Engineering:Low PIM antenna
Low PIM antenna is a term used in radio-frequency engineering for an antenna whose passive intermodulation (PIM) performance is specified and guaranteed by the manufacturer at defined test power levels. "PIM" (passive intermodulation) refers to unwanted intermodulation products generated when two or more carrier signals pass through passive components with nonlinear behaviour, such as junctions, contacts, and ferromagnetic or contaminated materials.[1]
Why PIM matters
In cellular networks, downlink transmit carriers can mix in passive devices and generate third-order products (2f1−f2, 2f2−f1) that fall in or near the uplink receive band. Because the uplink is power-limited, even very weak PIM products can raise the effective noise floor and reduce uplink sensitivity. The effect is most visible in dense urban deployments where multiple carriers at high power share the same antenna.
Units and test conditions
PIM is expressed in dBc, i.e. decibels below the carrier power, and it is meaningless without the test power: a product level measured at two 43 dBm carriers (2 × 20 W) cannot be compared directly with one quoted at lower power, because third-order PIM rises approximately 3 dB for every 1 dB increase in carrier power. A specification therefore needs the PIM level, the test power, the carrier frequencies, and the order of the product. A typical cellular specification is −150 dBc at 2 × 43 dBm.[2]
Sources of PIM in antennas
- Dissimilar metals in contact and galvanic corrosion at junctions
- Solder joints, screws and other current-path discontinuities
- Plating materials with nonlinear magnetic properties
- Contaminated or oxidized contact surfaces, moisture ingress
- Loose or under-torqued connectors and cable assemblies feeding the antenna
Design measures
Antenna-level measures include one-piece radiating elements, current paths designed to avoid threaded joints, choice of plating, laser welding instead of mechanical fasteners in high-current areas, and symmetric structures that cancel unwanted coupling. Because every junction between the combiner and the antenna contributes to a measured PIM value, cable assemblies and assembly torque must follow a low-PIM build specification.
Measurement
The two-tone test method is standardized in the IEC 62037 series: two amplified carriers are combined and fed to the device under test, and the returning intermodulation product is measured with a dedicated analyzer; for antennas, the test is typically performed as a reflected-PIM measurement with the antenna in an anechoic chamber or on an open test range.[1]
See also
References
- ↑ 1.0 1.1 IEC 62037 series, Passive RF and microwave devices, intermodulation level measurement.
- ↑ What is Low PIM antenna? — RF element
