Cable capacitance is the silent performance thief in ultrasound and NDT systems. Every picofarad per meter of cable capacitance acts as a shunt to ground for the signal — attenuating high-frequency echo signals before they reach the beamformer or flaw detector input. At 10 MHz over a 2-meter cable, the difference between 95 pF/m (ePTFE dielectric) and 120 pF/m (FEP dielectric) translates to approximately 0.8 dB additional signal loss. At 20 MHz, that gap widens to 1.5 dB.
For a 256-channel ultrasound probe, 1.5 dB of additional loss per channel means the system must increase transmit voltage or receiver gain by 1.5 dB to maintain the same image quality — reducing dynamic range, increasing power consumption, and accelerating transducer wear. For NDT PAUT inspection, it means reduced detection sensitivity for small defects at depth.
This guide explains how cable capacitance affects signal integrity, why dielectric material selection is the primary lever for minimizing capacitance, and how to specify low-capacitance
Micro Coaxial Cable
for your application.
What Determines Cable Capacitance
Cable capacitance per unit length is determined by the geometry and material of the dielectric layer between the center conductor and the shield:
C = (2π × ε₀ × Dk) / ln(D/d)
Where:
- C = capacitance per unit length (pF/m)
- ε₀ = permittivity of free space (8.854 pF/m)
- Dk = dielectric constant of the insulating material
- D = shield inner diameter
- d = conductor outer diameter
Two factors control capacitance:
1. Dielectric constant (Dk) — directly proportional. Lower Dk = lower capacitance. This is the primary lever.
2. D/d ratio — inversely proportional (logarithmic). Larger ratio = lower capacitance. But the D/d ratio is constrained by the impedance requirement (50 Ω), so it cannot be independently adjusted without changing impedance.
For a 50 Ω cable at a given AWG, the D/d ratio is fixed by the impedance specification. The only way to reduce capacitance at constant impedance is to reduce the dielectric constant.
Dielectric Material Dk Capacitance at 42 AWG 50 Ω VoP Cost Phase Match Capability ePTFE (expanded) ≈ 1.45 ≤ 95 pF/m ≥ 78% Highest Excellent (±0.05 Dk) Foamed FEP ≈ 1.55 ≤ 100 pF/m ≥ 75% High Good (±0.08 Dk) Solid FEP ≈ 2.1 ≤ 120 pF/m ≥ 69% Medium Moderate (±0.10 Dk) Solid PTFE ≈ 2.1 ≤ 120 pF/m ≥ 69% Medium Moderate (±0.10 Dk) Polyethylene (PE) ≈ 2.25 ≤ 130 pF/m ≥ 66% Lowest Poor (±0.15 Dk)
ePTFE achieves the lowest capacitance by exploiting its microporous structure — the expanded PTFE matrix contains approximately 50–70% air (Dk = 1.0) by volume, producing an effective Dk of approximately 1.45 compared to 2.1 for solid PTFE/FEP. This 31% reduction in Dk translates directly to approximately 21% lower capacitance at the same cable geometry.
Signal Impact — Capacitance vs. Attenuation
Cable capacitance causes frequency-dependent signal attenuation. Higher frequencies are attenuated more than lower frequencies because the capacitive reactance (Xc = 1/2πfC) decreases with frequency — allowing more signal current to leak through the capacitive path to ground rather than traveling to the receiver.
Attenuation comparison at 42 AWG, 50 Ω, 2-meter cable:
| Frequency | ePTFE (95 pF/m) | FEP (120 pF/m) | PE (130 pF/m) | ePTFE Advantage |
| 1 MHz | 0.3 dB | 0.4 dB | 0.4 dB | 0.1 dB |
| 5 MHz | 1.0 dB | 1.3 dB | 1.4 dB | 0.3–0.4 dB |
| 10 MHz | 1.6 dB | 2.1 dB | 2.3 dB | 0.5–0.7 dB |
| 15 MHz | 2.1 dB | 2.8 dB | 3.1 dB | 0.7–1.0 dB |
| 20 MHz | 2.6 dB | 3.5 dB | 3.9 dB | 0.9–1.3 dB |
At 5 MHz (cardiac ultrasound), the ePTFE advantage is modest: 0.3 dB. At 15 MHz (linear MSK/vascular), the advantage grows to 0.7–1.0 dB — significant when multiplied across 256 channels.
The crossover point: for Application below 5 MHz with cable lengths under 1.5 m, the capacitance difference between ePTFE and FEP has negligible clinical impact. Above 10 MHz or with cable lengths above 2 m, ePTFE becomes strongly recommended.
Capacitance and Phase Matching
Capacitance uniformity — not just absolute capacitance — is critical for phase-matched systems. Phase matching requires uniform propagation velocity across all channels. Propagation velocity is determined by Dk:
VoP = 1 / √Dk (expressed as fraction of speed of light)
If Dk varies between cable elements, VoP varies, and electrical length varies — producing phase errors even in cables cut to identical physical length.
| Dielectric | Dk Variation (1σ) | VoP Variation | Phase Error per Meter | Phase-Matchable |
| ePTFE | ±0.05 | ±0.17% | ±1.7 ps/m | ±1.0% achievable |
| Foamed FEP | ±0.08 | ±0.27% | ±2.7 ps/m | ±1.5% achievable |
| Solid FEP | ±0.10 | ±0.34% | ±3.4 ps/m | ±2.0% marginal |
| PE | ±0.15 | ±0.50% | ±5.0 ps/m | Not practical |
ePTFE is the only dielectric that reliably achieves ±1.0% phase matching — the standard specification for
ultrasound probe cables
. FEP can achieve ±1.5–2.0%, which may be acceptable for some NDT Application but not for premium ultrasound imaging. PE cannot be practically phase-matched.
When Low Capacitance Justifies the ePTFE Premium
ePTFE dielectric cable costs approximately 40–60% more than FEP dielectric at the same AWG. The question is whether the signal quality improvement justifies the cost.
Use ePTFE when:
- Operating frequency > 10 MHz (high-frequency linear, IVUS)
- Cable length > 2 m (long probe umbilical)
- Phase matching required (±1.0% or tighter)
- Maximum SNR required (premium probe positioning)
- 256+ channels (cumulative capacitance budget is tight)
Use FEP when:
- Operating frequency < 5 MHz (cardiac, abdominal)
- Cable length < 1.5 m (compact/portable probe)
- Phase matching not required (endoscope video, single-element NDT)
- Cost optimization is a priority (single-use endoscope, high-volume industrial)
Capacitance at Different AWG Gauges
| AWG | Cable OD | ePTFE Capacitance | FEP Capacitance | Difference |
| 36 | 0.75 mm | ≤ 82 pF/m | ≤ 100 pF/m | 18 pF/m |
| 40 | 0.57 mm | ≤ 90 pF/m | ≤ 112 pF/m | 22 pF/m |
| 42 | 0.48 mm | ≤ 95 pF/m | ≤ 120 pF/m | 25 pF/m |
| 44 | 0.38 mm | ≤ 110 pF/m | ≤ 135 pF/m | 25 pF/m |
| 46 | 0.30 mm | ≤ 125 pF/m | ≤ 155 pF/m | 30 pF/m |
Capacitance increases at finer gauges because the D/d ratio decreases (conductor and shield are closer together). At 46 AWG, the capacitance penalty of FEP versus ePTFE is 30 pF/m — the largest gap in the table. This is why ePTFE is essentially mandatory for IVUS and catheter Application at 46+ AWG, where every picofarad matters in the tight signal budget at 20–60 MHz.
Frequently Asked Questions
Q: What is the lowest capacitance
Micro Coaxial Cable
available?
A: Our 36 AWG with ePTFE dielectric at ≤ 82 pF/m. At 42 AWG (the most common gauge): ≤ 95 pF/m. These are among the lowest capacitance values available for 50 Ω
Micro Coaxial Cable
at these gauges. Lower capacitance would require either vacuum dielectric (impractical for flexible cable) or non-standard impedance.
Q: Can I reduce capacitance by using 75 Ω instead of 50 Ω?
A: Yes — 75 Ω cable has approximately 30% lower capacitance than 50 Ω at the same AWG (67 pF/m vs. 95 pF/m at 42 AWG ePTFE). However, using 75 Ω cable in a 50 Ω system creates impedance mismatch that causes more signal degradation than the capacitance improvement saves. See our 50 Ohm vs 75 Ohm guide.
Q: Does cable capacitance change with temperature?
A: Minimally for fluoropolymer dielectrics. ePTFE Dk temperature coefficient is approximately +0.01% per °C — at operating range of −40°C to +200°C, total Dk change is < 2.5%, producing < 3% capacitance change. This is negligible for all practical Application.
Q: Is there a foamed ePTFE option for even lower capacitance?
A: ePTFE is inherently "foamed" — its microporous structure contains 50–70% air. Further expansion to achieve Dk < 1.3 is possible in laboratory settings but produces mechanically fragile dielectric unsuitable for flexible cable production. At ≤ 95 pF/m (42 AWG), our ePTFE is already at the practical minimum for production micro coaxial cable.
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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:
Send your drawing or spec sheet and get a quote within 48 hours. Prototype quantities start at 5 assemblies.
