Answer first: the CD250 is a self-contained dual-band licence-exempt repeater that any owner, tenant or small-business manager can install without an RF contractor, a drilled roof panel, or a paid frequency licence, because a 250 m2 footprint needs one outdoor omni, one indoor whip, a 10 m outdoor cable, a 5 m indoor cable and a standard UK mains socket. The whole job breaks into ten factual stages: (1) confirm a usable outdoor 4G reading on your network's band, between -97 and -105 dBm, plus a usable 2G voice reading where calls matter; (2) choose a window, fascia or low-roof mounting point for the 5 dBi omni at 3 to 5 m height with a clear path to the serving mast; (3) keep the outdoor omni and the indoor whip at least 10 m apart vertically; (4) connect the included 10 m outdoor coax to the BS base-station donor port; (5) connect the included 5 m indoor coax to the MS mobile-station server port; (6) extend the outdoor run to 20, 30, 40 or 50 m only when the chosen point cannot be reached with 10 m; (7) place the amplifier chassis near the centre of the 250 m2 floor plan; (8) connect every antenna first, then power on through the AC 100-240 V to DC 5 V adapter into a UK mains socket; (9) read the LED indicators for 3 to 5 minutes while the automatic gain control locks onto the donor cells; (10) test a voice call and a 4G speed session in the weakest room before calling the job finished. The sections below expand each stage with the exact dBm ranges, heights, port labels, cable lengths, LED states and timings the CD250 needs for 250 m2.

CD250 Snapshot Before You Touch a Screw

The CD250 is rated for up to 250 m2 (2,691 ft2) of indoor coverage across a medium family home, a shop floor, a small office, a cafe or a pub, and for up to 25 simultaneous active phones. It ships with a 5 dBi omni-directional outdoor antenna, a 3 dBi whip indoor antenna, a 10 m low-loss outdoor coaxial cable, a 5 m low-loss indoor coaxial cable, and an AC 100 to 240 V 50/60 Hz adapter stepping down to DC 5 V with a UK 3-pin plug. It delivers at least 60 dB uplink gain and at least 65 dB downlink gain on both Band 20 (800 MHz) and Band 3 (1800 MHz), with a maximum output power of at least 17 dBm and a noise figure of no more than 5 dB. It holds UKCA, CE and RoHS certification and Ofcom licence-exempt status as a mobile repeater; no frequency licence, annual fee or formal installer sign-off is required.

The dBm ranges the CD250 expects at the outdoor antenna

Walk the boundary of your property with the phone screen visible and read the received signal strength in field-test mode. The CD250 amplifies two independent services, Band 20 4G plus Band 3 4G and 2G voice, so read the band your network actually uses rather than trust the subjective bar count on the home screen.

-97 dBm on 4G is a healthy reading that the omni will turn into a loud indoor signal once the >=65 dB downlink stage restores wall loss. -105 dBm on 4G sits at the edge of the recommended band but still produces a stable LTE session once amplified, because the CD250's larger downlink budget absorbs plasterboard, brick, double-glazing and even a floor slab more comfortably than a small unit. -112 dBm on 4G is the practical floor: below roughly -112 dBm the LTE receiver loses the control channel, the phone falls back to a slower service or shows no service, and no booster can manufacture a donor the outdoor antenna never receives. The >=17 dBm output margin absorbs wall and floor loss inside the home, but it cannot invent a band the outdoor survey never picked up.

How to read dBm rather than bars

Phone bars are subjective and differ between iPhone and Android; the dBm figure is the measurement the amplifier actually sees. On an iPhone, enter field-test mode from the dialer and watch the RSRP or RSRP value while standing still, then move around with the screen visible. On Android, use the engineering or network diagnostics menu under About Phone, or a free field-test app that lists RSRP by band. Hold the phone away from your body, because a hand and head absorb 3 to 5 dB, and read the value on the road-facing side rather than the rear garden if the mast sits on the road side.

Where to survey: front, rear, sides, upstairs and each window

Stand at the front gate, the rear fence, each side wall, the upstairs landing window, and at the proposed window or fascia position. Record the reading at each spot for your own SIM, because the strongest combined spot is where the omni goes. On UK two-to-three-storey homes the ground-floor windows read weak on Band 20 but the upper side elevation reads better; on a rural cottage the field-side gable usually beats the wall sheltered by a mature hedge. For a cafe or pub facing a high street, survey the street-facing door and front window at opening hours, because passing traffic does not change the tower but interior metal counters and fridge units do absorb signal.

Survey warning: if your band reads below -112 dBm at every outdoor position on your land, the CD250 cannot manufacture that band indoors. You may still improve voice on the other network's service, but the missing band stays missing. In that case raise the mounting point, add height, or consider the optional high-gain directional Yagi before committing rather than driving twice.

Why 3 to 5 m height matters at 250 m2

The omni should sit at 3 to 5 m above ground level on a wall bracket, drainpipe clip, fascia rail, low roof edge or chimney stack side, whichever gives the shortest clean run back to the chassis. Three metres clears garden fences, parked cars and hedge tops; five metres lifts the antenna above a single storey extension or a cafe's flat roof where a ground-level window reads weak. Extra height is useful only up to the point the donor becomes visible; beyond that the cable route back grows longer and adds passive loss without changing the received band.

Window frame, fascia bracket, drainpipe or low roof edge

A window-frame bracket suits a terraced or semi-detached home where drilling is limited; a fascia bracket suits a bungalow or cafe frontage; a drainpipe clip suits a party wall; a low roof edge or side gable suits a rural cottage and pub garden room. Keep the omni away from metal guttering, satellite dishes, air-con units and metal chimney pots by half a metre or more, because a metal object within the near field absorbs the 5 dBi pickup. Face the clear opening toward the strongest survey reading; with an omni the orientation is about clearance rather than pointing.

What to avoid at the mounting point

Do not mount the omni inside a sealed metal box, behind a solid stone chimney breast, under a tiled roof overhang that blocks the horizon, or on the rear wall facing a solid garden wall when the mast sits on the road side. Do not let tree leaves grow within a foot of the antenna at summer fullness, because deciduous canopy at 800 MHz still removes several dB. For a listed or rented building, a temporary window-sill bracket held with clips often satisfies both the landlord and the survey.

Why vertical beats horizontal separation

UK housing geometry usually provides vertical separation cheaply: the omni on the front first-floor fascia and the whip on the ground-floor hall or back office naturally sit more than 10 m apart through the floors. Horizontal separation on a single open-plan floor rarely reaches 10 m, and two antennas on the same wall within a few metres invite feedback. For a single-storey cafe or pub where vertical distance cannot reach 10 m, place the omni at one extreme end of the building and the whip at the far opposite end, route the donor toward the street, and keep metal shelving between them.

What the >=17 dBm output means for the separation rule

A higher output power fills more of the room, but it also makes feedback slightly easier to trigger, so the 10 m separation rule is not relaxed on the CD250. Keep the whip pointed toward the centre of the floor plan, never back toward the omni, and never mount both on the same elevation. If the LED for a band begins to flash rapidly during the AGC warm-up, separation is the first suspect, followed by a shortened cable or a mismatched port.

The 10 m outdoor cable goes to the BS port

BS stands for base station donor: this port faces the outdoor omni and carries the weak tower signal into the amplifier. The included 10 m outdoor coax runs from the omni down the wall, through a drainpipe or under a window sill, and into the chassis BS socket. Only the outdoor run is ever extended, because the outdoor antenna distance from the building varies; the indoor server distance is fixed by the chassis position.

The 5 m indoor cable goes to the MS port

MS stands for mobile station server: this port faces the indoor whip and carries the amplified signal out to the users. The included 5 m indoor coax leads from the chassis MS socket to the whip. Keep it intact and do not extend it, because every extra metre of passive indoor cable removes radiated power from the >=17 dBm output budget without reaching new floors; the upgrade ladder applies to the outdoor donor run, never the indoor whip lead.

BS and MS labels and swapping them

Swapping the two produces no coverage: the weak donor signal would be sent to the whip as if it were output, and the amplified output would be fed back toward the omni. The chassis and connectors are labelled on the unit itself; match the cable from the omni to BS and the cable to the whip to MS before tightening the adapters.

20, 30, 40 and 50 m outdoor upgrades

Every extra 10 m of outdoor coax costs roughly 2 dB of passive loss that the >=60 dB uplink and >=65 dB downlink budget absorbs but never returns. Measure the physical route along the wall, downpipe and corners, not the straight-line map distance, and buy the length that matches rather than the longest available.

Whip direction on an open-plan business floor

On a cafe or pub floor, place the whip where customers actually sit, not where the chassis is convenient, and point the whip vertically upright through an open ceiling space rather than burying it inside a metal cabinet. On a medium home, centre the whip in the hallway or main living room so both wings share the field; the >=17 dBm output reaches farther through partition walls than a smaller unit, but walls, metal fridges and a solid bar counter still attenuate.

Why placement beats a longer MS cable

If the furthest back office or garden terrace remains weak after correct placement, the answer is to move the whip toward that zone or add a second unit, not to extend the 5 m MS lead. Extending the indoor cable steals radiated output from the whole 250 m2 instead of solving one corner.

3 to 5 minute automatic gain control

After power on, the CD250 runs a 3 to 5 minute automatic gain control routine that listens for the donor cells on Band 20 and Band 3 and sets stable gain on both bands. During this window the LEDs settle rather than reach final brightness; do not adjust cables or unplug during the warm-up.

Reading the LEDs

A steady green band light means the band donor is locked and stable. A flashing band light means the donor is weak, the band is searching, or the unit is backing off from feedback. A rapidly flickering light across both bands points to separation too close or a reversed port. After 5 minutes, confirm a steady green on the band your network uses before starting the zone test.

How >=17 dBm output interacts with AGC

The automatic gain control still controls radiated output at the source; the >=17 dBm rating describes the maximum the chassis can deliver when the donor is weak and the indoor floor needs filling, not a fixed loud setting. When the donor is strong, AGC lowers gain automatically, so the higher output never overshoots into interference or oscillation.

What success looks like

A steady 3 to 4 bars and a clear call in the furthest room proves the >=65 dB downlink margin and the >=17 dBm output carried the signal through partition walls, brick, plasterboard and the floor slab. Test a second SIM on a different network too, because the dual-band chassis carries O2, Vodafone and Three on Band 20 and EE family on Band 3; a household or business mixing providers needs both bands locked.

What a weak corner means

A room stuck at zero bars after the walk needs a moved whip or a second unit, not a re-aimed omni, because the donor side was never the problem there. A whole band absent on the LEDs points back to the outdoor survey rather than indoor placement.

Think of gain as the volume a microphone picks up and output power as the loudspeaker reaching the back of the room. The CD200 already listens well, but its >=10 dBm loudspeaker reaches 200 m2; the CD250 keeps the same good listening and raises the loudspeaker to >=17 dBm, which pushes the field across extra partition walls, a solid bar counter, a shop floor or a back office to cover 250 m2 and serve 25 sessions. The <=5 dB noise figure stays the same, so the louder output arrives without a louder hiss, and VoLTE calls remain clean rather than merely louder.

Where the extra output goes across the 250 m2

Indoor free space cost budget starts at the whip and accumulates: plasterboard partitions cost a few dB each, an external brick wall costs 10 to 20 dB, a store or kitchen wall with appliances costs more, and a ceiling slab between floors adds its own loss. The >=17 dBm reserve absorbs that cumulative loss so the phone still shows bars in the upstairs bedroom, the back desk and the garden room, where a >=10 dBm unit would run dry around 200 m2. The output budget does not substitute for a healthy donor; it stretches an already-amplified signal farther through structure, not out of nothing.

Choose the Yagi when no indoor corner reaches but the outdoor reading is poor; choose a larger-rated second unit when the outdoor signal is healthy but the area clearly exceeds 250 m2; never solve an oversized footprint with a Yagi or a longer cable. The Yagi is a donor-side fix, the second unit is a geometry fix, and the >=17 dBm output is what carries a good donor across the existing 250 m2.

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