Free RF Power Attenuator Pad Calculator
Design symmetrical Pi, Tee, and Bridged-Tee RF attenuators. Calculate exact resistor values, standard 1% E96 matches, and thermal power dissipation per element.
📉 Attenuator Specifications & Topology
For low-attenuation pads (≤ 6 dB), a Pi Pad is preferred because its series resistor is small, minimizing parasitic series inductance. For high attenuation (≥ 20 dB), Tee pads or cascaded 10 dB pads prevent shunt resistor values from falling below a couple ohms.
Nearest 1% E96: R1 = 150 Ω, R2 = 37.4 Ω
• Symmetrical Layout: Input and output are fully reciprocal; pad can be connected in either direction.
• Component Selection: Use thick-film SMD (1206 or 2512) or coaxial flanged resistors. Never use wirewound resistors due to high parasitic inductance.
Cascading for High Power: High-power 20 dB or 30 dB pads dissipate >90% of all RF power in the very first input shunt resistor. To distribute heat evenly, cascade two 10 dB pads or three 6 dB pads separated by 50Ω microstrip traces.
Recommended Tools & Equipment
Tested hardware and components for high reliability
RF Symmetrical Attenuator Mathematics
An attenuator reduces signal amplitude by a designated decibel ratio while presenting a matched characteristic impedance $Z_0$ to both the source and load ($SWR = 1.0:1$).
Governing Equations
Pi Pad: R_shunt = Z_0 × (K + 1) / (K - 1)
R_series = Z_0 × (K² - 1) / (2K)
Tee Pad: R_series = Z_0 × (K - 1) / (K + 1)
R_shunt = Z_0 × (2K) / (K² - 1)
Bridged-Tee Pad: R_bridge = Z_0 × (K - 1)
R_shunt = Z_0 / (K - 1)
Frequently Asked Questions
Why must wirewound resistors never be used in RF attenuators?
Wirewound resistors are miniature inductors (coils of Nichrome resistance wire). At radio frequencies, their inductive reactance swamps the resistance, completely destroying the impedance match and reflection coefficient. Always use carbon film, thin film, or metal oxide resistors.
What is the difference between a Pi pad and a Tee pad?
Electrically, both provide identical attenuation and 50 Ohm matching. Physically, Pi pads have higher resistor values that are easier to match with standard SMD components for low attenuation. Tee pads have lower resistor values, making them better suited for high attenuation.
How does an attenuator improve source-load SWR mismatch?
Any reflected wave travels through the attenuator twice (once forward, once reflected). A 6 dB pad reduces reflected power by 12 dB, converting an awful 3.0:1 SWR load into a gentle 1.25:1 SWR seen by the transmitter.