Tee Attenuator Calculator

Design symmetric Tee (T-pad) attenuators for RF and audio applications. Compute exact resistor values from characteristic impedance and desired attenuation.

Design formulas (matched, symmetric Tee):

k = 10^(dB/20) (voltage ratio)

R1 = Z₀ · (k-1)/(k+1)

R2 = Z₀ · (2k)/(k²-1)

R3 = R1 (symmetric)

Ω
Common values: 50 Ω (RF), 75 Ω (video), 600 Ω (audio)
dB
Positive value; typical range 0.1 – 60 dB
50Ω / 3dB 50Ω / 6dB 50Ω / 10dB 50Ω / 20dB 75Ω / 6dB 600Ω / 10dB
Calculating...

Understanding the Tee Attenuator

A Tee attenuator (or T-pad) is a passive resistive network that reduces signal amplitude while maintaining matched impedance on both ports. It is symmetric, meaning the input and output impedances are equal to the characteristic impedance Z₀ when properly terminated.

Design equations (for symmetric Tee):

Let the desired attenuation in dB be A. The voltage attenuation ratio k = 10^(A/20).

R₁ = R₃ = Z₀ · (k-1)/(k+1)

R₂ = Z₀ · (2k)/(k²-1)

These formulas ensure that the input impedance looking into either port equals Z₀ when the other port is terminated with Z₀.

Frequency Response and Limitations

Ideal resistors are frequency‑independent, so a Tee attenuator theoretically works from DC to microwaves. In practice, parasitic capacitance and inductance of real resistors limit high‑frequency performance. For RF applications, surface‑mount thin‑film resistors are preferred. The attenuator is also linear and introduces negligible phase shift.

Advantages of Tee attenuator:

  • Simple and inexpensive.
  • Provides precise attenuation if resistors are accurate.
  • Maintains constant impedance, preventing reflections.
  • Symmetric design allows bidirectional use.

Power Dissipation and Resistor Ratings

Each resistor dissipates a portion of the input power. The worst‑case dissipation often occurs in the shunt resistor R₂ for low attenuation values. For high‑power applications, calculate the maximum voltage/current and choose resistors with appropriate power ratings (derate by 50% for reliability). The calculator does not compute power automatically, but you can estimate using the voltage across each resistor.

Comparison with Pi Attenuator

The Pi attenuator (π‑pad) uses two shunt resistors and one series resistor. Tee attenuators are preferred when the series arm can be split (e.g., for balanced lines) and often have higher power handling in the shunt element. Both networks provide the same attenuation and impedance matching; the choice depends on physical layout and component availability.

Applications

  • RF & microwave systems: Adjust signal levels in receivers, transmitters, and test equipment.
  • Audio engineering: Line level matching and volume control (using stepped attenuators).
  • Laboratory measurements: Extend the range of meters and oscilloscopes.
  • Impedance matching: Can be used to reduce reflections in mismatched lines (though an attenuator always reduces signal).

? About This Tool

Developed by GetZenQuery RF Engineering Team — experienced in passive network design. The formulas are derived from classic textbooks and verified with industry standards.

  • Pozar, D. M. (2012). Microwave Engineering (4th ed.). Wiley.
  • Bahl, I. J. (2009). Fundamentals of RF and Microwave Transistor Amplifiers. Wiley.
  • ANSI/IEEE Std 145-1983, "IEEE Standard Definitions of Terms for Radio Wave Propagation".

Questions? Contact us at [email protected].

Frequently Asked Questions

Yes, the standard Tee attenuator is designed for unbalanced lines where one side is ground. It is widely used in coaxial systems. For balanced lines, a balanced Tee (with two series arms and a shunt across the lines) is required; our calculator assumes unbalanced topology.

The formulas still work, but the resistor values become extreme: R1 becomes very close to Z₀, and R2 becomes very small. Parasitic effects and resistor tolerances may cause deviations. It is often better to cascade multiple lower‑value attenuators (e.g., two 30 dB pads) to achieve high attenuation with better accuracy.

For precise attenuation, use 1% tolerance or better. Metal‑film or thin‑film resistors are recommended. At RF frequencies, surface‑mount resistors have lower parasitics. If you need exact values, you can combine resistors in series/parallel or use trim pots for adjustable attenuation.

Yes, a symmetric Tee attenuator is reciprocal. The attenuation is the same regardless of which port is input, provided both ports are terminated with Z₀. This is useful for bidirectional systems.

Attenuation can be arbitrarily close to 0 dB, but as A → 0 dB, k → 1, R1 → 0, and R2 → ∞ (open circuit). In practice, very small attenuations (like 0.1 dB) require impractically small series resistors (fractions of an ohm) and very large shunt resistors (kΩ). A 1 dB pad is usually the smallest practical design.