83 pF/m versus 58 pF/m. Two cables, same AWG, same impedance, same shield coverage—but the second one uses ePTFE dielectric instead of solid PTFE, and that 25 pF/m difference changes how the ultrasound image looks on a 15 MHz linear probe. Coaxial cable capacitance is one of those parameters that gets listed on every datasheet but rarely gets the engineering attention it deserves. For probes operating below 5 MHz with short cable runs, it's a secondary concern. But as transducer frequencies climb—15 MHz for superficial imaging, 20 MHz for ophthalmology, 40+ MHz for intravascular ultrasound—cable capacitance pF per meter becomes a first-order design constraint that directly affects the image you see on screen.
Let's look at why capacitance matters, what determines it, and how to choose the right cable construction for your frequency range and performance targets.
How Capacitance Acts as a Low-Pass Filter
A coaxial cable is a distributed capacitor. The center conductor and the shield are two conductors separated by a dielectric—the classic parallel plate capacitor geometry, just rolled into a cylinder. This distributed capacitance loads the signal source (the transducer element) and creates an RC low-pass filter with the source impedance.
The -3 dB cutoff frequency of this RC filter is approximately f₃ = 1 / (2π × R_source × C_cable). For a typical ultrasound transducer element with about 200Ω source impedance driving a 2-meter cable at 85 pF/m (170 pF total), the cutoff frequency is roughly 4.7 MHz. That doesn't mean the cable blocks signals above 4.7 MHz—it means the cable starts attenuating them. At 10 MHz, the cable-induced roll-off is about 3 dB. At 15 MHz, it's about 5 dB. At 20 MHz, it's 7+ dB.
Switch to a 58 pF/m cable (116 pF total at 2 meters), and the cutoff frequency moves to about 6.9 MHz. Now the attenuation at 15 MHz is only about 3 dB instead of 5 dB. That 2 dB improvement at 15 MHz preserves more of the high-frequency echo content, which translates directly to better axial resolution in the image.
The effect compounds with cable length. A 1-meter cable at 85 pF/m (85 pF total) has a cutoff at 9.4 MHz—significantly higher than the same cable at 2 meters. This is one reason portable ultrasound probes with shorter cables often produce noticeably sharper images at high frequencies than cart-based probes with longer cables, even when using identical transducer arrays.
What Determines Cable Capacitance
The coaxial capacitance formula is C = (2π × ε₀ × Dk) / ln(D/d), where Dk is the dielectric constant, D is the shield inner diameter, and d is the conductor outer diameter. Two levers to pull: reduce Dk, or increase D/d ratio.
| Dielectric Material | Dk (Dielectric Constant) | Typical Capacitance | Velocity of Propagation | Temperature Rating | Relative Cost |
|---|---|---|---|---|---|
| Solid PTFE | 2.1 | 83–87 pF/m | 69–70% | 260°C | 1.0× |
| Solid FEP | 2.0 | 78–82 pF/m | 70–71% | 200°C | 0.9× |
| Foamed PE (50% foam) | 1.4–1.6 | 62–72 pF/m | 80–85% | 80–105°C | 0.6× |
| ePTFE (75% porosity) | 1.2–1.5 | 55–65 pF/m | 82–90% | 260°C | 3–5× |
| Spiral PTFE tape | 1.5–1.8 | 65–75 pF/m | 75–82% | 260°C | 1.5–2× |
The D/d ratio is the other lever. A thicker dielectric wall increases D (shield inner diameter) relative to d (conductor outer diameter), reducing capacitance. But this directly increases the cable element OD—and for Bundled Micro Coaxial Cable s where dozens or hundreds of elements pack together, every tenth of a millimeter on element OD matters enormously. Switching to a lower-Dk dielectric is almost always more space-efficient than adding dielectric wall thickness.
Capacitance vs. Impedance: They're Connected
Here's a nuance that trips up some engineers: you can't independently specify impedance and capacitance on a coaxial cable. They're linked through the dielectric constant and geometry. For a given impedance target (50Ω), reducing capacitance by using a lower-Dk dielectric automatically changes the required D/d ratio to maintain 50Ω. The impedance formula is Z = (138 / √Dk) × log(D/d). With lower Dk, the required D/d ratio for 50Ω decreases—which means the dielectric wall can actually be thinner, partially offsetting the OD increase from the lower Dk.
In practice, this means that switching from solid PTFE (Dk=2.1) to ePTFE (Dk=1.3) for a 50Ω cable reduces capacitance by roughly 30% while only increasing element OD by about 5-8%. That's a favorable trade-off in most designs.
Capacitance Uniformity: The Hidden Specification
Most engineers focus on absolute capacitance value—"under 70 pF/m"—but for bundled ultrasound cables, the uniformity of capacitance across all channels matters as much as the average value. Capacitance variation between channels corresponds to velocity of propagation variation, which directly affects phase matching. A bundle where channels range from 58 to 72 pF/m (14 pF/m spread) will have worse phase coherence than a bundle at a uniform 80 pF/m, even though the average capacitance of the first bundle is lower.
We specify capacitance with both a maximum value and a maximum channel-to-channel variation. For standard ultrasound probe cables , ±5 pF/m variation across the bundle is typical. For tight phase matching Applications (±1%), we tighten this to ±2 pF/m, which requires pre-screening individual cable elements by measured capacitance before bundling—the same sorting process we use for propagation velocity matching.
This uniformity requirement is one reason that choosing a consistent dielectric material matters more than chasing the absolute lowest Dk. ePTFE with well-controlled tape wrapping gives excellent capacitance uniformity (±2-3 pF/m typical). Foamed PE, despite potentially lower average capacitance, tends to have wider variation (±5-8 pF/m) due to foam cell density inconsistency. The uniform dielectric produces a better imaging result than the lower-average but inconsistent one.
Real-World Impact: Measured Data from Probe Builds
We built matched sets of 128-channel ultrasound probe cable assemblies using identical everything except the dielectric: solid PTFE (84 pF/m) and ePTFE (59 pF/m). Both cables were 2.0 meters, 42 AWG, 50Ω, braided shield, FEP jacket. The cables were tested on the same probe and system platform, switching only the cable assembly between measurements.
At 5 MHz (abdominal imaging frequency), the image quality difference was subtle—about 0.5 dB improvement in SNR with the ePTFE cable, and no visually detectable resolution difference. The solid PTFE cable was already adequate at this frequency.
At 12 MHz (vascular/musculoskeletal frequency), the difference became meaningful. The ePTFE cable delivered 1.8 dB better SNR and measurably sharper axial resolution—the -6 dB pulse width shortened from 0.21mm to 0.18mm on a wire target phantom. Clinically, this showed up as better definition of small vessel walls and tendon fiber structure.
At 18 MHz (superficial/dermatology frequency), the ePTFE cable showed 3.2 dB better SNR and a dramatic difference in high-frequency content preservation. The solid PTFE cable was visibly rolling off the 18 MHz signal, producing softer images with reduced resolution. The ePTFE cable maintained image sharpness comparable to what you'd see with a 1-meter cable—effectively reclaiming the bandwidth that the cable capacitance was stealing.
When Low Capacitance Matters and When It Doesn't
Not every application needs the lowest possible capacitance. The frequency and cable length determine whether capacitance is a first-order or second-order concern:
Capacitance is critical when operating frequency exceeds 10 MHz AND cable length exceeds 1.5 meters. This covers high-frequency ultrasound probes (superficial, ophthalmologic, intravascular), broadband probes needing to preserve bandwidth up to 15-20 MHz, and phased array NDT at 10-15 MHz with long cables.
Capacitance matters but isn't dominant when operating frequency is 5-10 MHz with 1.5-2.5m cables. This is the sweet spot for most cart-based medical ultrasound. Choosing ePTFE over PTFE gives a measurable improvement but isn't make-or-break. The cost premium of ePTFE needs to be weighed against the performance gain.
Capacitance is secondary when operating frequency is below 5 MHz or cable length is under 1 meter. Standard solid PTFE dielectric is more than adequate. The money is better spent on tighter impedance tolerance or better phase matching rather than lower capacitance.
Specifying Capacitance on Your Cable RFQ
State capacitance as a maximum value per meter at the cable level (not per assembly), measured at 1 kHz or 1 MHz per IEC 61196. Specify the test frequency because capacitance is slightly frequancy-dependent due to dielectric relaxation effects—though for PTFE-family materials, the variation between 1 kHz and 1 MHz is typically less than 2%.
If your application falls in the "capacitance is critical" category, specify both the maximum pF/m and the dielectric material. Saying "max 65 pF/m" leaves ambiguity about how the manufacturer achieves it—thicker dielectric wall (larger OD) or lower-Dk material (higher cost). Better to specify "ePTFE dielectric, max 65 pF/m" so the manufacturing approach is clear.
If you're evaluating whether a lower-capacitance cable would improve your probe or system performance, send us your current cable specification and operating frequency range . We can provide comparative samples with measured bandwidth data showing the expected performance difference—the data is usually more convincing than the theory, especially for system designers who haven't previously isolated the cable's contribution to bandwidth limiting.
The direction the industry is moving is clear: transducer frequencies are increasing, probe bandwidths are widening, and cable capacitance is becoming a more significant bottleneck. Five years ago, ePTFE dielectric was a premium option requested by maybe 20% of our ultrasound cable customers. Today it's closer to 45%, and the trend is accelerating as probe designers push toward higher-frequency, wider-bandwidth imaging.
Related Products
Our production covers 4 to 512 cores in 36-50 AWG, with ePTFE dielectric and 100% electrical inspection on every channel. Related products:
Have an existing cable to match or replace? Send us the sample or spec for a like-for-like quote.
