Tee attenuator basics for matched transmission lines
A tee (T) attenuator uses three resistors arranged in a “T” shape: two series resistors in the signal path and one shunt resistor to ground. When the resistor values are chosen correctly, the T pad provides a defined attenuation while maintaining the desired input and output impedance.
In a system with characteristic impedance Z0 (for example 50 Ω, 75 Ω or 600 Ω), a properly designed tee attenuator can:
- Reduce signal level by a specified number of decibels (dB)
- Present approximately Z0 to both source and load
- Improve return loss and reduce reflections
- Protect sensitive stages from excessive power
Attenuation and resistor equations
For a symmetrical T attenuator with equal input and output impedances Z0 and target attenuation AdB, define the voltage attenuation factor:
K = 10(AdB / 20) (K > 1 for attenuation)
The required resistor values are:
-
Series resistors (Rs, one at input and one at output):
Rs = Z0 × (K − 2) / (K + 2) -
Shunt resistor (Rsh to ground at the T junction):
Rsh = 4 × K × Z0 / (K² − 4)
These expressions yield a constant-impedance pad when the load is Z0. The calculator applies them automatically so you only need to specify Z0 and AdB.
Note that a symmetrical T pad requires a minimum attenuation of a few dB (K must be > 2 for positive Rs). For very small trims, other topologies or variable attenuators are often used.
Common attenuation levels and power ratios
The table below shows how voltage and power change for popular attenuation values:
| Attenuation (dB) | Power ratio (Pout / Pin) | Voltage ratio (Vout / Vin) | Typical application |
|---|---|---|---|
| 6 dB | ≈ 0.25 | ≈ 0.50 | General level reduction, modest isolation |
| 10 dB | 0.1 | ≈ 0.316 | Protecting receivers, line-up gain trimming |
| 20 dB | 0.01 | 0.1 | Strong isolation between stages |
| 30 dB | 0.001 | ≈ 0.0316 | Very low-level test signals, precise measurements |
Using the Tee Attenuator Calculator
- Set the system impedance: Enter Z0 to match your line or equipment (50 Ω, 75 Ω, 600 Ω, etc.).
- Enter the desired attenuation: Type the attenuation in dB (for example 6, 10, 20). The calculator derives K and uses it in the design equations.
- Read the resistor values: The tool outputs Rs (used at both input and output) and Rsh (the shunt resistor to ground).
- Select real components: Choose the nearest standard resistor values and verify the resulting attenuation and match, especially for precision RF or audio work.
Worked example – 20 dB T pad in a 50 Ω system
- Z0 = 50 Ω
- AdB = 20 dB → K = 10(20/20) = 10
Series resistors:
Rs = 50 × (10 − 2) / (10 + 2) = 50 × 8 / 12 ≈ 33.3 Ω
Shunt resistor:
Rsh = 4 × 10 × 50 / (10² − 4) = 2000 / 96 ≈ 20.8 Ω
In practice you might choose E24 values such as 33 Ω for Rs and 20 Ω or 21 Ω for Rsh, then verify the exact attenuation and input impedance with a simulator or network analyzer.
Design tips for tee attenuators
- Check resistor power ratings: For high-level signals, calculate power in each resistor at maximum input and apply generous derating.
- Watch cumulative loss: When multiple pads are cascaded, total insertion loss can be significant; plan the gain structure around the worst-case attenuation.
- Consider frequency range: At high frequencies, parasitic inductance and capacitance affect performance. Use suitable RF resistors and a compact layout.
- Choose between T and π pads wisely: T pads may be easier to implement in some layouts or balanced-like conditions, while π pads are often convenient in unbalanced coaxial systems. The calculator for each topology helps you compare options quickly.
This tee attenuator calculator speeds up the transition from target attenuation in dB to real resistor values, helping you build matched pads for RF chains, audio lines, lab fixtures and production test setups.
