What is the difference between insertion loss and attenuation?

Attenuation refers specifically to the dissipative loss: power absorbed and converted to heat within the component. Insertion loss is a broader measurement that includes both dissipative loss and mismatch loss (power reflected due to impedance mismatch). For a well-matched component (return loss above 20 dB), mismatch loss is negligible (less than 0.04 dB) and insertion loss approximately equals attenuation. For a poorly matched component (return loss of 10 dB), mismatch loss alone contributes 0.46 dB, and the total insertion loss is the sum of mismatch and dissipative contributions. A VNA measures S21, which captures both effects combined.

Why does insertion loss increase with frequency?

Several mechanisms contribute. Conductor loss increases as the square root of frequency due to skin effect, which reduces the effective conductor cross-section. Dielectric loss increases linearly with frequency due to increased molecular friction in the insulating material. Radiation loss from discontinuities (bends, transitions, connectors) increases with frequency because the physical dimensions become a larger fraction of the wavelength. For PCB traces, surface roughness contributes additional loss above about 5 GHz as the skin depth approaches the roughness height. A typical FR-4 microstrip line loses 0.1 dB/cm at 1 GHz but 0.8 dB/cm at 10 GHz.

How does insertion loss before the LNA affect receiver sensitivity?

Any passive loss before the LNA adds directly to the receiver noise figure. A 1 dB insertion loss from a preselector filter adds 1 dB to the system NF, reducing sensitivity by 1 dB (the receiver can detect signals 1 dB weaker without the filter). This is why the component order in a receiver front-end matters enormously: a 2 dB filter followed by a 0.5 dB NF LNA gives a system NF of 2.5 dB. Reversing the order (LNA first) gives approximately 0.7 dB system NF but loses the preselector's blocker protection. The trade-off between selectivity and sensitivity is one of the fundamental decisions in receiver architecture.

RF Design

Insertion Loss

IL (dB)

Every component between the transmitter and the antenna steals power. A preselector filter takes 1.5 dB. Two SMA connectors take 0.3 dB each. The cable takes 2 dB. A circulator takes 0.4 dB. Total: 4.5 dB. That means only 35% of the transmitter's power reaches the antenna. At the receiver, the same losses add directly to the noise figure, degrading sensitivity. Insertion loss, the ratio of output power to input power expressed in dB, is the tax every passive component charges for the privilege of being in the signal path. Minimizing it is not an optimization; it is a survival requirement.

Category: RF Design

Formula: IL = −20·log(|S₂₁|)

Ideal: 0 dB (no loss)

What Every Component Costs You

Component	IL at 1 GHz	IL at 10 GHz	IL at 28 GHz	Primary Loss Mechanism
SMA connector (pair)	0.1 dB	0.3 dB	0.5 dB	Conductor, radiation
2.92mm connector (pair)	0.05 dB	0.15 dB	0.25 dB	Conductor
PCB via transition	0.1 dB	0.3 dB	0.5 to 1 dB	Radiation, mismatch
50 Ω microstrip (FR-4, 10 cm)	0.1 dB	0.8 dB	2 to 3 dB	Dielectric (tanδ)
50 Ω microstrip (Rogers, 10 cm)	0.04 dB	0.2 dB	0.5 dB	Conductor, roughness
Preselector filter (ceramic)	1 to 2 dB	2 to 3 dB	3 to 5 dB	Resonator Q, coupling
Circulator	0.3 dB	0.4 dB	0.5 dB	Ferrite loss

Insertion loss decomposition:
IL_total = IL_dissipative + IL_mismatch
IL_mismatch = −10·log(1 − |S₁₁|²) dB

Impact on receiver NF:
NF_sys = IL_pre-LNA + NF_LNA + (F₂−1)/G_LNA + ...
1 dB of IL before the LNA = 1 dB worse NF = 1 dB less sensitivity

Common Questions

Frequently Asked Questions

IL vs. attenuation?

Attenuation = dissipative loss only (heat). IL includes mismatch loss too. At 20 dB return loss, mismatch adds <0.04 dB. At 10 dB RL, mismatch alone is 0.46 dB. VNA S₂₁ captures both together.

Why does IL increase with frequency?

Conductor loss scales as √f (skin effect). Dielectric loss scales linearly with f. Radiation from discontinuities increases with frequency. PCB roughness adds loss above 5 GHz when skin depth approaches roughness height.

How does pre-LNA IL affect sensitivity?

1 dB filter loss before LNA = 1 dB worse system NF = 1 dB less sensitivity. Filter before LNA: NF = 2 + 0.5 = 2.5 dB. LNA before filter: NF ≈ 0.7 dB. The trade-off: preselector protects LNA from blockers.

Related Terms

← Input Impedance Intermodulation →

Loss Budget

System Insertion Loss Accumulator

Stack cables, connectors, filters, and transitions to compute the total insertion loss from source to destination at any frequency. See the NF impact on receiver sensitivity.

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