The C100 covers Band 20 (800 MHz) only, so it boosts O2 and Vodafone 4G but not EE Band 3 4G; the D250 covers Band 3 (1800 MHz) only, so it boosts EE 4G and voice but not O2/Vodafone Band 20 4G. The CD100 combines both bands in one box, covering O2, Vodafone, Three (via O2 MOCN) and EE 4G across up to 100 m² for 10 users. Choose the CD100 when your household or office mixes operators, the C100 if you are O2/Vodafone-only, and the D250 if you are EE-only and want its larger 250 m² footprint.

Band 20 (800 MHz) is the rural propagation band used by O2 and Vodafone for 4G, reaching further over distance and through obstacles; Three 4G rides Band 20 over the shared O2 MOCN. Band 3 (1800 MHz) carries EE 4G alongside the EE 2G GSM layer for calls and SMS. The CD100 amplifies both simultaneously, so an O2 phone and an EE phone in the same room both get boosted 4G without switching boxes.

Up to 100 m² (1,076 ft²) means a single open-plan room, a one-bedroom flat, a cottage living room plus kitchen, a home office or a compact shop. It is not a whole-house solution for a three- or four-bedroom detached or a two-storey property; the 4-star review from Lisa P. notes it covers the living room and kitchen but a bigger model would be needed for the whole house. Solid walls, metal shelving and distance from the whip shrink the usable area.

The CD100 supports up to 10 phones or data devices connected at the same time, sharing the boosted Band 20 and Band 3 capacity. This suits a couple or small family, a solo home office with a few staff phones, or a small reception. Beyond 10 active users the shared RF capacity thins out; a busy shop floor or a whole team needs a higher-capacity unit. It is connected capacity, not a guaranteed 10 fast-speed lines at once.

Gain in dB measures how much the amplifier strengthens the signal: ≥45 dB uplink lifts your phone's signal back up to the mast, and ≥50 dB downlink lifts the mast's signal down to your phone. Output power in dBm measures the final re-broadcast strength; the CD100 caps at ≥10 dBm (about 10 mW) under Ofcom's licence-exempt limit. That is plenty for 100 m² indoors but deliberately lower than a high-power repeater, which is why it cannot cover a large building.

UK licence-exempt mobile repeaters are restricted to a low maximum output (≥10 dBm per this unit's class) to protect the mobile network from interference; the Wireless Telegraphy (Mobile Repeater) Exemption framework sets these bounds so no operator licence or paid spectrum authorisation is needed. A higher-output amplifier would require licensing and is not what the CD100 is designed for. The trade-off is that it is legal, plug-and-play and low-cost, but confined to a small 100 m² area.

The supplied 5 dBi omni works well when you have a clear, unobstructed mounting spot with 1–2 bars outside and do not need to aim at a specific mast. Where donor signal is very weak, or the mast lies in one direction behind a ridge or trees, the paid high-gain directional aerial (+£50) focuses the pickup on that mast and often makes the difference between marginal and solid. Start with the omni, read the bar count, and upgrade only if signal stays weak.

At least 1–2 bars of usable outdoor signal on Band 20 and/or Band 3 at the omni mounting point, checked with a phone outside on the wall, roof or eave. A booster amplifies the donor signal it receives; it cannot create signal where none exists outside. If your phone shows zero bars and emergency calls only even at the mounting point, this unit will not work, and neither will any Ofcom-legal booster at this price.

The CD100 carries a 365-day money-back guarantee: if it does not work for your location, return it by mail within 365 days for a refund; return shipping is deducted from the refund (ReturnShippingFees), so it is not a free-return label. Standard delivery is free: handling takes 1–2 working days and UK transit 10–15 working days, so it is not next-day, and the same policy covers 15 European countries (GB, DE, FR, IT, ES, NL, BE, AT, SE, DK, FI, IE, PT, PL, CZ).

The CD100 boosts 4G LTE data and 2G calls/SMS; calls over 4G work when your handset and network offer VoLTE (Voice over LTE), which O2, Vodafone, EE and Three support on compatible plans. It does not boost 5G NR on any band and it does not boost 3G UMTS Band 1 (2100 MHz), so a device stuck on 3G fallback will not be helped. If 5G speed or 3G fallback is the main requirement, a different unit on those bands is needed.