RF Attenuator Calculator (Pi, Tee, Bridged-T)
Design passive resistor attenuation networks for RF/microwave transceivers, test instrument protection, and antenna matching. Enter target attenuation in dB, system impedance (50Ω / 75Ω), and RF input power to get theoretical resistor values, closest E96 1% standard values, realized attenuation, VSWR, and power dissipation.
3dB Pi Attenuator: Shunt Resistors R1 = R2 = 294 Ω, Series Resistor R3 = 17.8 Ω (Actual: 3.02 dB, VSWR ≈ 1.02);
6dB Pi Attenuator: R1 = R2 = 150 Ω, Series R3 = 37.4 Ω (Actual: 5.99 dB);
10dB Pi Attenuator: R1 = R2 = 95.3 Ω, Series R3 = 71.5 Ω;
20dB Pi Attenuator: R1 = R2 = 61.9 Ω, Series R3 = 249 Ω.
Layout Rule: Place dual ground stitching vias immediately adjacent to shunt resistor pads to minimize parasitic inductance.
Results & E96 (1%) Matching
| Position | Calculated | E96 Standard | Power Loss |
|---|---|---|---|
| R1 (Shunt Input) | — Ω | — Ω | — mW |
| R2 (Shunt Output) | — Ω | — Ω | — mW |
| R3 (Series Bridge) | — Ω | — Ω | — mW |
Formulas
K = 10(AdB / 20)
Pi: R1=R2 = Z0 × (K+1)/(K-1); R3 = Z0 × (K²-1)/(2K)
50Ω Standard RF Attenuator Lookup Table (E96 1% Precision)
| Attenuation (dB) | Pi: R1, R2 (Shunt) | Pi: R3 (Series) | Tee: R1, R2 (Series) | Tee: R3 (Shunt) | Application |
|---|---|---|---|---|---|
| 1.0 dB | 866 Ω | 5.76 Ω | 5.76 Ω | 432 Ω | Fine power level trim |
| 2.0 dB | 432 Ω | 11.5 Ω | 11.5 Ω | 215 Ω | Impedance buffer pad |
| 3.0 dB [Half Power] | 294 Ω | 17.8 Ω | 17.8 Ω | 140 Ω | Isolation, 50% power reduction |
| 5.0 dB | 178 Ω | 30.1 Ω | 28.0 Ω | 88.7 Ω | Transceiver output backoff |
| 6.0 dB [Half Voltage] | 150 Ω | 37.4 Ω | 33.2 Ω | 68.1 Ω | Precision 50% voltage cut |
| 10.0 dB [10:1 Power] | 95.3 Ω | 71.5 Ω | 51.1 Ω | 34.8 Ω | Spectrum analyzer input protection |
| 20.0 dB [100:1 Power] | 61.9 Ω | 249 Ω | 82.5 Ω | 10.0 Ω | Power measurement & high attenuation |
Frequently Asked Questions on RF Attenuators (FAQ)
Reflected power travels through the attenuator twice: once forward, then bounces off the mismatched load, and passes back through the pad. A 3dB pad attenuates the forward wave by 3dB and the return wave by another 3dB, cutting reflected power by 6dB.
1. Keep shunt ground vias directly adjacent to resistor pads with at least two 0.3mm vias each.
2. Carve out a ground void directly underneath series resistor R3 to minimize parasitic shunt capacitance to ground.
3. Maintain continuous 50Ω coplanar ground shielding around the pad.
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