Most engineers assume that the lowest dielectric constant always wins. It makes intuitive sense—lower Dk means higher velocity of propagation, lower capacitance, and wider bandwidth. So ePTFE with its Dk of 1.3–1.7 should be the obvious choice over FEP (Dk 2.15) or PFA (Dk 2.06) every time, right?
Not even close. In the 800+ cable assembly designs we've worked on, FEP is the dielectric in roughly 60% of production micro coaxial cables . ePTFE accounts for maybe 25%, and PFA fills the remaining 15%. The "inferior" dielectric materials win more often because dielectric constant is only one variable in a design that has to balance impedance control, sterilization compatibility, flex life, phase matching consistency, manufacturing yield, and cost.
The ePTFE vs FEP vs PFA decision is more nuanced than the data sheets suggest. Here's how we think about it.
What Each Dielectric Actually Is
Before comparing performance, it helps to understand what you're comparing. These three materials are all fluoropolymers, but their structures and manufacturing processes are fundamentally different.
ePTFE (expanded PTFE) starts as standard polytetrafluoroethylene that gets stretched under controlled heat and tension. This expansion creates a microporous matrix—billions of tiny air pockets within the PTFE structure. Since air has a Dk of 1.0, the composite Dk drops well below solid PTFE's 2.1. The expansion ratio determines the final Dk: more stretching means more air, lower Dk, but also weaker mechanical properties. Most cable-grade ePTFE targets Dk 1.4–1.5.
FEP (fluorinated ethylene propylene) is a fully fluorinated copolymer that can be melt-processed—unlike PTFE, which doesn't truly melt. This melt-processability means FEP can be extruded directly onto conductors at production speed using conventional extrusion equipment. The result is a uniform, void-free, solid dielectric layer with consistent wall thickness. Dk sits at 2.15 across a wide frequency range, and the loss tangent (Df) is typically 0.0007 at 1 GHz.
PFA (perfluoroalkoxy) is chemically similar to FEP but with slightly higher melt viscosity and better mechanical properties at elevated temperatures. PFA can also be melt-extruded, though processing temperatures run about 20–30°C hotter than FEP. Dk is 2.06—marginally lower than FEP—and PFA's continuous use temperature extends to 260°C versus FEP's 200°C. In practice, we use PFA when the cable needs to survive temperatures above 180°C or when slightly better flex resistance is needed.
How Dielectric Choice Affects Impedance Design
Impedance in a coaxial cable is determined by the ratio of outer conductor to inner conductor diameter and the dielectric constant of the insulation between them. The formula is Z₀ = (138/√Dk) × log(D/d), where D is the outer conductor ID and d is the center conductor OD.
Here's what this means practically: to hit 50Ω with a 42 AWG conductor (0.064 mm OD), each dielectric requires a different wall thickness.
With FEP (Dk 2.15), you need approximately 0.15 mm of dielectric—this is a comfortable thickness to extrude consistently. With ePTFE (Dk 1.45), the required dielectric thickness drops to about 0.09 mm. Extruding—or more accurately, wrapping—a 0.09 mm ePTFE layer with consistent density and thickness is significantly harder. Wall thickness variation of even ±0.01 mm shifts impedance by ±2–3Ω.
That's the theory. In practice, this means FEP-insulated cables have tighter impedance tolerance off the production line. We hold ±2Ω on FEP cables routinely; ePTFE cables typically come in at ±3Ω with careful process control. Not a huge difference, but for 128-channel phase-matched bundles , those extra ohms of variation propagate into measurable VoP spread across channels.
Performance Comparison: The Real Trade-Offs
| Property | ePTFE | FEP | PFA |
|---|---|---|---|
| Dielectric Constant (Dk) | 1.3–1.7 ✓ | 2.15 △ | 2.06 △ |
| Loss Tangent (Df @ 1 GHz) | 0.0003–0.0005 ✓ | 0.0007 △ | 0.0006 △ |
| Velocity of Propagation | 77–83% ✓ | 69% △ | 70% △ |
| Max Temperature (continuous) | 260°C ✓ | 200°C △ | 260°C ✓ |
| Autoclave Compatibility | ✓ | ✓ | ✓ |
| Wall Thickness Uniformity | ±0.01–0.015 mm ✗ | ±0.005–0.008 mm ✓ | ±0.006–0.010 mm ✓ |
| Impedance Tolerance (50Ω) | ±3Ω typical ✗ | ±2Ω typical ✓ | ±2.5Ω typical △ |
| VoP Consistency (batch) | ±0.8–1.2% ✗ | ±0.3–0.5% ✓ | ±0.4–0.6% ✓ |
| Flex Life Impact | Best (microporous) ✓ | Good △ | Very good ✓ |
| Processing Speed | 3–8 m/min ✗ | 15–30 m/min ✓ | 12–25 m/min ✓ |
| Relative Cable Cost | 1.4–1.6× ✗ | 1.0× (baseline) ✓ | 1.1–1.2× △ |
When ePTFE Is Worth the Premium
ePTFE earns its price tag in two specific scenarios. The first is bandwidth-critical Applications where cable capacitance directly limits system performance. Lower Dk means lower capacitance per unit length—a 42 AWG cable with ePTFE dielectric (Dk 1.45) has roughly 30% lower capacitance than the same cable with FEP (Dk 2.15). For high-frequency ultrasound above 15 MHz or broadband pulse Applications , that capacitance reduction translates directly to wider bandwidth and shorter rise times.
The second scenario is flex life. ePTFE's microporous structure makes it inherently more compliant than solid FEP or PFA. The dielectric layer compresses slightly during bending rather than resisting it, which reduces strain on the conductor-dielectric interface. In our flex testing, ePTFE-insulated cables consistently outlast FEP-insulated equivalents by 30–50% at equivalent bend radius—a meaningful advantage for endoscope cables and surgical robot arms that see tens of thousands of flex cycles in service.
But Watch the Phase Matching
Bit of a tangent, but this is relevant: if you're building a 128-channel ultrasound probe cable and you specify ePTFE for its low capacitance, make sure your supplier can actually hold the phase matching spec across all channels. The ePTFE expansion process introduces inherent variability in porosity from batch to batch—and even within a single production run, the Dk can drift by ±0.05–0.10 depending on tension control, temperature, and stretch ratio.
We've seen projects where an OEM specified ePTFE for its electrical superiority, then couldn't get consistent phase matching across their 128-channel bundle. The solution was to switch to FEP dielectric with slightly worse Dk but dramatically better consistency, and compensate for the higher capacitance with modest changes to the system electronics. Engineering is about trade-offs, not optimizing any single parameter.
When FEP Is the Smarter Choice
FEP isn't the "cheap option"—it's the engineered option for most production micro coaxial cables. Its advantages are all on the manufacturing side: melt extrusion is fast, repeatable, and produces uniform wall thickness. This translates to tighter impedance tolerance, better batch-to-batch consistency, and higher production yield.
For medical devices requiring sterilization, FEP handles every common method—steam autoclave (134°C), EtO, hydrogen peroxide (Sterrad), and gamma radiation. We've validated FEP-insulated cables through 1,000 autoclave cycles with impedance drift of less than 0.5%. PFA is equally sterilization-compatible, but FEP processes at lower temperature (saving energy cost in production) and costs about 10–15% less as raw material.
FEP is also the default choice when your cable specification calls out biocompatibility. FEP passes ISO 10993 cytotoxicity and sensitization testing without qualification—it's inherently bioinert. ePTFE also passes, but the microporous structure can theoretically trap particulates during sterilization, which complicates the biocompatibility argument for implant-adjacent Applications .
PFA: The Underappreciated Third Option
PFA gets overlooked because it sits between ePTFE and FEP on almost every metric—not the lowest Dk, not the cheapest, not the fastest to process. But PFA has a quiet advantage: it's the best fluoropolymer dielectric for high-temperature applications.
PFA's continuous use temperature of 260°C matches ePTFE and exceeds FEP's 200°C. More importantly, PFA maintains its mechanical properties at elevated temperature better than FEP. At 180°C, FEP's tensile strength drops by about 40%—not a failure, but enough that the dielectric can deform under mechanical stress and shift impedance. PFA retains about 75% of room-temperature tensile strength at the same temperature.
We use PFA dielectric primarily in cables for sterilization-intensive reusable devices (where 200°C headroom might not be enough for certain flash sterilization cycles) and in industrial applications where ambient temperature around the cable exceeds 150°C—think intraoperative imaging probes near electrosurgical instruments, or NDT cables on hot pipelines.
The Decision Framework
After working through this comparison, the selection logic is straightforward—but you have to answer the application questions in order:
Does your cable need to operate or survive above 180°C continuously? If yes, PFA or ePTFE. FEP is marginal above 180°C.
Is the cable for a multi-channel bundle requiring tight phase matching (±1%)? If yes, FEP gives you the most consistent VoP across channels. ePTFE is achievable but requires a supplier with very tight process control on the expansion step.
Do you need the lowest possible capacitance for bandwidth-critical signal paths? If yes, ePTFE. The 30% capacitance reduction vs. FEP matters for frequencies above 15 MHz or for very long cable runs where cumulative capacitance degrades rise time.
Is this a high-volume production cable (>5,000 units/year) where cost matters? If yes, FEP unless the application specifically demands ePTFE. The 40–60% cost premium of ePTFE adds up fast at volume.
If you need help deciding, send us your operating frequency, channel count, sterilization method, and temperature range —we'll recommend the dielectric that best fits your overall design constraints, not just the one with the best single-parameter spec.
Frequently Asked Questions
Why does ePTFE have a lower dielectric constant than solid PTFE?
ePTFE is stretched under controlled conditions to create billions of microscopic air pockets within the PTFE matrix. Since air has a Dk of 1.0 and solid PTFE is 2.1, the composite material's Dk drops proportionally with porosity—typically to 1.3–1.7 depending on the expansion ratio. Higher expansion means lower Dk but reduced mechanical strength.
Can FEP dielectric micro coaxial cable be autoclaved?
Absolutely. FEP handles steam autoclave at 134°C/18 minutes without degradation, and we've validated it through 1,000 cycles with less than 0.5% impedance change. FEP is also compatible with EtO, hydrogen peroxide, and gamma sterilization—making it the go-to dielectric for reusable medical devices.
Which dielectric gives the best phase matching in multi-channel ultrasound cables?
FEP, in most production scenarios. Melt extrusion produces highly uniform wall thickness with channel-to-channel VoP variation of ±0.3–0.5%. ePTFE has better absolute electrical properties but wider VoP variation (±0.8–1.2%) due to porosity inconsistency in the expansion process. For 128+ channel bundles, FEP is the lower-risk choice for phase matching.
We're currently evaluating a next-generation ePTFE tape-wrapping process that uses in-line TDR monitoring to compensate for density variations during wrapping—essentially a closed-loop control system for dielectric consistency. Early results show VoP variation below ±0.5%, which would close the gap with FEP while retaining ePTFE's electrical advantages. We don't have enough production data to guarantee these numbers yet, but it's the most promising manufacturing advancement we've seen in this space in years.
Related Products
Need a cable built to this spec? FRS Technology produces ultra-fine coaxial assemblies from 36 to 50 AWG, with per-element shielding and phase matching to +/-1%. Related products:
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