Free RF Attenuator Pad Calculator

Calculate exact resistor values for Pi (π) and Tee (T) passive RF attenuator networks with 50Ω and 75Ω impedance matching.

📉 Attenuation & Impedance

Standard Attenuation Presets

Attenuation A (dB)

System Impedance Z_0 (Ω)

Input RF Power (Watts) Transmitter output entering the attenuator

📊 Resistor Values (Pi vs. Tee)

Output Power Delivered

0.100 W (100 mW)

-10.0 dB power reduction (90% dissipated as heat)

Pi (π) Topology

R1 (Parallel): 96.2 Ω

R2 (Series): 71.2 Ω

2× R1 shunts, 1× R2 series

Tee (T) Topology

R1 (Series): 26.0 Ω

R2 (Shunt): 35.1 Ω

2× R1 series, 1× R2 shunt

Voltage Ratio (V_out / V_in): 0.316 (31.6%)

Power Dissipated in Pad: 0.900 W (Requires 1W+ resistors)

Input Return Loss / SWR: > 35 dB (SWR < 1.05:1)

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Designing Passive RF Attenuators

Passive attenuator pads reduce signal amplitude without distorting the waveform while presenting a constant characteristic impedance ($Z_0$) to both source and load. They protect sensitive spectrum analyzers from overdrive, improve transmitter load matching, and isolate stages.

The Pi (π) and Tee (T) Pad Equations

K = 10^{\frac{A_{\text{dB}}}{20}} \quad \text{(Voltage Ratio Factor)}

\text{\bf Tee Pad: } R_{\text{series}} = Z_0 \cdot \frac{K - 1}{K + 1}, \quad R_{\text{shunt}} = Z_0 \cdot \frac{2K}{K^2 - 1}

\text{\bf Pi Pad: } R_{\text{shunt}} = Z_0 \cdot \frac{K + 1}{K - 1}, \quad R_{\text{series}} = Z_0 \cdot \frac{K^2 - 1}{2K}

Choosing Between Pi and Tee Topologies

At high attenuation values ($>15\text{ dB}$), the series resistor in a Tee pad becomes very close to $Z_0$ (e.g. $40.9 Omega$ on 50Ω) while the shunt drops to single-digit ohms ($10.1 Omega$), where parasitic lead inductance ruins frequency response. In contrast, Pi pads at high dB have large shunt resistors and a large series resistor, making them significantly easier to construct at microwave frequencies with SMD chip components.

Frequently Asked Questions

Why are carbon film or metal film resistors required instead of wirewound?

Wirewound resistors are coiled inductors that exhibit massive inductive reactance at radio frequencies, completely ruining input return loss and frequency flatness. Precision thin-film or SMD chip resistors have virtually zero parasitic inductance and operate reliably up into GHz frequencies.

How does an attenuator improve SWR?

Any reflected signal from a mismatched antenna must pass through the attenuator twice (once going forward, and once reflecting back). A 3 dB pad improves the return loss by 6 dB, reducing a high 3.0:1 SWR down to an acceptable 1.5:1 SWR seen by the transmitter.

What resistor wattage rating do I need?

The first resistor in the network absorbs the vast majority of dissipated power. For a 10 dB pad with a 1 Watt input, 900 mW is dissipated as heat. Choosing resistors with at least 2x power derating (2W total rating) ensures thermal stability.