Passive Components

Conformable Cable

Q: What is the difference between conformable cable and standard semi-rigid cable?

Standard semi-rigid cable uses a fully annealed seamless copper tube as the outer conductor and must be shaped on a mandrel or bending fixture; once formed it cannot be reworked easily. Conformable cable replaces that rigid tube with a softer solid outer conductor or a tin-soaked flat braid that an assembler bends by hand. A typical 0.085 inch conformable line such as the UT-085C-FORM class can be bent to a minimum radius near 2 to 3 mm and reshaped several times, whereas true semi-rigid is normally formed once. The trade-off is that conformable cable has slightly higher attenuation, roughly 0.1 to 0.3 dB per meter more at 18 GHz, and modestly poorer phase stability over flexing than a one-time-formed semi-rigid run.

Q: How does bending affect the electrical length and phase of conformable cable?

Bending changes the physical path length and slightly distorts the dielectric, both of which shift electrical length and therefore phase. Electrical length theta in degrees is 360 times the physical length L divided by the guide wavelength, where guide wavelength equals the free-space wavelength divided by the square root of the effective dielectric constant. For solid PTFE the effective dielectric constant is about 2.05, giving a velocity factor near 0.70. The in-cable wavelength at 18 GHz is only about 11.7 mm, so each millimeter of added or removed path moves the phase by roughly 31 degrees, and a 3 mm routing change can swing phase by nearly 90 degrees. For phase-critical work, assemblers form the cable to a marked length and avoid reflexing it after the initial fit.

Q: What is the maximum usable frequency of 0.085 inch conformable cable?

The cable itself is limited by the onset of the first higher-order TE11 coaxial mode, which appears when the mean circumference of the dielectric approaches one guide wavelength. For 0.085 inch (2.2 mm) cable with PTFE dielectric that cutoff is about 61 GHz, so the line stays single-mode well into millimeter wave. In practice the connector sets the rating: SMA limits a 0.085 inch jumper to about 26.5 GHz, while 2.92 mm connectors take the same cable to roughly 40 GHz. Smaller 0.047 inch conformable cable has its mode cutoff near 90 GHz and is used to about 65 GHz, and 0.020 inch micro-conformable lines run to 110 GHz. Once a mode beyond TEM can propagate, the cable produces resonant suckouts and unpredictable insertion loss.

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A hand-formable cousin of semi-rigid cable, this coaxial line swaps the rigid seamless copper tube for a soft solid outer conductor or a tin-soaked flat braid, so an assembler can bend it by hand to route a path inside a module and have it hold that shape without a mandrel or bending fixture. Built on the same solid-PTFE dielectric as coaxial cable, it keeps a velocity factor near 0.70 and low insertion loss, paying only a small attenuation and phase-stability penalty for the convenience of reshaping. The 0.085 inch size is the workhorse for board-to-connector jumpers, with a TE11 mode cutoff near 61 GHz so the connector, not the cable, sets the rating; thinner 0.047 inch and 0.020 inch variants push usable performance to roughly 65 and 110 GHz.

Category: Passive Components

Common OD: 0.085 / 0.047 / 0.020 in

Velocity Factor: ≈ 0.70 (PTFE)

How Conformable Cable Differs From a Rigid Coax Tube

Conformable cable grew out of a manufacturing pain point. Traditional semi-rigid coax uses a fully annealed seamless copper tube as its outer conductor, which gives superb shielding and phase stability but requires a hand bender or CNC tube-forming fixture to shape, and it cannot be reflexed without kinking. Conformable lines keep the same center conductor and solid PTFE dielectric, then replace the hard tube with either a softer solid outer wall or a tin-soaked flat-braid jacket. The result bends cleanly by hand to a minimum radius near the cable diameter itself, roughly 2 to 3 mm for a 0.085 inch line, holds its formed shape, and tolerates several reshaping cycles before fatigue becomes a concern.

Electrically, the cost of that flexibility is modest. A 0.085 inch conformable line typically shows 0.1 to 0.3 dB per meter higher attenuation than a freshly formed semi-rigid run at 18 GHz, because the seamed or braided outer wall presents slightly more resistive loss than a seamless tube. Shielding effectiveness for flat-braid types runs near 90 to 100 dB versus better than 120 dB for solid tube, which is still ample for most module-level interconnects. The dielectric is the same solid PTFE used throughout the family, so the velocity of propagation and characteristic impedance are essentially unchanged from semi-rigid.

Velocity Factor, Loss, and Phase

Because solid PTFE sets the effective dielectric constant near 2.05, signals travel at about 70 percent of the speed of light, and the guide wavelength is correspondingly shorter than free space. That is why a small routing change produces a large phase change at high frequency: at 18 GHz the free-space wavelength is 16.7 mm but the in-cable wavelength is only about 11.7 mm, so each millimeter of path length is worth roughly 31 degrees of phase. Assemblers form phase-critical jumpers to a scribed length and avoid reflexing them once fitted.

Governing Relationships

Velocity factor and effective permittivity:
VF = 1 / √ε_eff ≈ 1 / √2.05 ≈ 0.70

Characteristic impedance (coaxial):
Z₀ = (138 / √ε_r) × log₁₀(D / d) Ω

Electrical length / phase per unit length:
θ = 360° × L × √ε_eff / λ₀

TE₁₁ mode cutoff (upper usable limit):
f_c ≈ c / [π × (D + d)/2 × √ε_r]

Where ε_r = dielectric constant (PTFE ≈ 2.05), D = inner diameter of outer conductor, d = center conductor diameter, L = physical length, λ₀ = free-space wavelength, c = speed of light. Example: 0.085 in cable (D ≈ 1.68 mm, d ≈ 0.51 mm) → f_c ≈ 61 GHz, so the SMA or 2.92 mm connector, not the cable, sets the practical rating.

Conformable Cable Size Comparison

Type / Size	Outer Dia.	Atten. @ 18 GHz	TE₁₁ Cutoff (single mode)	Min Bend Radius	Typical Use
Conformable 0.085 in	2.2 mm	1.3 to 1.6 dB/m	≈ 61 GHz	2 to 3 mm	Board-to-connector jumpers
Conformable 0.047 in	1.2 mm	2.5 to 3.0 dB/m	≈ 90 GHz	1.5 mm	Dense module interconnect
Conformable 0.020 in	0.5 mm	5 to 7 dB/m	> 110 GHz	0.8 mm	MMIC / chip-level routing
Semi-rigid 0.085 in	2.2 mm	1.1 to 1.3 dB/m	≈ 61 GHz	fixture only	Fixed, phase-stable runs
Flexible (braid) RG-405	2.2 mm	1.8 to 2.2 dB/m	≈ 61 GHz	flexes freely	Repeated-flex test leads

Common Questions

Frequently Asked Questions

What is the difference between conformable cable and standard semi-rigid cable?

Semi-rigid uses a seamless annealed copper tube that must be shaped on a mandrel and is formed once; conformable replaces it with a soft solid wall or tin-soaked flat braid that bends by hand to a 2 to 3 mm radius and tolerates several reshapes. The penalty is roughly 0.1 to 0.3 dB/m more loss at 18 GHz and slightly poorer phase stability over flexing.

How does bending affect the electrical length and phase of conformable cable?

Bending changes path length and slightly distorts the dielectric, shifting electrical length θ = 360° × L × √ε_eff / λ₀. With PTFE (VF ≈ 0.70) the in-cable wavelength at 18 GHz is about 11.7 mm, so each millimeter of routing is worth roughly 31° of phase. Form phase-critical jumpers to a scribed length and avoid reflexing them after fit.

What is the maximum usable frequency of 0.085 inch conformable cable?

The cable itself is limited by the first higher-order TE₁₁ coaxial mode, about 61 GHz for 0.085 in (2.2 mm) PTFE cable, so the line stays single-mode well into millimeter wave. In practice the connector sets the rating: an SMA jumper is good to about 26.5 GHz and a 2.92 mm version to roughly 40 GHz on the same cable. The 0.047 in size has its cutoff near 90 GHz (used to about 65 GHz) and 0.020 in micro-conformable runs to 110 GHz. Once a mode beyond TEM can propagate the cable becomes multimode, producing resonant suckouts and erratic insertion loss.

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