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RF Attenuator Pad Resistor Sizer

Calculate a π or T pad for equal source and load impedances. Compare ideal resistors with standard values, then review the change in attenuation, port match, and resistor heating.

π and T networksE24 / E96 valuesPower distributionTolerance corner checks

01 / Define the pad

Design inputs

Single-ended, purely resistive networks. Both ports use the same real reference impedance.

Pad topology

System impedance Z₀ (Ω)Use 50 or 75 Ω, or another equal source / load resistance. Not a 50-to-75 Ω matching tool.

Target attenuation (dB)Positive insertion loss, referenced to a direct matched connection.

Resistor value series

Resistor tolerance (± %)Independent of the value series. Eight endpoint combinations plus nominal are evaluated.

Available source power (dBm)CW power into a matched Z₀ load: 0 dBm = 1 mW; 20 dBm = 100 mW. Not a pulsed peak-power rating.

Maximum rating utilization (%)Your power allowance. At 50%, the effective derated rating target is twice the highest sampled dissipation.

Calculated locally in your browser. This tool does not send your inputs to YURUNOX.

02 / Review the candidate

Resistors & circuit response

Waiting for the calculator.

Enter your requirements and calculate the pad.

Selected-value attenuation—Relative to your target.

Nominal input return loss—Resistor-only result; not an RF measurement.

Nominal pad dissipation—Sum of R1, R2, and R3 heating.

The external source and load are not part of the three-resistor BOM. Power values assume excitation from the left.

Resistor values

Ideal and selected nominal values
Ref.	Connection	Ideal resistance	Selected resistance

Each resistor is independently rounded to the nearest series value by absolute resistance difference. This is not an optimized combination or a stock check.

Match & delivered power

Input impedance, output terminated in Z₀: —
Output impedance, source terminated in Z₀: —
Input / output VSWR: —
Nominal output return loss: —
Available source power: —
Power delivered to Z₀ load: —
Power reflected toward the source: —
Loaded output voltage, sine-wave RMS: —

Tolerance & power review

CW power, left-to-right excitation
Ref.	Nominal dissipation	Highest sampled dissipation	Effective rating target ≥

The rating target is a calculation, not an available wattage or package recommendation. Compare it with a resistor’s usable rating after temperature, mounting, and frequency derating. Corner samples are not a certified worst-case bound.

Before building the RF circuit

Includes inputs, resistor values, match, power, sampled tolerances, and assumptions.

Understand the network

One attenuation target, two topologies

Let K = 10^A/20, where A is the positive attenuation in dB. These formulas assume equal, real source and load impedances Z₀ and ideal resistors.

π attenuator

Two shunts, one series resistor

R1 = R3 = Z₀ × (K + 1) / (K − 1)
R2 = Z₀ × (K² − 1) / (2K)

R1 and R3 connect to the return. Their resistances are equal in a symmetric design, but their dissipation is not equal when the signal enters from one side.

T attenuator

Two series resistors, one shunt

R1 = R3 = Z₀ × (K − 1) / (K + 1)
R2 = 2Z₀K / (K² − 1)

R1 and R3 sit in the signal path. R2 connects their junction to the return. The resistor positions in the diagram and report follow this numbering.

π-pad equations and practical RF implementation: Analog Devices / Hittite — HMC199MS8 attenuator application note, Figure 6 and Equation 1. The symmetric T network is the equivalent star transformation of the π network.

What the results mean

Recalculate after choosing standard values

Loss is referenced to the matched source

Insertion loss is −20 log₁₀|S21|, using Z₀ at both ports. It is not simply −20 log₁₀(Vout/Vin) after resistor rounding introduces mismatch.

A Thevenin source resistance of Z₀ and a load of Z₀ are included. The open-circuit source RMS voltage is 2√(Pavailable × Z₀).

Heating is solved per resistor

The tool solves the resistive circuit, then uses V²/R for each element. Reflected power is separated from heat and delivered load power.

Pavailable = Preflected + Ppad + Pload
Rating target = highest sampled Pd / utilization

Tolerance is sampled, not certified

Each resistor is evaluated at its positive and negative tolerance limit, giving eight combinations, plus nominal. The displayed ranges describe these nine evaluations only.

The tolerance does not change automatically with E24 or E96. Confirm the actual tolerance, tracking, TCR, and available resistance range of the chosen part.

Preferred-value reference: Vishay — E-series standard resistance values. The chosen series specifies nominal values, not guaranteed product availability.

Reference check

A 6 dB pad in a 50 Ω system

With exact resistor values and 20 dBm available CW source power, the calculated load power is about 25.12 mW. Both pads dissipate about 74.88 mW. Corresponding R1, R2, and R3 positions have equal dissipation in this ideal matched example, despite different connections and resistances.

Calculated ideal example · no parasitics · left-to-right drive
Topology	R1	R2	R3	Total pad heat
π pad	150.48 Ω	37.35 Ω	150.48 Ω	74.88 mW
T pad	16.61 Ω	66.93 Ω	16.61 Ω	74.88 mW

The calculator starts with this target and E96 rounding enabled. Switch to “Exact theoretical values” to reproduce the ideal example.

RF implementation

Resistance is only the starting point

Package and layout: review resistor parasitics, pad geometry, ground-via inductance, and coupling around the pad.
Bandwidth: the model has no frequency response. Check manufacturer RF data or simulate and measure S21, S11, and S22 across your actual band.
Terminations: the results assume Z₀ at both ports. An unterminated oscilloscope input or a mismatched load changes the response.
High attenuation: compare a single pad with staged or purpose-built attenuation, including interstage match, shielding, and heat distribution.

Component selection

Check more than the wattage label

Effective power rating: apply the manufacturer’s ambient, PCB, mounting, and RF derating conditions before comparing to the target.
Voltage and pulse limits: review working voltage, overload behavior, crest factor, and pulse energy separately. This calculation uses CW RMS quantities.
Direction of drive: reversing a symmetric pad swaps the end-resistor heating. Specify both directions if the application can transmit either way.
Receiver signal budget: attenuation reduces signal level before the following stage; include the pad in sensitivity and noise-budget review.

Common questions

RF attenuator sizing questions

Should I choose a π pad or a T pad?

For the same ideal attenuation and equal port impedances, both produce the same matched transmission. Compare resistor ranges, layout, return paths, and per-resistor dissipation. Neither topology is universally better at every RF frequency.

Can this calculator match a 50 Ω source to a 75 Ω load?

No. This tool uses equal real source and load impedances. Unequal-impedance pads need a different synthesis and have additional feasibility constraints. Do not use a midpoint impedance as a substitute.

Why does a 6 dB pad not give exactly half the voltage?

An exact factor of two in voltage corresponds to approximately 6.0206 dB under the same impedance conditions. A 6 dB pad has a matched voltage transmission ratio of approximately 0.5012 and a power ratio of approximately 0.2512.

Why are the input and output resistors rated differently?

Equal resistance does not mean equal voltage or current. A left-to-right signal loses power as it crosses the network. If the pad may be driven in reverse, evaluate that direction too; in a symmetric pad, the two end-resistor duties exchange.

Does E96 guarantee 1% attenuation accuracy?

No. E96 is a set of preferred nominal resistor values. Resistor tolerance, ratio errors, parasitics, and terminations determine the actual attenuation and match. The selected resistor’s tolerance is a separate specification.

Can I build a microwave attenuator directly from these results?

Use the values as a starting point, not a bandwidth qualification. A frequency-independent resistor model cannot predict package, trace, via, or coupling effects. Validate the implementation over frequency, temperature, and power.

What does an infinite return-loss result mean?

It means the ideal resistive model is matched to numerical precision, so the computed reflected wave is zero. The tool labels this as “Ideal match,” not as a realizable infinite broadband RF return loss.

From design values to a component BOM

Ready to source the RF components?

Share the approved resistor or attenuator part numbers, tolerance, package, power conditions, quantity, and delivery requirements.