Answer first: the CVCD is a self-contained dual-band licence-exempt vehicle repeater an owner can fit at the kerb without a workshop, a roof drill or a paid frequency licence, because a cabin or small caravan interior starts with one magnetic omni, one whip, a 5 m coax and a 5 V DC lighter socket, with an optional 240 V AC adapter for a static pitch. The whole job breaks into ten factual stages: (1) read the outdoor 4G dBm on Band 20 and Band 3 at the kerb before mounting; (2) place the 5 dBi magnetic omni at the centre or front of a steel roof, or on an adhesive pad and screw bracket for aluminium and fibreglass roofs; (3) lay the 5 m coax through the door rubber seal or a window gap rather than pinching it in a closing door; (4) connect the outdoor cable to the BS base-station donor port; (5) connect the whip to the MS mobile-station server port; (6) plug the 5 V DC cigarette-lighter adapter last in the sequence; (7) switch on and let the automatic gain control settle for 3 to 5 minutes; (8) read the steady green band LEDs; (9) test a call and a 4G session from the driver's seat and passenger seats; (10) check the magnetic hold at highway speed before a long rural route.

CVCD Snapshot Before You Fix the Magnet

The CVCD is rated for vehicle cabin and small caravan interior improvement, roughly a 100 m2 space inside the vehicle or pitch unit, and for up to 15 simultaneous active phones. It ships with a 5 dBi magnetic-mount omni-directional outdoor antenna, a 3 dBi whip indoor antenna, a 5 m low-loss coaxial cable, and a 5 V DC car cigarette-lighter adapter; an optional 240 V AC adapter powers a static caravan off the mains hookup. It delivers at least 45 dB uplink gain and at least 50 dB downlink gain on both Band 20 (800 MHz) and Band 3 (1800 MHz), with a 10 dBm maximum output, a noise figure of no more than 5 dB, and automatic gain control. It holds UKCA, CE and RoHS certification and Ofcom licence-exempt mobile-repeater status, and fits a cigarette-box size chassis on the dash or a seat pocket.

The dBm ranges the CVCD expects at the roof antenna

Stand beside the vehicle with the phone screen visible and read the received signal strength in field-test mode. The CVCD 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 the magnet turns into a usable cabin signal once the >=50 dB downlink gain and 10 dBm output restore the body-shell loss. -105 dBm on 4G sits at the edge of the recommended band but still produces a stable LTE session once amplified, because the gain margin absorbs steel, tinted glass and the pitch-side shelter more comfortably than a phone held to the window. -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 roof antenna never receives. The >=45/50 dB gain recovers a weak roadside donor better than holding a phone to the glass, but it cannot invent a band the survey never picked up.

How to read dBm rather than bars in a vehicle

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 value while standing beside the vehicle. 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 side of the vehicle facing the mast rather than the sheltered kerb side.

Where to survey: road side, campsite pitch, motorway layby

Stand on the road-facing side, the open field side at a campsite, and the far side of the layby; record the reading for your own SIM and a passenger's SIM on a different network, because the strongest combined spot is where the magnet goes. On a rural lane the open-field gable or roof edge usually beats the side sheltered by a hedge or a bank; on a motorway the hard-shoulder side facing the tower usually beats the slip-road side.

Survey warning: if your band reads below -112 dBm at every position beside the vehicle, the CVCD cannot manufacture that band inside. You may still improve voice on the other network's service, but the missing band stays missing. In that case re-park facing the open horizon or move the pitch, rather than expecting the amplifier to close a gap the roof never sees.

Steel roof: centre or front, clear horizon

Wipe the roof dry, peel any wax or polish from the contact circle, and press the magnet to the centre or front third of the roof, away from roof rails, sunroof seams and the tailgate spoiler. The front-third position favours forward travel on rural routes; the centre position balances all directions for a campsite pitch. Keep the omni at least a metre clear of roof-mounted air horns, spotlights and satellite dishes, because metal near-field objects absorb the 5 dBi pickup.

Aluminium and fibreglass roofs: adhesive pad or screw bracket

A magnet will not hold on an aluminium van roof or a fibreglass motorhome shell. Clean the chosen patch, use the supplied adhesive mounting pad, or fit the small screw bracket where a permanent fix is acceptable; the same 5 dBi omni then sits on the pad or bracket rather than a bare magnet. Never rely on the magnet alone on a non-magnetic roof, even at low speed.

The fishtail three-magnet upgrade for high speed

The optional fishtail aerial with three magnets and a longer 3 m coax spreads the hold over three contact points, which is preferred for long HGV or motorway runs where a single disc magnet can shift in crosswind. It is an upgrade rather than a requirement for a short commute, and it keeps the same Band 20 and Band 3 tuning.

Door rubber seal route

Feed the cable from the roof, down the A-pillar channel or along the gutter, and tuck it under the rubber door seal at the front corner; the seal grips the cable while leaving the door able to close. Never trap the cable in the metal pinch point where the door meets the body, because a closed door cuts the coax and damages the shielding over time.

Window-gap route for tinted or sealed vans

For a panel van or a vehicle with sealed doors, lower one side window a few centimetres, lay the cable through the gap, and close the window onto the insulated section rather than the bare copper; use a window weather strip to keep the gap draught-free overnight.

Dash tidy and avoiding the driver's feet

Tuck the cable under floor mats and along the door sill, then bring it up to the chassis on the passenger side so it never crosses the brake or accelerator pedal. A seat pocket or a small dash mount holds the cigarette-box chassis, and the whip sits on the dash or a rear parcel shelf rather than on the driver's line of sight.

BS donor port faces the roof omni

BS stands for base station donor: this socket faces the outdoor magnetic omni and carries the weak tower signal into the amplifier. The cable from the roof magnet plugs into the BS port before any other connection.

MS server port faces the whip

MS stands for mobile station server: this socket faces the indoor whip and carries the amplified signal out to the phones. The whip lead plugs into the MS port.

Swapping the two and what it looks like

If the roof cable lands on MS and the whip on BS, the weak donor signal is sent to the whip as if it were output, and the amplified field feeds back toward the omni; the LEDs stay dim or flicker and the cabin shows no improvement. Match the labelled ports, finger-tighten the adapters, and check once more before powering on.

5 V DC in motion

Plug the lighter adapter into the 12 V socket after both antennas are connected, not before. Most vehicle lighters are live with the ignition, so the chassis powers up and down with the key; run a short extension only where the socket sits behind a seat and never leave the adapter loose under the pedals.

Optional 240 V AC on a static pitch

A touring or static caravan on hookup uses the optional 240 V AC adapter instead of the lighter socket, so the repeater runs all evening without draining the leisure battery. The same BS/MS cabling applies; only the power brick changes, and the 5 V output to the chassis stays identical.

Low radiation and the dash placement

The 10 dBm output is a low-power in-cabin field; place the whip a short distance from passengers rather than directly beside an ear, and keep the chassis on the dash or seat pocket rather than under the seat where heat builds. The operating range -10 to +55 C suits a parked vehicle cabin without extra cooling.

3 to 5 minute automatic gain control

After power on, the CVCD 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 unplug or re-position during the warm-up, and keep the vehicle parked facing the open horizon.

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 the whip placed too near the roof omni, a reversed port, or the vehicle parked inside a metal garage. After 5 minutes, confirm a steady green on the band your network uses before driving off.

How >=45/50dB gain and 10dBm output interact with AGC

The automatic gain control balances recovered donor signal against the 10 dBm output so a strong donor backs off automatically and a faint roadside donor is recovered fully; the >=45/50 dB rating describes amplification of the weak donor, and 10 dBm describes the low-power in-cabin field. The two work together so the chassis never overshoots into oscillation even when parked close to a mast.

The hold test before the motorway

After the magnet is pressed home, tug gently from all four sides and confirm no edge lifts; on the fishtail three-magnet version, check all three discs sit flat. Start at 40 mph, watch through a mirror on a clear stretch, then build to 70 mph only after the first hold test passes. A single disc magnet on a waxed roof can slide in crosswind, so re-wipe the contact circle after every car wash.

What happens if the cable pulls

The coax strain relief at the magnet should take tension off the disc itself; route the cable with a small loop at the roof edge so wind catches the loop, not the antenna. If the fishtail upgrade is used for long HGV runs, secure the extra magnets with the tie points supplied with the kit.

What success looks like

A steady 3 to 4 bars and a clear call on the driver's seat, the co-driver seat, the rear bench and the caravan dinette proves the >=50 dB downlink margin and 10 dBm output carried the signal through tinted glass and the body shell. 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 mixed-carrier family trip needs both bands locked.

What a weak seat means

A rear seat stuck at zero bars after the test means the whip needs moving toward that row rather than re-aiming the roof 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 whip placement.

Think of gain as the microphone's sensitivity and output as the loudspeaker's volume. The >=45 dB uplink and >=50 dB downlink gain lift the weak donor the steel roof hides, while the 10 dBm output spreads the recovered signal across the driver, co-driver and passenger seats. The <=5 dB noise figure keeps the recovered call legible rather than louder but garbled, which matters when several family members or colleagues share the trip.

Where the gain and output go inside the vehicle

Tinted glass costs a few dB, a steel door skin costs more, and a caravan's insulated wall and window frame add their own loss. The >=50 dB downlink margin absorbs that cumulative loss so the phone shows bars on the rear bench and the dinette, while the 10 dBm output stays low enough for the licence-exempt vehicle class and keeps radiation modest inside the cabin. This is an in-vehicle improvement, not a building repeater: the rated space is the cabin or a small caravan interior, roughly 100 m2, not an open farmyard.

Gain cannot fix a missing band or a sealed underground car park

Even the full >=45/50 dB gain cannot pull in a donor the roof antenna never sees, so a sealed multi-storey or underground car park stays dead regardless of the booster, and a band the survey missed remains missing indoors. The gain recovers a faint but present roadside signal; it does not synthesise a new layer.

Choose a better parking or pitching position when no seat reaches but the outdoor reading is poor; choose a larger building-rated unit only when the vehicle is permanently sited and the space clearly exceeds the cabin or small caravan limit; never solve a missing donor by routing a longer cable or raising the output. The re-positioning move is a donor-side fix, and the >=45/50 dB gain plus 10 dBm output carries a good donor across the existing interior.

Article Keywords

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