Source of Loss
Coaxial cables generate heat loss due to the dielectric loss between the inner and outer conductors and the resistance of the conductors. The higher the frequency and the longer the cable, the greater the insertion loss. A typical RG174 thin cable can reach several dB per meter in the GHz frequency band, while low-loss foam dielectric or corrugated hardline cables perform significantly better over the same length. For IoT gateways, if the module is placed inside a cabinet and the antenna is mounted outdoors, the feeder can reach several meters. If a low-loss cable is not used, the uplink EIRP may be insufficient to maintain the base station's reception threshold.
Relationship with Impedance and Matching
The characteristic impedance of the cable should match the rest of the RF link (usually 50 Ω). Connectors and adapters must be well-matched, otherwise reflections and additional loss will occur at the joints. Poor-quality connectors or moisture-affected cables not only increase loss but may also introduce intermodulation products, interfering with reception sensitivity. During installation, avoid sharp bends and compression, and maintain the connector torque specification.
Engineering Estimation
In link budget calculations, feeder loss should be accounted for separately at the transmitting end and the receiving end: deducting from module-to-antenna power in the uplink direction, and from antenna-to-module received power in the downlink direction. If using a multi-band antenna, the worst-case loss should be estimated according to the highest operating frequency. For multiple diversity paths, asymmetrical feeder lengths can introduce phase differences, affecting MIMO performance, so symmetric wiring or compensation should be implemented in the design.
Alternatives
Under permissible conditions, shortening the feedline or placing the module closer to the antenna can reduce loss; if EMC and heat dissipation permit, onboard antennas or FPC direct connection can be used. If a long feedline must be used, higher-grade cable should be selected and margin reserved in the budget to avoid critical coverage. Outdoor installations need to consider the effects of UV and water ingress on aging loss.
Engineering Practice Supplement
Regarding 'feedline loss,' it is recommended to distinguish between real-time control links and non-real-time telemetry links in the architecture: control and interlock should be locally closed-loop whenever possible, with cellular used for monitoring, configuration, and OTA. On the cloud platform side, use observability metrics (connection duration, message latency distribution, error codes, reconnection reasons) to drive optimization, rather than just evaluating 'whether it works.' Documentation deliverables include interface specifications, error code tables, extreme operating conditions, and known limitations; test deliverables include test cases, data, and conclusions, facilitating third-party retesting or audits.
Take the link budget at the highest band
Feeder loss means the coaxial cable turns part of the transmitted and received energy into heat. The longer the run, the higher the frequency, and the thinner the cable, the greater the loss. It comes straight out of the link budget: on the uplink it reduces power arriving at the antenna, and on the downlink it reduces power arriving at the module. The conditions are that the cable’s characteristic impedance matches the stages on either side, that the connector types match, and that the bend radius has not been crushed. On a higher band, each extra length of thin flexible cable can push indoor coverage that was barely usable past the base station’s receive threshold.
A multiband antenna cannot be estimated on the lowest band alone. The highest band usually loses the most, so the margin should be left there. If two diversity runs are one long and one short, the unequal loss and delay hurt combining. Route them as equal as practical, not whichever side is convenient.
A damp connector adds loss later
One VSWR measurement at installation does not mean the same result after the rainy season. Water and oxidation turn a connector into extra resistance and a reflection point. Receive sensitivity worsens first, then transmit efficiency. A sharp bend, a footstep, and an over-tight cable tie change the dielectric, and the loss is added slowly. A poor adapter can also produce intermodulation. The interference does not always look like “no signal at all.”
When the module can be moved closer to the antenna, shorten the feeder before choosing a higher-power module. When a long run is required, use a lower-loss cable and leave aging margin in the budget. A matching network does not compensate for heat loss in the cable. Port reflection is handled in impedance and matching. If the product’s radiation is over the limit, first confirm the feeder and connectors have not become an accidental radiator, then compare with conducted spurious emissions.
Count adapters and arresters in the loss
A budget often counts only cable meters and leaves out adapters, lightning arresters, and weatherproof connectors at both ends. Each adapter has insertion loss. At high frequency, a few of them stacked together can cancel the benefit of a thicker cable. An outdoor arrester used on the wrong impedance or band also becomes an extra reflection. The installation drawing should list every part from the module port to the antenna port, and acceptance should check against that list, not only the type printed on the cable jacket. Omitting an arrester, or adding an adapter that was not in the design, means the link is no longer the one in the budget. Photograph the model of every adapter and arrester at acceptance. Do not photograph only the antenna. Miss one, and a retest cannot be compared with the design budget.