Passive-intermodulation (PIM) is the silent killer of multi-carrier systems. Cheap antennas generate –120 dBc of self-noise, enough to wipe out a weak uplink. Indoor MIMO omni antennas are built with etched brass, silver-plated screws, and tuned gaps so PIM stays below –150 dBc with two 20 W carriers. The payoff is visible in stadiums where 500 phones transmit side-by-side yet the base-station still decodes a 0 dBm signal from the upper deck.
Inside the radome, the architecture mimics a miniature distributed antenna system. Four micro-strip arrays are printed on a Rogers 4350B substrate, then sandwiched to an aluminum ground plane only 0.8 mm thick. The entire assembly is 55 mm high and weighs 0.8 kg, light enough to clip into a standard 60 cm ceiling grid without extra seismic support. A laser-etched RF absorber ring between arrays suppresses edge currents that would otherwise raise the back-lobe and increase PIM.
Thermal stability is often overlooked. Office HVAC can swing from 18 °C at night to 30 °C on weekends. The radome is injection-molded ABS blended with 15 % glass fiber so its coefficient of expansion matches the PCB. Over –30 °C to +60 °C, the center frequency drifts < 20 MHz at 3.5 GHz, well inside the bandwidth of the matching network . That means no seasonal re-tuning, no truck roll, no angry finance director
