Key Takeaways
- 3 dB/m threshold: When cable loss exceeds 3 dB/m at the operating frequency, switch to a lower-loss type — no downstream amplifier recovers the noise figure lost in the feed cable.
- 50 Ω vs 75 Ω is non-negotiable: A 50 Ω / 75 Ω mismatch produces a 14 dB return loss and degrades receiver noise figure by up to 1 dB — never mix impedance standards in an RF chain.
- Velocity factor governs electrical length: Solid-PE cable (VF = 0.66) is electrically 52 % longer than its physical length; always divide target wavelength by VF before cutting quarter-wave stubs or delay lines.
- Double-braid above 1 GHz: Single braid delivers ~55 dB shielding; foil-plus-braid reaches 90 dB or more, the minimum for CISPR 25 Class 5 automotive compliance.
What Is a Coaxial Cable?
A coaxial cable is a high-frequency transmission line featuring a central conductor and one or more concentric outer shields. Sharing a common geometric axis, these layers allow the cable to carry signals with controlled impedance and minimal radiation loss.
Internal Construction and Materials
- Center Conductor: Bare copper, silver-plated copper, or copper-clad steel (CCS) for high tensile strength.
- Dielectric: Materials like Solid PE, Foamed PE (lower loss), or PTFE (high-temp) govern the Velocity Factor (VF) and signal attenuation.
- Shielding & Jacket: Braided copper or aluminum foil layers provide protection, finished with a PVC, PE, or LSZH jacket.
Why Coaxial Cable Is Indispensable for Engineers
Coaxial cable’s self-shielding geometry suppresses EMI and external interference, outperforming unshielded twisted pairs above 100 MHz. It is the primary interconnect for antenna feed lines, CATV, vehicular RF, and instrumentation operating up to 18 GHz.
What Are the Key Features and Advantages of Coaxial Cable?
| Feature | Description | Engineering Benefit |
| Controlled characteristic impedance | Impedance set by conductor diameter ratio and dielectric constant: Z0 = (138 / sqrt(Er)) x log10(D/d), standardised at 50 Ω or 75 Ω | Eliminates reflections and standing waves when matched to source and load; critical for flat frequency response across multi-octave RF links |
| Low attenuation vs. frequency | Attenuation rises with sqrt(f) due to skin effect; RG-58 loses 0.66 dB/m at 100 MHz, LMR-400 loses 0.22 dB/m at the same frequency | Lower cable loss directly extends wireless link range and reduces required transmit power, shrinking PA heat sink size and supply current |
| High shielding effectiveness | Foil-plus-braid shields achieve 90–100 dB isolation from 10 MHz to 3 GHz; measured per IEEE 1560 or IEC 62153-4-3 | Meets CISPR 25 Class 5 automotive RF immunity and FCC Part 15 emission limits without additional EMI filtering in the RF signal path |
Why Attenuation Coefficient Drives Cable Selection More Than Impedance
Because attenuation scales with sqrt(f), a cable acceptable at 100 MHz can lose three times as much power at 900 MHz. Always evaluate the attenuation specification at the highest operating frequency and budget a further 3 dB for connector loss and temperature derating before finalising cable type.
What Are the Technical Specifications to Watch?
| Parameter | RG-58 / Typical | LMR-400 / Typical | Unit | Compliance |
| Characteristic impedance | 50 ± 2 | 50 ± 1 | Ω | IEC 61196-1 |
| Attenuation @ 100 MHz | 0.66 | 0.22 | dB/m | IEC 61196-1 |
| Attenuation @ 1 GHz | 2.3 | 0.73 | dB/m | IEC 61196-1 |
| Velocity factor (VF) | 0.66 | 0.85 | — | IEC 61196-1 |
| Max operating voltage | 500 | 1,000 | V RMS | UL 444 |
| Shielding effectiveness | 55 (single braid) | 90 (foil+braid) | dB | IEC 62153-4-3 |
| Operating temperature | -40 to +80 | -40 to +85 | °C | UL 444 / MIL-C-17 |
| Centre conductor | Cu, 0.9 mm solid | Cu, 2.74 mm solid | — | ASTM B33 |
| RoHS / REACH | Compliant | Compliant | — | EU 2011/65/EU |
How Do These Specifications Affect Real-World Performance?
- Attenuation with temperature: Attenuation rises ~0.2 % per °C above 25°C; a 10-metre RG-58 run at 6.6 dB loss at 25°C reaches 7.9 dB at 85°C, enough to drop a marginal GPS link.
- Velocity factor and electrical length: Resonant structures (quarter-wave stubs, balun windings) must be cut to physical length = target wavelength x VF; a 10 % VF error shifts a 900 MHz notch filter 90 MHz off-band.
- Shielding and CISPR 25: Single-braid RG-58 at 55 dB shielding fails CISPR 25 Class 5 by 35 dB above 1 GHz; foil-plus-braid or semi-rigid construction is required.
What Are the Cable Types and Configuration Options?
Cable Form Factors
Flexible cables (RG-58, LMR-series) are ideal for field routing and repeated flexing. For radar and base stations, semi-rigid cable offers maximum shielding (>100 dB) and phase stability (<1 °/°C} drift), while conformable versions provide similar performance with hand-formability. For space-constrained UAV or smartphone payloads, micro-coax (0.81 mm OD) is the standard .
Dielectric and Jacket Material Variants
Solid PE is the economical choice for sub-1 GHz use, while foamed PE reduces attenuation by 40% for longer runs above 500 MHz. PTFE is mandatory for MIL-C-17 aerospace applications up to +200°C. For protection, specify PVC for indoors, PE for burial/UV, or LSZH for rail and public safety compliance (IEC 60332-1).
How Is Coaxial Cable Used in Real-World Application Scenarios?
- 5G Small-Cell Antenna Feed: At 3.5 GHz (5G NR Band n78), a 5-metre feed cable must limit attenuation to 2 dB to preserve link budget; LMR-240 (0.38 dB/m at 3.5 GHz) achieves this in 5 m while fitting within the 19 mm diameter constraint of a small-cell radome enclosure.
- Automotive GNSS/LTE Roof-Mounted Antenna: In-vehicle RF cables must pass CISPR 25 Class 5 shielding, survive −40°C to +125°C under-hood excursions, and maintain impedance within 50 ± 2 Ω throughout; RG-58 variants with foil-plus-braid shield and XLPE jacket qualified to ISO 14572 are the standard solution.
- Industrial IoT Gateway to External Antenna: An outdoor LoRaWAN gateway at 868 MHz with a 15-metre mast run requires LMR-400 (0.22 dB/m) to limit cable loss to 3.3 dB, preserving the 125 dB link budget needed for 5 km rural coverage; RG-58 in the same run would dissipate 9.9 dB, cutting range by more than half.
- Medical Ultrasound Probe Interconnect: High-frequency ultrasound transducers at 5–15 MHz use miniature 50 Ω micro-coax (0.81 mm OD) assemblies with silver-plated Cu centre conductors and PTFE dielectric to achieve the sub-0.1 dB insertion loss and phase matching (< 2° variation) required for coherent beamforming across a 128-element phased array.
Find Your Coaxial Cable on LCSC
LCSC stocks coaxial cable assemblies and raw coaxial cable from Amphenol, Molex, and Hirose, alongside competitively priced Asian manufacturers including Cvilux, RONGLAN, and Junkonn, covering impedances of 50 Ω and 75 Ω, outer diameters from 0.81 mm micro-coax to 10.3 mm LMR-400 equivalents, and connector terminations in SMA, SMB, BNC, MCX, and U.FL/IPEX formats.
Key sourcing filters available on LCSC:
- Characteristic impedance (50 Ω / 75 Ω)
- Cable outer diameter / form factor (micro-coax, RG-58 equiv., LMR-400 equiv.)
- Connector type (SMA / BNC / MCX / U.FL / MMCX)
- Operating frequency range (up to 1 GHz / up to 6 GHz / up to 18 GHz)
How Do 50 Ω and 75 Ω Coaxial Cables Compare?
| Parameter | 50 Ω Coaxial | 75 Ω Coaxial | Best For |
| Impedance standard | 50 Ω ± 1–2 Ω | 75 Ω ± 1–2 Ω | 50 Ω for RF transmit/receive; 75 Ω for video and CATV |
| Attenuation (RG-58 vs RG-59 equiv.) | 0.66 dB/m @ 100 MHz | 0.56 dB/m @ 100 MHz | 75 Ω slightly lower loss at equivalent OD |
| Power handling | Higher (lower impedance = more current capacity) | Lower power rating at equal conductor size | 50 Ω for transmitters, amplifiers, antenna tuners |
| Ecosystem / connectors | SMA, N, BNC, TNC, SMP widely available | F, RCA, BNC-75 for broadcast and CATV | 75 Ω for HDTV, CATV, satellite; 50 Ω for all RF comms |
Quick Selection Guide
- RF transmitter or transceiver application? → Always 50 Ω; power handling and connector ecosystem strongly favour it over 75 Ω.
- Video distribution or CATV/satellite? → Always 75 Ω; the 75 Ω standard is embedded in every HDTV, CATV amplifier, and satellite LNB interface.
- Loss is the binding constraint above 1 GHz? → Select LMR-400 or equivalent low-loss 50 Ω cable; its foamed PE dielectric cuts attenuation by 65 % versus RG-58.
- Shielding must meet CISPR 25 Class 5? → Specify foil-plus-braid construction (90 dB SE minimum); single-braid types fail automotive EMC limits above 1 GHz.
- Operating temperature above +85°C? → Switch to PTFE-dielectric cable rated to +200°C; PE-dielectric cables soften and fail above +80°C under sustained load.
Conclusion: Choosing the Right Coaxial Cable for Your Design
Coaxial selection hinges on the trade-off between attenuation, cost, and form factor. While low-loss cables require larger diameters and foamed dielectrics, standard RG-58 cables prioritize flexibility and low cost. The critical decision threshold is 3 dB total loss; exceeding this requires a higher-grade cable or an LNA. Key selection factors include operating temperature, shielding effectiveness, and bend radius. The governing principle is that 3 dB of feed-line loss halves signal power, and post-amplification cannot recover the resulting SNR degradation.
Frequently Asked Questions
Q: How do I calculate the maximum cable run length before a signal amplifier is needed?
Divide the total allowable cable loss from the link budget (total path loss minus connector loss minus 3 dB margin) by the cable’s attenuation in dB/m at the highest operating frequency. A 6 dB budget at 1 GHz allows 2.6 m of RG-58 (2.3 dB/m) or 8.2 m of LMR-400 (0.73 dB/m) before an LNA is required.
Q: Does it matter if I mix 50 Ω and 75 Ω components in the same RF chain?
A 50 Ω to 75 Ω mismatch produces a reflection coefficient of 0.2 and a return loss of only 14 dB, degrading receiver noise figure by up to 1 dB — enough to drop a GPS or LTE link below minimum detectable signal in a tightly budgeted RF front end. Always use matching pads or maintain a single impedance standard throughout an RF chain.
Q: How should coaxial cable be routed to meet CISPR 25 automotive shielding requirements?
CISPR 25 Class 5 requires shielding effectiveness of at least 90 dB above 1 GHz, met only with foil-plus-braid or solid-outer (semi-rigid) cable construction; the shield must terminate to chassis with a 360° bond at both ends, as pigtail grounds fail above 100 MHz. Maintain at least 50 mm separation from CAN, LIN, and ignition harnesses, and route along grounded chassis members where separation is not feasible.
Q: How do I second-source a coaxial cable assembly if the original supplier cannot deliver?
Match five parameters from the original datasheet: impedance (± 1 Ω), attenuation at the highest operating frequency (± 5 %), velocity factor (± 0.01), connector interface standard (SMA per MIL-PRF-39012 or BNC per IEC 61169-8), and jacket temperature rating. Confirm with insertion loss and return loss sweeps from 100 MHz to the maximum operating frequency before switching production.
Q: What bending radius rules apply to coaxial cable to avoid impedance degradation?
The minimum bend radius for flexible coaxial cable is 10 times the outer diameter (OD) under static conditions and 20 times OD for repeated flexing per IEC 61196-1. Bending tighter deforms the dielectric, shifts impedance by 2–5 Ω at the kink, and creates a return-loss null at the frequency where the deformed length equals a quarter wavelength — a fault that shows up only under cable movement and requires a VNA to locate.