📡 Coaxial Cable and RF Passive Components: Engineering Reference

Characteristic Impedance: 50Ω vs. 75Ω

Coaxial cable characteristic impedance Z₀ is determined by the ratio of outer to inner conductor diameters and the dielectric constant of the insulating medium:

Z₀ = (138 / √εᵣ) · log₁₀(D/d)

where D is the inner diameter of the outer conductor, d is the outer diameter of the inner conductor, and εᵣ is the relative permittivity of the dielectric. The 50Ω standard arose as a practical compromise between minimum attenuation (achievable around 77Ω in air-dielectric) and maximum power handling (achievable around 30Ω). 50Ω is universally used in RF communications, test equipment, and broadcast transmitters. 75Ω cable (e.g., RG-59, RG-6) minimises attenuation in video and cable television distribution systems and is not interchangeable with 50Ω systems without impedance matching. Mismatching 75Ω cable into a 50Ω system causes a VSWR of 1.5:1 and an 8% power reflection — acceptable for test purposes but not for permanent installations.

Coaxial Cable Types and Specifications

Selection guide for common RF coaxial cables:

• RG-8 / RG-8X: 50Ω, polyethylene dielectric. Attenuation: ~2.5 dB/30 m at 400 MHz. Flexible, inexpensive. Suitable for low-power runs to mobile antennas and test leads. Velocity factor: 0.66.
• RG-213: 50Ω, same electrical performance as RG-8 but MIL-spec construction. Common in military and public safety portable equipment.
• LMR-400 (Times Microwave): 50Ω, PE foam dielectric, copper-clad aluminium inner conductor. Attenuation: 0.68 dB/30 m at 450 MHz; 1.34 dB/30 m at 2.4 GHz. Excellent flexibility for a low-loss cable. Velocity factor: 0.85. Industry standard for rooftop cable runs in DAS and BDA installations.
• LMR-600: 50Ω, larger (15 mm diameter), lower loss. Attenuation: 0.43 dB/30 m at 450 MHz. Stiffer than LMR-400; requires larger bend radius (38 mm minimum). Used for longer trunk runs where LMR-400 loss budget is insufficient.
• 7/8" HELIAX (Andrew/CommScope FSJ4-50B or equivalent): Air-dielectric (helical polyethylene support). Attenuation: 0.25 dB/30 m at 450 MHz. Extremely low loss but semi-rigid — requires special bending tools and corrugated-to-connector transitions. Standard for BTS-to-antenna trunk runs and DAS riser cables in large buildings.
• 1-5/8" HELIAX: The largest practical indoor cable. Attenuation: 0.13 dB/30 m at 450 MHz. Used for head-end to riser transitions in high-power broadcast and large-venue DAS applications. Requires dedicated cable management and connector tools. Weight: approximately 1.5 kg/m.

Attenuation vs. Frequency

Coaxial cable attenuation increases with frequency due to the skin effect in the conductors (proportional to √f) and dielectric loss (proportional to f). The combined effect means attenuation roughly doubles for each quadrupling of frequency. Designers must run the link budget at the highest operating frequency to determine the worst-case insertion loss. For a 50 m run of LMR-400:

• 150 MHz: ≈ 0.7 dB
• 450 MHz: ≈ 1.1 dB
• 900 MHz: ≈ 1.6 dB
• 2400 MHz: ≈ 2.2 dB

Temperature de-rating is important for outdoor installations. Most cable specifications are at 25°C; attenuation increases approximately 0.2% per °C above this. In a desert environment at 60°C, attenuation may be 7% higher than rated — worth accounting for in tight link budgets.

Velocity of Propagation

The velocity factor (VF) is the ratio of signal propagation velocity in the cable to the speed of light in vacuum. VF = 1/√εᵣ. Solid PE dielectric: VF ≈ 0.66. Foam PE: VF ≈ 0.80–0.85. Air dielectric (HELIAX): VF ≈ 0.92–0.93. VF affects electrical length calculations critical for phased array feeding, impedance matching stubs, and delay equalisation in simulcast systems. For simulcast timing alignment, cable electrical lengths must match to within ±100 ns — equivalent to ±15 m of LMR-400.

RF Connector Types

• Type N: 50Ω or 75Ω, threaded coupling, frequency to 18 GHz. Weatherproof when mated. Standard for base station antennas, LMR-400/600, and HELIAX transitions (via adapter). Industry workhorse.
• TNC: Threaded N-style, smaller body, to 12 GHz. Used where N is too large (handie-talkies, small antennas). Better vibration resistance than BNC.
• BNC: Bayonet coupling, to 4 GHz. Quick-connect for test equipment, video, and low-frequency patch cables. Not weatherproof.
• PL-259 / SO-239 (UHF connector): Misnomer — originally designed for HF (< 30 MHz). Acceptable to 150 MHz; above that, the un-constant impedance geometry causes reflections. Ubiquitous in amateur radio but avoided in professional LMR systems above VHF.
• DIN 7-16 (4.3-10): The modern replacement for 7-16 DIN in cellular base station equipment. Smaller than 7-16, same electrical performance to 6 GHz. Now the predominant connector at macro-cell base station ports globally due to its compact size and field-installable interface.
• SMA: Sub-Miniature A, to 18 GHz. Standard in PCB-mounted RF modules, small antennas, and portable test equipment. Not rated for outdoor weathering without covers; inadequate for high-cycle mating environments.

Splitters, Combiners, and Directional Couplers

A 2-way splitter divides input power equally between two output ports: each port receives −3 dB (one-half) of input power, plus typical excess insertion loss of 0.5 dB, giving a total of 3.5 dB per port. A 4-way splitter is two 2-way splitters in series: −6 dB + 1 dB excess = 7 dB per port. An 8-way adds another stage: 10.5 dB per port. Splitters are reciprocal — they function identically as combiners (joining multiple signals), with the same insertion losses.

A directional coupler samples a fixed fraction of power from a through-line with high port-to-port isolation, useful for monitoring or distributing a small tap of the main signal. Coupling values: −10 dB, −15 dB, −20 dB, −30 dB. The through-line has minimal insertion loss (0.5–1.5 dB depending on coupling value). Used in DAS for unequal power splitting where one antenna needs less power than the trunk path.

Attenuators, Circulators, and Surge Protectors

Fixed attenuators (pads) reduce signal level to prevent receiver overload, equalise levels in a distribution network, or improve return loss by padding impedance mismatches. Values: 1, 2, 3, 6, 10, 20 dB. Power ratings: 1 W to 100 W for standard pads; high-power versions to 1 kW for transmit applications. Circulators (3-port ferrite devices) route signals in one direction only: port 1 → port 2 → port 3 → port 1. Used to isolate transmitter from antenna VSWR reflections — reflections from a mismatched antenna exit port 3 (a dummy load) rather than re-entering the transmitter output stage.

Coaxial lightning arrestors (surge protectors) are installed at every cable entry point where an outdoor antenna cable enters a building or equipment enclosure. They provide a low-impedance path to ground for lightning-induced surges while presenting low RF insertion loss (< 0.2 dB) and low VSWR (< 1.2:1) during normal operation. Gas discharge tube (GDT) and hybrid (GDT + TVS diode) types are available; hybrid types provide better response to fast transients. Arrestors must be bonded to the building grounding electrode system per NEC Article 810 and TIA-222 requirements.

Weatherproofing: IP67/IP68 and Self-Amalgamating Tape

Outdoor connector weatherproofing is critical for long-term reliability. Unsealed N-type connectors in outdoor environments allow moisture ingress, oxidation, and eventually connector failure within 1–3 years. Methods:

• Self-amalgamating tape (Scotch 23 or equivalent): Stretch and wrap over the connector body and cable, then overvrap with vinyl electrical tape. Simple, field-repairable, effective if applied correctly.
• Heat-shrink boots: Pre-installed before connector assembly; heat-shrunk after mating. Clean appearance but difficult to reverse without cutting.
• IP67/IP68 rated connectors: Sealed connector designs (common on 4.3-10 DIN, waterproof SMA variants) with integrated O-ring seals providing full ingress protection when mated. Preferred for permanent installations requiring < 1 m water ingress protection (IP67) or continuous submersion (IP68). IP68 connectors are required for below-grade antenna installations in utility vaults and underground DAS components.

Power handling de-rating in weatherproofed assemblies: connector body temperature rise due to I²R loss limits the maximum continuous power. LMR-400 with N-type connectors: continuous power limit at 450 MHz is approximately 1,100 W at 40°C ambient. For most LMR and DAS applications (< 50 W per port), power handling is not the limiting design constraint — signal level and loss are.