PTFE and FEP can both be used between the center conductor and shield of a coaxial cable, but they lead to different design and manufacturing choices. PTFE usually provides lower dielectric loss, a wider temperature range, and more options for low density dielectric structures. FEP supports melt extrusion and can form thin, uniform solid insulation around very small conductors.
- PTFE and FEP Do Not Always Serve the Same Cable Function
- Electrical Differences That Matter in a Coaxial Line
- Dielectric Structure Changes Velocity Factor and Capacitance
- PTFE Usually Has Lower Dielectric Loss
- Processing Method Changes the Final Cable Geometry
- Temperature Capability and Phase Stability
- Where PTFE Makes More Sense
- Where FEP Makes More Sense
- FAQ
- Conclusion
PTFE and FEP Do Not Always Serve the Same Cable Function
The dielectric is the material directly surrounding the center conductor. It controls the electrical spacing between the center conductor and shield. The jacket sits outside the shield and mainly provides environmental and mechanical protection.
FEP is often seen as an outer jacket on PTFE dielectric coaxial cable. This can create confusion when a cable is described simply as an FEP cable. The same material can also be extruded around the center conductor and used as the RF dielectric in another cable design.
A complete cable construction drawing identifies these layers separately:
- Center conductor material and diameter
- Dielectric material, density, and diameter
- Shield construction
- Outer jacket material
Without this separation, a PTFE and FEP comparison may compare a dielectric in one cable with a jacket in another.
Electrical Differences That Matter in a Coaxial Line
Solid PTFE and solid FEP both have relative dielectric constants roughly around 2.0 to 2.1, depending on material grade, frequency, temperature, and test method. Their dielectric constants are close enough that either material can be used in a 50 ohm coaxial cable.
For a simplified solid dielectric coaxial line, characteristic impedance can be expressed as:
Z₀ ≈ (60 / √εᵣ) × ln(D / d)
Where:
- Z0 is the characteristic impedance
- εr is the effective relative dielectric constant
- D is the inner diameter of the outer conductor
- d is the outer diameter of the center conductor
This relationship shows why the dielectric material cannot be changed without reviewing the cable dimensions. If the effective dielectric constant changes while D and d remain unchanged, the impedance also changes.
Because solid PTFE and solid FEP have similar dielectric constants, the dimensional change required when moving between them may be small. A larger difference appears when solid FEP is compared with expanded, microporous, or low density PTFE.
Dielectric Structure Changes Velocity Factor and Capacitance
Velocity factor is related to the effective dielectric constant:
VF ≈ 1 / √εr,eff
A solid fluoropolymer dielectric normally produces a lower velocity factor than a low density dielectric containing controlled air spaces. Air has a much lower dielectric constant than PTFE or FEP, so adding air to the dielectric structure lowers the effective dielectric constant.
Expanded or microporous PTFE can therefore provide:
- Higher velocity factor
- Lower capacitance per unit length
- Lower dielectric contribution to attenuation
- Shorter electrical delay for the same physical cable length
A solid FEP dielectric may have a similar bulk dielectric constant to solid PTFE, but it does not automatically produce the same velocity factor as a low density PTFE cable. The dielectric density and air content can create a larger electrical difference than the polymer names themselves.
PTFE Usually Has Lower Dielectric Loss
PTFE generally has a lower dielectric loss tangent than FEP under comparable frequency and temperature conditions. As frequency increases, this difference can contribute to lower dielectric loss in microwave cable.
Material loss is still one part of total attenuation. Conductor diameter, conductor plating, shield construction, cable size, and connector transition also contribute to insertion loss. This means a small PTFE cable can have higher attenuation than a larger FEP dielectric cable.
Processing Method Changes the Final Cable Geometry
PTFE is not processed in the same way as a conventional melt processable thermoplastic. PTFE dielectric structures are commonly made through paste extrusion, tape wrapping, expansion, and sintering.
These methods allow PTFE to form solid, low density, expanded, or microporous dielectric structures. They also create manufacturing variables related to density, sintering, tape overlap, void distribution, and dimensional recovery.
FEP is melt processable. It can be extruded continuously around the center conductor as a solid and smooth dielectric layer. This process can be useful when a design needs:
- Thin dielectric walls
- Small cable diameter
- Continuous insulation around a fine conductor
- Controlled surface quality
- Consistent dimensions over a production length
FEP extrusion can support uniform small diameter structures, while PTFE offers more freedom to reduce dielectric density.
Temperature Capability and Phase Stability
PTFE generally provides a higher continuous temperature capability than FEP. This gives PTFE a clear advantage when the dielectric is exposed to elevated ambient temperature, conductor heating, or thermal cycling beyond the suitable range of FEP.
Temperature rating describes whether the cable materials can continue operating without unacceptable physical or electrical degradation. It does not describe how much the electrical length changes while the temperature moves within that range.
PTFE can show nonlinear dielectric behavior around a transition region near room temperature. In a phase sensitive cable, this behavior can appear as a change in phase slope or hysteresis during heating and cooling.
A standard PTFE cable may have excellent high temperature durability while still showing too much phase movement for a phase matched assembly. A cable designed for phase stability uses a controlled dielectric structure and needs phase versus temperature data.
Where PTFE Makes More Sense
PTFE is usually the stronger option when the cable needs one or more of the following characteristics:
- A wider operating temperature range
- Lower dielectric loss at microwave frequencies
- A low density or microporous dielectric
- Higher velocity factor
- Lower capacitance for a given cable structure
- Compatibility with an existing high temperature coaxial cable family
The specific PTFE structure still needs to be defined. Solid PTFE, expanded PTFE, microporous PTFE, and wrapped PTFE can have different velocity factor, capacitance, flexibility, attenuation, and phase behavior.
Where FEP Makes More Sense
FEP can be a suitable dielectric when the cable design places more emphasis on melt extrusion and dimensional control than on the highest temperature capability or the lowest possible dielectric loss. FEP is worth considering for:
- Small diameter solid dielectric coaxial cable
- Thin dielectric walls around fine conductors
- Continuous extrusion with controlled concentricity
- Cable structures that remain within the FEP temperature range
- Designs supported by measured attenuation and flex data
Some flexible and continuous motion coaxial cables use FEP dielectric, but this does not make FEP inherently more resistant to repeated flexing. Flex life also depends on the center conductor, shield, cable diameter, jacket, bend radius, and movement pattern.
FAQ
Conclusion
PTFE and FEP are both suitable RF cable dielectrics, but they support different design priorities. PTFE provides lower dielectric loss, a wider temperature range, and access to low density structures with higher velocity factor. FEP supports melt extrusion and can form thin, uniform solid dielectric layers in small cable constructions. The final comparison needs to separate solid and low density structures, use matching cable dimensions and test conditions, and review phase behavior when temperature changes affect the signal path. For a custom RF cable assembly, the Bafitop engineering team can review the target frequency, cable diameter, attenuation limit, temperature range, velocity factor, phase requirement, and connector interface before confirming the cable construction.