the bands repeat; they confirm the unit may be sold and used without a separate permit.
VoLTE and 2G Voice: How Calls Stay Connected Across 250 m2
Modern phones place calls over the 4G data layer using Voice over LTE (VoLTE), while older handsets and areas without VoLTE fall back to circuit-switched 2G GSM. The CD250 supports both routes where the network offers them, but its support differs by band.
VoLTE on Band 20 for O2, Vodafone and Three
Where the host network has enabled VoLTE on Band 20, a modern O2, Vodafone or Three handset places the call inside the boosted 800 MHz LTE session and the CD250 repeats that session like any other data call. The booster does not negotiate VoLTE itself; the phone and network do, and the unit simply carries the already established 4G call across the room where the >=17 dBm margin reaches.
2G circuit-switched voice on Band 3 for EE
EE maintains a 2G layer in the 1800 MHz region, and the CD250's Band 3 path repeats circuit-switched voice and SMS for EE, ASDA, BT, Orange, T-Mobile, The People's Operator and Virgin handsets that cannot or do not use VoLTE. A basic feature phone, an elderly relative's handset, or a phone in a VoLTE-not-yet-enabled area still rings and completes a normal call over the boosted 2G layer.
What the CD250 does not carry for calls
The unit does not tune 3G Band 1 at 2100 MHz, so a network that still routes voice or data fallback through 3G in your location receives no boost on that layer. Where VoLTE is unavailable and the network relies on 3G circuit fallback, a call may not complete indoors even though 4G data shows bars; checking VoLTE support on your handset before ordering avoids that surprise, and the higher output cannot invent a band the chassis does not tune.
Honest Limitations: No 3G Band 1, No 5G, and Only 250 m2
A guide that lists only benefits misleads a buyer. The CD250 has three clear limits that should match your situation before you order, and neither the >=17 dBm output nor the >=60/65 dB gain removes any of them.
No 3G Band 1 at 2100 MHz
The CD250 tunes only Band 20 and Band 3. It does not repeat Band 1 at 2100 MHz, the 3G UMTS layer some networks still use for data fallback or circuit-switched voice. A home that relies on 3G where 4G fails will see that fallback unboosted indoors; raising output to 17 dBm cannot manufacture a band the hardware never receives, and this is the single most important technical limit for areas already served only by Band 20 or Band 3 4G rather than 3G-only locations.
No 5G on any band
The CD250 does not tune n78 (3.5 GHz), n8 (900 MHz) or any other 5G band, and it does not convert 4G into 5G. A 5G phone still receives its normal unboosted 5G signal from the macro tower, and the CD250 only lifts the 4G Band 20 plus Band 3 layers. Expect reliable 4G calls and data across 250 m2, not boosted 5G speeds, no matter how high the output or gain.
Only 250 m2 for 25 simultaneous users
The rated footprint of 250 m2 (2,691 ft2) fits a two-to-three-bedroom home, a larger flat, a cottage, a small office or a cafe and pub, and supports up to 25 active sessions. A whole house across multiple floors, a large detached property over several wings, a shop floor clearly beyond 250 m2, or a property clearly beyond this area needs a larger unit or a second unit rather than more output from this chassis. The 25-user count means active calls and data sessions, not every registered device in the home, and beyond that ceiling the network itself schedules phones.
In plain terms: the CD250 buys both UK 4G layers in one box at the larger domestic and small-business footprint: Band 20 at 800 MHz for O2, Vodafone and Three via 832-862 MHz uplink and 791-821 MHz downlink, and Band 3 at 1800 MHz for EE plus 2G voice via 1710-1785 MHz uplink and 1805-1880 MHz downlink, with >=60 dB uplink and >=65 dB downlink gain and a higher >=17 dBm output than the >=10 dBm CD100 and CD200. It covers all four network families across 250 m2 for 25 sessions under Ofcom licence-exempt status with UKCA, CE and RoHS, but it does not tune 3G Band 1, does not carry 5G, and should not be stretched past 250 m2; choose the CD100 for a single-network 100 m2 room, the CD200 for a 200 m2 mixed-network home, and the C250 only when Band 20 alone on one host network is the entire requirement.
Frequently Asked Questions
What does >=17 dBm output mean versus the CD200's >=10 dBm, and why does the 7 dBm step drive 250m2?
The 7 dB step from 10 to 17 dBm represents about five times the radiated power after cable and wall loss. Where the CD200's >=10 dBm field fills 200 m2, the CD250's >=17 dBm pushes the same already-amplified signal an extra 50 m2 through partitions, counters and floors to reach 250 m2 and 25 sessions; gain stays at >=60/65 dB on both, so the reach comes from output loudness, not extra amplification.
Why does the CD250 share the CD200's >=60/65dB gain but reach a larger 250m2 footprint?
Gain and output are separate figures: gain amplifies the weak donor before re-radiating, output sets the final indoor loudness. The CD250 keeps the CD200's >=60 dB uplink and >=65 dB downlink but raises maximum output to >=17 dBm, which stretches the footprint from 200 to 250 m2 and the session count from 20 to 25; the bigger area is bought with watts, not a higher gain stage.
How does the CD250's dual Band 20 plus Band 3 compare to the single-band C250 on 800 MHz at 250m2?
The C250 reaches 250 m2 on Band 20 alone and suits a building where every SIM rides O2, Vodafone or Three, but it leaves the EE Band 3 layer and 2G voice absent. The CD250 keeps both bands, raises output to >=17 dBm, and carries all four network families plus EE voice in one chassis, which matters the moment a household or business mixes providers.
Does the higher 17dBm output keep the CD250 within Ofcom licence-exempt rules, and what balances it?
Yes. The automatic gain control caps the radiated field so the >=17 dBm output sits inside the permitted-class limits, and no spectrum licence, fee, registration number or annual renewal is required as long as the unit is used as supplied, kept at least 10 m from its own donor antenna, and not modified to raise output. The higher figure is a rated maximum, not a fixed loud setting.
Why does the extra output reach the back office and garden room while gain stays identical to the CD200?
Once the donor signal is amplified by the same >=65 dB downlink margin, the remaining bottleneck is how loud that field arrives past walls. The >=17 dBm output supplies extra reach past the partition, bar counter and floor slab, so the back office and garden room still show bars where the CD200's >=10 dBm runs dry; identical gain handles the donor, higher output handles the distance indoors.
How does VoLTE on Band 20 and 2G voice on Band 3 behave across a 250m2 cafe or small office?
VoLTE calls from O2, Vodafone or Three handsets ride the boosted Band 20 LTE session, while EE-family handsets that do not use VoLTE complete circuit-switched calls over the repeated Band 3 2G layer. The booster carries whichever call the phone and network set up; it does not negotiate VoLTE itself, and a busy cafe with mixed SIMs gets both voice paths repeated across the 250 m2.
Why can higher output watts not create the 3G Band 1 or 5G layer the chassis does not tune?
Output power makes a tuned band louder indoors; it cannot synthesise an untuned band. The CD250 receives only Band 20 and Band 3 at the donor antenna, so raising output to 17 dBm cannot pull in Band 1 3G at 2100 MHz or any 5G band, and a 3G-only fallback or a 5G phone receives those services unboosted from the macro tower.
What is the practical CD100 to CD200 to CD250 ladder on area, users, output and gain?
The CD100 covers 100 m2 for 10 users at >=45/50 dB gain and >=10 dBm output; the CD200 covers 200 m2 for 20 users at the same >=10 dBm output but >=60/65 dB gain; the CD250 keeps the >=60/65 dB gain and raises output to >=17 dBm to cover 250 m2 for 25 users. The ladder steps area and users with output, while the gain jump happens once between the CD100 and CD200.
How does >=17dBm output pair with the <=5dB noise figure to keep calls clean at 25 sessions?
The <=5 dB noise figure keeps added hiss low while the louder >=17 dBm field fills the room, so 25 concurrent calls and data sessions arrive legible rather than merely loud. A high noise figure would make the phone select a noisy cell and degrade the uplink; the combination of controlled output and low noise is what keeps VoLTE clear across a busy office or cafe rather than raising the noise floor.
How does the 25-session ceiling work at a busy cafe or office, and what limits it beyond that?
The 25 rating counts active calls and data sessions on the boosted layer, not every registered device on the premises. Past 25 concurrent sessions the macro network itself schedules phones and assigns capacity, so the booster does not become the limit; the unit serves a lunch rush or meeting room, and a venue with far more concurrent users needs a larger-rated or second system rather than louder output.
Article Keywords
CD250 Band 20 Band 3 explained, 800MHz 1800MHz UK 4G booster, 17dBm output vs 10dBm, O2 Vodafone Three Band 20, EE Band 3 1800MHz, 832-862 791-821 1710-1785 1805-1880 MHz, 60dB 65dB gain, Ofcom licence exempt mobile repeater, dual band vs C250 single band, 250m2 25 users UK
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