Every
Micro Coaxial Cable
has a characteristic impedance — the ratio of voltage to current for a traveling electromagnetic wave, determined by conductor diameter, dielectric constant, and shield geometry. The two standard impedances are 50 Ω and 75 Ω. Choosing incorrectly causes impedance mismatch reflections that degrade signal quality, reduce measurement accuracy, and can damage sensitive transducers.
This guide explains when to specify 50 Ω versus 75 Ω in micro coaxial cable systems, focusing on the application most relevant to fine-gauge (36–50 AWG) cables: ultrasound imaging, NDT inspection, video transmission, and RF instrumentation.
The Physics — Why Two Standard Impedances Exist
The characteristic impedance of a coaxial cable depends on three physical parameters: the ratio of shield inner diameter (D) to conductor outer diameter (d), and the dielectric constant (Dk) of the insulating material between them.
Z₀ = (138 / √Dk) × log₁₀(D/d)
For air dielectric (Dk = 1.0), two special impedances emerge from electromagnetic theory:
77 Ω — minimum attenuation. The D/d ratio that minimizes signal loss per unit length. Rounded to 75 Ω for practical standardization.
30 Ω — maximum power handling. The D/d ratio that maximizes the power a cable can carry before dielectric breakdown.
50 Ω is the geometric compromise between minimum attenuation (75 Ω) and maximum power handling (30 Ω). It was standardized for RF/microwave systems where both moderate power handling and reasonable attenuation matter.
For modern micro coaxial cables using ePTFE dielectric (Dk ≈ 1.45), the minimum-attenuation impedance shifts to approximately 65 Ω, but the 50/75 Ω standards remain unchanged for historical compatibility.
50 Ω vs. 75 Ω — Quick Decision Guide
| Application | Standard Impedance | Why |
| Medical ultrasound (all types) | 50 Ω | Transducer impedance matching, beamformer standard |
| NDT ultrasound (PAUT, TOFD, UT) | 50 Ω | Instrument input impedance, pulser matching |
| IVUS / ICE catheters | 50 Ω | Transducer element impedance |
| Analog video (composite, HD-SDI) | 75 Ω | Broadcast standard, minimum attenuation |
| RF instrumentation (VNA, spectrum) | 50 Ω | Test equipment standard |
| Antenna feed | 50 Ω | Power handling + attenuation compromise |
| Cable TV / CATV | 75 Ω | Video distribution, minimum loss |
| Digital high-speed (USB, HDMI) | Per protocol | USB: 90 Ω diff; HDMI: 100 Ω diff |
For micro coaxial cables in medical and NDT , the answer is almost always 50 Ω. The 75 Ω option exists for specific video and RF Application where the system impedance is designed around 75 Ω.
Why Ultrasound Uses 50 Ω
Ultrasound transducer elements are not purely resistive — they have complex impedance that varies with frequency. However, the electrical matching networks (inductors and transformers) between the transducer element and the cable are designed to present approximately 50 Ω to the cable. The beamformer transmit/receive electronics also present 50 Ω input/output impedance.
This creates a 50 Ω system: transmitter (50 Ω) → cable (50 Ω) → matching network → transducer element → matching network → cable (50 Ω) → receiver (50 Ω).
If a 75 Ω cable is inserted into this 50 Ω system, the impedance mismatch at each end creates reflections. The voltage reflection coefficient:
Γ = (75 − 50) / (75 + 50) = 0.20
This means 4% of signal power is reflected at each connector interface (20% voltage reflection, squared for power). Over a round trip (transmit and receive), the effective signal loss from mismatch alone is approximately 0.35 dB — before accounting for multiple reflections that create time-domain artifacts in the ultrasound image.
For phase-matched ultrasound systems, the mismatch reflections are even more damaging because they arrive at different times on each channel (depending on cable length variation), creating systematic phase errors that degrade beamforming accuracy.
When to Use 75 Ω Micro Coaxial
75 Ω micro coaxial cable is specified when the system impedance is designed around 75 Ω:
Analog video transmission: Composite video (NTSC/PAL), HD-SDI, and 3G-SDI are 75 Ω systems. Endoscopy video processors using analog video interfaces may require 75 Ω cables from camera head to processor.
Specific NDT instruments: Some older or specialized flaw detectors have 75 Ω input impedance. Always verify your instrument specification. Modern PAUT instruments (OmniScan, TOPAZ, etc.) are uniformly 50 Ω.
RF measurement at 75 Ω: Network analyzers and impedance meters configured for 75 Ω measurements (common in cable TV, broadcast, and antenna testing).
Custom sensor interfaces: Some research transducers and sensors are designed for 75 Ω to minimize cable attenuation (75 Ω has approximately 12% lower attenuation than 50 Ω at the same frequency in the same cable geometry).
Physical Differences — 50 Ω vs. 75 Ω at Same AWG
| Parameter | 42 AWG, 50 Ω | 42 AWG, 75 Ω | Difference |
| Conductor diameter | 0.064 mm | 0.064 mm | Same |
| Dielectric OD (ePTFE) | 0.22 mm | 0.30 mm | 75 Ω needs larger D/d |
| Cable OD | 0.48 mm | 0.55 mm | 75 Ω is ~15% larger |
| Capacitance | 95 pF/m | 67 pF/m | 75 Ω has 30% lower C |
| DC resistance | Same | Same | Conductor unchanged |
| Attenuation at 10 MHz | 0.8 dB/m | 0.7 dB/m | 75 Ω has ~12% lower |
| Shield braid coverage | ≥ 85% | ≥ 82% | Larger shield diameter |
To achieve 75 Ω, the dielectric OD must increase (larger D/d ratio in the impedance formula). This increases overall cable OD by approximately 15%. For space-constrained Application (catheters, endoscopes), this size increase may be unacceptable — another reason 50 Ω dominates in medical micro coaxial.
Impedance Mismatch — What Actually Happens
When cable impedance does not match the system impedance, reflections occur at every impedance transition (connector, splice, cable-to-instrument interface). Effects include:
Signal loss: Energy reflected back toward the source reduces delivered signal amplitude. At 50/75 Ω mismatch: 0.18 dB loss per transition (0.35 dB round trip).
Time-domain artifacts: In pulsed systems (ultrasound, NDT), reflections appear as ghost echoes at times corresponding to the cable round-trip delay. In ultrasound imaging, these appear as horizontal line artifacts.
Phase distortion: Reflected signals interfere with the primary signal, creating frequency-dependent phase distortion. Degrades image resolution in ultrasound and defect sizing accuracy in NDT.
Standing waves: At specific frequencies where cable length equals integer multiples of half-wavelength, standing waves create peaks and nulls in the frequency response. Affects broadband ultrasound transducers operating at 1–20 MHz.
Return loss: The ratio of incident to reflected power. At perfect match: return loss = ∞ (no reflection). At 50/75 Ω mismatch: return loss = 14 dB. Acceptable for many Application; marginal for precision phase-matched systems.
Frequently Asked Questions
Can I use a 75 Ω cable in a 50 Ω ultrasound system?
Technically the system will function — 14 dB return loss is not catastrophic. However, the 4% power reflection per interface reduces SNR, and the time-domain reflections can create image artifacts. For research or prototype work with short cables (< 0.5 m), the impact may be acceptable. For production probes with 2+ meter cables and phase-matching requirements, always use 50 Ω.
Why does lower capacitance not make 75 Ω better for ultrasound?
Lower capacitance (67 pF/m vs. 95 pF/m) would reduce signal attenuation — but this advantage is completely negated by the 4% reflection loss at each impedance mismatch. A matched 50 Ω system has lower total insertion loss than a mismatched 75 Ω cable in a 50 Ω system, at any cable length relevant to medical ultrasound.
Is there a 50 Ω vs. 75 Ω cost difference?
Minimal. Same conductor, same shield, slightly different dielectric OD. The cost difference is < 5% at same AWG. The choice should always be driven by system impedance matching, not cost.
Do you stock both 50 Ω and 75 Ω?
50 Ω is stocked in 42 AWG and 44 AWG (the vast majority of demand). 75 Ω is made-to-order at any AWG with 2–4 week lead time.
What about 93 Ω or 95 Ω cables?
These exist in the larger coaxial cable world (RG-62) for specific legacy Application (ARCNET, IBM pulse systems). We do not manufacture 93/95 Ω micro coaxial — there is no demand in the micro coaxial market. If you need non-standard impedance, we can engineer custom D/d ratios to achieve any impedance from 30 Ω to 120 Ω.
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Related Products
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