LMR Feedline Moisture Intrusion and Hidden Coverage Loss in Aging Public Safety Tower Infrastructure

LMR Feedline Moisture Intrusion and Hidden Coverage Loss in Aging Public Safety Tower Infrastructure

Moisture intrusion into feedlines is typically treated as a maintenance-related failure of the Feedline itself; however, in public safety lmr systems, it has evolved into a systematic coverage degradation process. Water entering coaxial transmission lines, jumper assemblies, weather sealing transition areas, and antenna feedpoint transitions causes the electrical behavior of the RF path to change. Typically, this results in a coverage loss that is not immediately apparent (i.e., there is no instantaneous failure). Instead, the degradation occurs gradually as the insertion loss of the Feedline grows due to increased absorption and/or reflection losses caused by the presence of water. The degradation can also manifest as unstable impedance matching between components in the RF path.

As towers age, the exposure to potential Feedline failures increases. Public safety agencies continue to use VHF, UHF, 700 MHz, and 800 MHz lmr systems and add Broadband in-building coverage, microwave systems, and interoperability systems at the same sites. With higher loads on tower tops and associated structures, cable lengths increase along with the number of connectors used to connect cables together. Additionally, with more people moving about the tower and associated structures (e.G., engineers, climbers), the likelihood of someone accidentally disturbing a connection increases. The result can be progressive, yet potentially unobvious degradation of coverage performance. Marginal audio quality issues may initially present themselves as missed affiliation calls, increased bit error rate (ber) or poor performance of hand held devices at the edges of the coverage footprint.

Changes caused by electrical properties of a wet transmission Line

Coaxial cables were designed with consistent dielectric properties, precise conductor spacing, and a well-defined impedance profile. However, once a coaxial cable is exposed to moisture, the dielectric properties of the insulation surrounding the center conductor are altered. Specifically, moisture will alter the dielectric constant and loss tangent of the material. These changes will create an increase in attenuation and a change in impedance. The impedance change will typically produce a value that deviates significantly from its nominal value (typically 50 ohms for LMR type coaxial cables).

The effects of increased attenuation are felt equally in both directions. In the forward direction (from transmitter to antenna), additional loss in the Feedline will decrease the amount of effective radiated power (ERP) delivered to the antenna prior to radiation. Conversely, in the reverse direction (from antenna to receiver), the same loss will occur before the received signal is processed by the receiver’s front-end. Since loss in excess of 10 dB in the receive path can decrease system sensitivity in proportion to the amount of loss, a relatively small amount of loss in the receive path (prior to processing) can damage the receive system more severely than would be experienced if similar amounts of loss occurred further down-stream in the receive chain after amplification/filtration.

Additionally, moisture can produce frequency-dependent behavior. Attenuation may be greater at different frequencies within the band-pass resulting in distortion that cannot be measured using a single insertion loss measurement taken at only one frequency. In multi-channel LMR systems, this means that certain channels may perform satisfactorily while adjacent channels exhibit reduced margins.

VSWR change and antenna protection logic

Moisture entry into a coaxial transmission Line usually creates VSWR changes because the feed-Line no longer provides a stable impedance match to either the transmitter/duplexer/combiner/antenna. As a result, increasing levels of reflected power are created as the mismatch between the antenna/feed-point and Feedline grows. Most modern transmitters contain protective logic which reduces transmitter power when reflected power exceeds predetermined thresholds. Although transmitter power reduction protects the transmitter from possible damage (overheating), it simultaneously decreases coverage and does so without necessarily generating an easily recognizable alarm at the dispatcher’s console.

Furthermore, in high-duty-cycle public-safety sites (those receiving large volumes of communication requests), temperature fluctuations can make the moisture-induced fault intermittent. A wet section of coaxial cable may behave differently under continuous transmitter loading versus periodic short-term maintenance tests. During transmitter operation, heat generated by transmitted RF power can cause redistribution of moisture within the cable and modify dielectric properties resulting in varying degrees of impedance mismatching with respect to transmitter/duplexer/combiner/antenna ports.

Consequently, coverage can deteriorate in accordance with operational requirements rather than static test conditions. Combiner and duplexer networks may amplify such effects since they depend upon predictable impedance values at each port. Impedance mismatches at feed-lines can disrupt isolative and tuning behaviors thereby increasing insertion loss and reducing rejection of adjacent channels affecting multiple transmit/receive channels.

Impact to receive path and edge coverage performance

The receive path is particularly susceptible to feed-Line moisture because all receive-path loss occurs before reaching any first-stage active receive amplifier. In cases where receivers are mounted atop towers and water enters beneath the antenna feeding into the tower-mounted receiver, the feed-Line loss can still reduce carrier-to-noise ratio (CNR) before any filtering or distribution. In cases where receivers are mounted in shelters or buildings and water enters into feed-lines running vertically up to the receiver location, feed-Line loss before the first active amplifier can be even greater.

P25 systems require sufficiently good signal quality to reliably decode frames. Coverage does not fail simply when signal strength drops to zero. Rather, coverage degrades as snr/cinr falls below required receiver specifications. Moisture-induced feed-Line losses can move marginal portable radio locations into unreliable decoding areas while mobile radios or high-power subscriber units appear functional.

Therefore, it is common for a site to meet general transmitter power specification checks and yet under-perform for hand-held users in buildings or low-elevation terrain areas or overlapping coverage areas. Consequently, a “coverage problem” appears rather than a transmission-Line problem.

Environmental stress accumulation over time and aging tower structures

Ultraviolet degradation of jacket materials, freeze/thaw cycles of tower structures including cable routes, compressions of gaskets on connectors from repeated opening/closing, vibrations of tower structures from wind loads or climber footfall or other activities, wind-induced movements on cables from wind gusts or structural deflections etc., improper strain-relief techniques on cables attached to structures all contribute to an increased likelihood of water entering feed-lines. Once water enters a feed-Line, capillary action and gravity can transport water past the initial entry point.

Tower structures supporting older public-safety systems are increasingly being asked to support more RF functions than ever before supported. Additional antennas, cable trays, attachment hardware for additional antennas and Broadband equipment can create increased mechanical stresses on existing feed-lines. Bending radius violations on cables and connector overstress can develop as a result of added attachments to tower structures over time.

Pressure to modernize increases importance of failure mode

Pressure to modernize public-safety systems creates an additional importance on avoiding moisture-related degradation in passive RF infrastructure. While agencies are adding LTE, 5g satellite-backhaul and mission-critical Broadband services to their existing LMR voice services, it remains necessary for RF plants to maintain stability across both legacy communications services and new/emerging communications services.

Shared-site operations create shared-site interoperability risks

Multiple agencies often share common tower structures at shared-site locations. A faulty feed-Line for one agency’s system can indirectly affect how tower crews plan operations at the shared-site location. Shelter access and antenna positioning may be shared among agencies at shared-site locations. When degraded coverage is observed during mutual aid or regional interoperability exercises; it is possible that the source of the degraded coverage was a faulty passive component rather than an interoperability protocol issue.

Increased spectrum congestion makes consequences of hidden coverage degradation greater

Due to increased spectrum congestion within crowded 700 MHz and 800 MHz environments; public-safety agencies rely upon established link-budget models for predictably sized signals arriving at receivers from distant locations via established antenna patterns and noise floors that remain below impairment thresholds. Moisture-degraded passive RF infrastructure alters these assumptions; reducing receive sensitivities allows adjacent signals to interfere with impaired channels; reducing cinr thresholds allows Broadband noise to exceed previous impairment thresholds.

Adding Broadband capabilities to existing Hybrid LMR/Broadband deployments increases operational complexity. Increased physical densities at tower sites placed additional pressures on existing passive RF infrastructure. Existing LMR systems remain as primary voice-layer systems; however passive RF pathways must functionally operate within an increasingly dense and mechanically complex site environment.

Routine sweep-testing can identify many severe feed-Line faults; however detection of early moisture-entry into a cable may be challenging. A cable can remain within acceptable return-loss limits during periods when dry and at low test-power levels while exhibiting degradation during rainfall, freezing temperatures or high-load RF transmissions. Frequency-selective loss and intermittent impedance-shifting necessitate measurement methods capable of simulating realistic operating conditions.

Time-domain reflectometers can assist in locating impedance-discontinuities in cables; however interpretation is highly dependent on severity of fault(s) and testing conditions. Measuring received power at a shelter does not provide assurance that received power is sufficient for proper operation unless actual feed-Line loss is known and remains stable. Similarly comparing receiver-noise-floor readings over time may indicate degradation; however comparisons must be made relative to historic baseline measurements made at the same site using identical configurations.

Maintenance programs must consider trends in passive component condition

Reliable maintenance programs consider trends in passive-component condition rather than making single-time pass/fail assessments. All changes in insertion-loss-return-loss-reflected-power-receiver-sensitivity-and-complaints regarding coverage-performance should be correlated over time.

Maintaining moisture-related degradation of coverage performance requires careful design principles applied to passive RF components. Connector selection and controlled installation torques ensure reliable connections; quality weather-sealing ensures that connectors do not leak; ensuring proper strain-relief reduces bending-stresses on cables; placing drip-loops near feeds ensure that drips flow-away-from-components; ensuring adequate cable-routing minimizes exposure to environmental elements.

Low-insertion-loss designs minimize loss-of-margin available to environmental variations; thus minimizing degradation-of-performance over time. Accurate predictable designs reduce diagnostic burden when complaints occur

Using accurate designs with predictable filter/duplexer/combining/transmission-Line interface behaviors simplifies diagnostics when complaints occur. Technicians can rapidly isolate external problems because baseline performances are repeatable.

Tx-rx systems emphasizes design of RF passive infrastructure focusing upon low-insertion-loss-designs-mechanical-stability-long-term-passive-performance. Maintaining stability in aged-lmr-tower-environments helps protect remaining coverage-margin as system-density-increases-and-interoperability-expectations-grow.