Attenuation (Transmission Line)
Understanding Transmission Line Attenuation
Every meter of cable between your transmitter and antenna absorbs a fraction of the signal. At 100 MHz, the loss might be negligible. At 28 GHz, that same cable might consume half the signal power before it reaches the antenna. Transmission line attenuation is this fundamental, unavoidable loss mechanism that RF engineers must manage in every system design.
Conductor Loss and Skin Effect
At RF frequencies, current flows only in a thin skin on the conductor surface (the skin depth: ~2 μm in copper at 1 GHz). This concentrates the current into a smaller cross-sectional area, increasing the effective resistance. Conductor loss increases as √f because the skin depth decreases as 1/√f. Larger conductors have more surface area for current flow and lower loss — this is why low-loss cables are physically larger.
Dielectric Loss
The dielectric material between conductors absorbs energy through molecular polarization and relaxation. Dielectric loss increases linearly with frequency and is characterized by the material's loss tangent (tan δ). PTFE (Teflon) has an extremely low tan δ (~0.0002), making it the preferred dielectric for low-loss coaxial cables. FR-4 PCB material has tan δ of ~0.02 — 100× higher — which is why FR-4 traces are unusable at mmWave frequencies.
Key Equations
Transmission Line Attenuation is the loss of signal power per unit length as an RF signal propagates along a transmission line (coaxial cable, microstrip, stripline,...
Key specifications:
100 MHz | 28 GHz | 2 μm | 1 GHz | 2 dB | 1 dB
Z0: = √(L/C) = √((R+jωL)/(G+jωC))
Comparison
| Aspect | Attenuation (Transmission Line) Spec | Typical Range | Impact | Design Note |
|---|---|---|---|---|
| Primary function | For a coaxial cable, conductor attenuati... | Application-dep. | Critical | Verify in sim |
| Operating range | Dielectric attenuation is proportional t... | Application-dep. | Critical | Verify in sim |
| Performance | Line selection in RF system design is fu... | Application-dep. | Critical | Verify in sim |
| Integration | Understanding Transmission Line Attenuat... | Application-dep. | Critical | Verify in sim |
| Trade-off | At 100 MHz, the loss might be negligible... | Application-dep. | Critical | Verify in sim |
Frequently Asked Questions
How much loss is typical for common cable types?
At 1 GHz: RG-58 (small, flexible) ≈ 0.6 dB/m, RG-8 (medium) ≈ 0.2 dB/m, LMR-400 (low-loss) ≈ 0.1 dB/m, 7/8-inch hardline (rigid) ≈ 0.03 dB/m. At 28 GHz (mmWave): even the best coaxial cables exceed 1 dB/m, which is why mmWave 5G systems use very short cable runs or integrate the radio directly at the antenna to eliminate cable loss entirely.
What is the maximum usable frequency for coaxial cable?
The maximum frequency is limited by the onset of higher-order waveguide modes inside the coaxial structure. Above the TE₁₁ mode cutoff frequency (determined by the cable's outer conductor diameter), the cable supports multiple propagation modes, causing severe signal distortion. For a typical SMA-connectorized cable (outer diameter ~4mm), the cutoff is approximately 25 GHz. For 2.92mm (K) connectors, the cutoff extends to 40 GHz. Above these frequencies, waveguide becomes the preferred transmission medium.
How does microstrip attenuation compare to coaxial?
Microstrip lines on PCB substrates have significantly higher attenuation than coaxial cables due to higher conductor loss (thin copper traces vs. thick cable conductors) and higher dielectric loss (PCB substrates vs. PTFE). At 28 GHz on Rogers RO4003C substrate, microstrip attenuation is approximately 0.5 dB/cm — 50× higher per unit length than a high-quality coaxial cable. This is why mmWave PCB traces must be kept as short as possible, and why antenna-integrated modules (AiMs) that minimize trace length are the dominant 5G mmWave architecture.