RF Attenuator Calculator
Design Pi-pad, T-pad, and Bridged-T RF attenuators with exact resistor values, impedance verification, and power rating analysis.
About This Calculator
RF attenuators are essential passive components used to reduce signal amplitude without introducing significant distortion. They are widely employed in RF test setups, receiver front-ends, level control circuits, and impedance-matching networks. This calculator determines the exact resistor values for the three most common attenuator topologies: Pi-pad, T-pad, and Bridged-T, all designed to maintain a matched characteristic impedance at both input and output ports.
How to Use
- Select Attenuator Type — Choose Pi-pad (balanced, most common for general RF), T-pad (simpler PCB layout, fewer ground connections), or Bridged-T (low attenuation values with high precision).
- Set System Impedance Z₀ — Typically 50 Ω for most RF systems, 75 Ω for video/CATV, or 100 Ω for differential/twisted-pair applications.
- Enter Attenuation (dB) — The desired signal reduction in decibels. Range: 0.1 dB to 60 dB. A 3 dB attenuator halves the power; 10 dB reduces power to one-tenth; 20 dB to one-hundredth.
- Set Power Handling (W) — The maximum input power the attenuator must handle. The calculator derives the required power rating for each resistor to ensure reliable operation.
Attenuator Topologies
| Pi-pad (π) | Three resistors in a π-shaped network. Most widely used in general-purpose RF attenuation. Offers balanced performance, good bandwidth, and symmetric input/output impedance. Requires two ground connections for the shunt resistor. |
| T-pad (T) | Three resistors in a T-shaped network. Simpler PCB layout with only one ground connection. Slightly easier to fabricate at very high frequencies. Ideal when ground vias are at a premium. |
| Bridged-T | Four resistors in a bridged-T configuration. Best suited for low attenuation values (typically < 20 dB) where Pi-pad and T-pad resistor values become impractically large. Provides the highest precision at low attenuation. |
Formulas
All formulas use K = 10A/20, where A is the attenuation in dB and Z₀ is the system characteristic impedance. For a symmetric attenuator, input impedance Zin = output impedance Zout = Z₀ when terminated in Z₀.
| Pi-pad R₁ | R₁ = Z₀ × (K + 1) / (K − 1) |
| Pi-pad R₂ | R₂ = Z₀/2 × (K − 1) / K |
| T-pad R₁ | R₁ = Z₀ × (K − 1) / (K + 1) |
| T-pad R₂ | R₂ = 2 × Z₀ × K / (10A/2 − 1) |
| Bridged-T R₁ | R₁ = Z₀ × (K − 1) |
| Bridged-T R₂ | R₂ = Z₀ / (K − 1) |
| Bridged-T R₃ | R₃ = Z₀ (fixed) |
Power Rating Notes
The calculator computes the power dissipated in each resistor when the attenuator is driven at the specified input power into a matched Z₀ load. For continuous operation, select resistors with a power rating at least 2× the calculated dissipation. For high-power transmitters, consider using multiple resistors in series/parallel to share the thermal load. At microwave frequencies (> 1 GHz), parasitic inductance and capacitance become significant — use surface-mount chip resistors with minimal lead inductance and ensure a solid ground plane.