Why is corrugated cable lower loss than braided coax like RG-cables?

Three factors. First, the outer conductor of corrugated cable is a solid seamless copper tube rather than a woven braid, so there is no braid leakage and the full conductor cross section carries current, giving near 100 percent shielding effectiveness (better than 120 dB). Second, the corrugated tube allows large diameters (7/8 inch, 1-5/8 inch, and larger) that reduce conductor loss because attenuation scales roughly inversely with diameter at a fixed impedance. Third, the dielectric is usually low-density closed-cell foam polyethylene or an air-spaced spiral spacer, with a dielectric constant near 1.2 to 1.5 and a loss tangent below 0.0003, far lower than the solid PE used in flexible RG cables. The result is that 7/8 inch corrugated cable shows about 1.1 dB per 100 ft at 1 GHz, versus roughly 6 to 9 dB per 100 ft for RG-213.

How do I calculate attenuation and power rating for a corrugated cable run?

Attenuation in dB per 100 m is published per cable type and rises roughly with the square root of frequency below the onset of dielectric loss. Total feedline loss is alpha (dB per 100 m) times the run length divided by 100. A 7/8 inch cable is about 3.6 dB per 100 m at 1 GHz, so scaling by the square root of frequency gives roughly 5.1 dB per 100 m at 2 GHz; a 60 m run then loses about 3.1 dB. Average power handling is set by conductor heating and falls with frequency and ambient temperature; a 7/8 inch cable handles roughly 2.5 kW at 1 GHz dropping to about 1 kW at 4 GHz. Peak power is limited by voltage breakdown in the dielectric and by VSWR, since standing waves raise the local field. Always derate published average power by the run VSWR and by elevation if the cable is air-dielectric and unpressurized.

Passive Components

Corrugated Cable

Q: What is the difference between annular and helical corrugated cable?

Annular corrugated cable has ring-shaped corrugations perpendicular to the cable axis, with each corrugation forming a closed loop around the circumference. This geometry gives lower attenuation and slightly better return loss because the corrugations do not introduce a periodic phase tilt, but the cable is stiffer and the minimum bend radius is larger. Helical corrugated cable has a continuous spiral corrugation, like a screw thread, which makes the cable far more flexible and allows a much tighter bend radius (often 5 to 8 times the cable diameter for helical versus 10 to 20 times for annular). The helical geometry introduces a small periodic structure that marginally raises attenuation and can produce a return-loss spike near the frequency where the corrugation pitch equals a half wavelength, so most manufacturers keep that resonance well above the operating band.

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Used wherever a long antenna run demands low loss without the rigidity of hardline, this coaxial cable replaces the woven braid with a seamless corrugated copper outer conductor, either annular (ring-shaped) or helical (spiral) corrugations. The corrugations let a large-diameter tube bend in the field while a low-density foam or air transmission line dielectric keeps attenuation near that of rigid line. A 7/8 inch corrugated cable shows roughly 1.1 dB/100 ft at 1 GHz, handles 2 to 3 kW average power, and offers shielding better than 120 dB. Its velocity factor of 0.84 to 0.92 reflects the near-air dielectric. Corrugated feedlines are the standard for cellular base stations, broadcast towers, and radar antenna interconnects.

Category: Passive Components

Attenuation (7/8", 1 GHz): ≈1.1 dB/100 ft

Shielding: > 120 dB

How Corrugated Coaxial Cable Achieves Hardline Performance

The defining feature of corrugated cable is its outer conductor: instead of the woven copper braid found in flexible RG-style coax, the shield is a continuous seamless copper or aluminum tube that has been mechanically corrugated. The corrugations are what make a thick-walled metal tube flexible enough to ship on a reel and pull through a cable tray, yet the tube remains a solid conductor with no braid gaps. That gives shielding effectiveness above 120 dB and eliminates the braid-leakage and triboelectric noise that plague flexible cables, while the large tube diameter lowers conductor loss because skin-effect resistance per unit length falls as the conductor circumference grows.

Inside the tube, the center conductor is a smooth or corrugated copper tube or solid copper wire, supported by either a closed-cell foam polyethylene dielectric or an air-spaced spiral spacer. Both dielectric systems target a relative permittivity near 1.2 to 1.5 and a loss tangent below 0.0003, which is why corrugated cable approaches the loss of air-filled rigid line. Because the dielectric is mostly air or gas-blown foam, the cable must usually be sold with a sealed jacket or kept pressurized with dry air or nitrogen to keep moisture out, since water ingress raises both attenuation and the dielectric loss tangent dramatically.

Two corrugation geometries dominate. Annular cable has independent rings stamped around the circumference and delivers the lowest attenuation and cleanest return loss, at the cost of a larger minimum bend radius. Helical cable uses a continuous spiral, behaving like a coarse screw thread, which yields a much tighter bend radius and faster installation but introduces a small periodic structure that designers keep resonance-free across the operating band.

Attenuation and Velocity Factor Relations

Velocity Factor (foam/air dielectric):
VF = 1 / √ε_r ≈ 0.84 to 0.92 (for ε_r ≈ 1.18 to 1.42)

Total Feedline Loss:
L_total = α × (length / 100) dB, with α in dB/100 m

Conductor-Loss Frequency Scaling:
α_c ∝ √f (skin effect, below dielectric-loss onset)

Helical Corrugation Resonance (avoid in band):
f_res ≈ c × VF / (2 × p) where p = corrugation pitch

Example: 7/8" cable is ≈ 1.1 dB/100 ft (3.6 dB/100 m) at 1 GHz, so at 2 GHz the √f scaling gives α ≈ 5.1 dB/100 m; a 60 m run then loses L_total ≈ 3.1 dB. Pitch p = 8 mm, VF = 0.88 → f_res ≈ 16.5 GHz, far above any 2 GHz band.

Corrugated Cable Size and Performance Comparison

Cable Size	Atten. @ 1 GHz	Avg Power @ 1 GHz	Min Bend Radius	Velocity Factor	Typical Use
1/2 inch	~2.3 dB/100 ft	~0.8 kW	32 mm (helical)	0.88	Jumpers, tower-top runs
7/8 inch	~1.1 dB/100 ft	~2.5 kW	120 mm (annular)	0.89	Base station main feeder
1-1/4 inch	~0.8 dB/100 ft	~4 kW	200 mm (annular)	0.90	Broadcast, long runs
1-5/8 inch	~0.6 dB/100 ft	~6 kW	250 mm (annular)	0.92	High-power FM/TV
RG-213 (braided, ref.)	~6.5 dB/100 ft	~0.3 kW	25 mm	0.66	Short patch leads

Common Questions

Frequently Asked Questions

What is the difference between annular and helical corrugated cable?

Annular cable has independent ring-shaped corrugations and gives the lowest attenuation and cleanest return loss, but it is stiffer with a larger bend radius (10 to 20 cable diameters). Helical cable uses a continuous spiral corrugation, so it flexes far more easily and bends to 5 to 8 diameters, at a small attenuation penalty. The helical pitch creates a periodic structure, so makers keep its resonance (f_res ≈ c·VF / (2p), where p is the corrugation pitch) well above the operating band.

Why is corrugated cable lower loss than braided RG coax?

Three reasons. The seamless solid-tube outer conductor has no braid leakage and carries current over the full cross section, giving >120 dB shielding. Larger diameters (7/8", 1-5/8") cut conductor loss, which scales inversely with circumference. And the foam or air dielectric has ε_r ≈ 1.2 to 1.5 with loss tangent < 0.0003. Net result: 7/8" cable runs about 1.1 dB/100 ft at 1 GHz versus 6 to 9 dB for RG-213.

How do I calculate attenuation and power rating for a run?

Multiply the published α (dB/100 m) by length/100; attenuation rises roughly with √f below the dielectric-loss region. A 60 m run of 7/8" cable at 2 GHz (α ≈ 5.1 dB/100 m, scaled from 3.6 dB/100 m at 1 GHz) loses about 3.1 dB. Average power falls with frequency and ambient temperature (about 2.5 kW at 1 GHz, near 1 kW at 4 GHz for 7/8"), and you must derate for run VSWR and, with air dielectric, for altitude unless the line is pressurized.

Related Terms

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