T-Pad Attenuator Calculator – RF Resistor Pad Design

T-Pad Attenuator Calculator computes series and shunt resistor values for a symmetrical attenuator pad, along with exact voltage ratio, power ratio, and resistor heat dissipation.

Characteristic Impedance (Z₀) Ω Desired Attenuation (A_dB) dB

Series Resistors (R₁)

25.97 Ω

The two identical series resistors forming the top arms of the symmetrical T-pad topology.

Shunt Resistor (R₂)

35.14 Ω

Conductance (G₂) 28.46 mS

Network Nepers 1.15 Np

The central grounding resistor forming the core T-section of the topology.

Signal Dynamics

3.16 V/V (Voltage K)

Power Ratio (K²) 10.00 W/W

Transmission (S₂₁) 0.32 V/V

Linear scaling factors, power ratios, and the transmission coefficient of the resulting network.

Equivalent Pi-Pad

96.25 Ω (Shunts Rₚ)

Series Arm (Rₛ) 71.15 Ω

Parallel Eq (Rₚ/2) 48.12 Ω

Alternative standard resistor values to build a matched Pi-pad layout for the same attenuation.

Power Dissipation (@ 1W In)

900.00 mW (Total Heat)

Input R₁ Heat 519.49 mW

Shunt R₂ Heat 328.56 mW

The exact milliwatts of heat dissipated by the input and shunt resistors when a 1W signal is applied.

Network Solved

Analysis successfully computed exact T-Pad resistor values, equivalent Pi-pad transforms, and thermal power dynamics.

T-Pad Attenuator Calculator: Resistor Values for Impedance-Matched RF Pads

This calculator computes the series and shunt resistor values for a symmetrical T-pad attenuator that reduces signal level by a set number of decibels while keeping the input and output impedance matched to $Z_{0}$ . RF technicians, broadcast engineers, and amateur radio operators use it to build fixed attenuator pads for signal generators, receivers, and test equipment.

T-Pad Calculator Inputs and Outputs

Enter the characteristic impedance $Z_{0}$ in ohms and the desired attenuation $A_{d B}$ in dB. The calculator returns the two series resistors $R_{1}$ and the shunt resistor $R_{2}$ in ohms, the voltage and power ratios, the equivalent Pi-pad resistor set, and the heat dissipated by each resistor for a given input power.

How the T-Pad Formula Works

Per the ARRL Handbook for Radio Amateurs, T-pad design starts with the voltage ratio, or K-factor:

$K = 1 0^{A_{d B} /20}$

The two series arms and the shunt arm are then:

$R_{1} = Z_{0} (\frac{K - 1}{K + 1}) R_{2} = Z_{0} (\frac{2 K}{K ^{2} - 1})$

Attenuation in nepers is $ln (K)$ , power ratio is $K^{2}$ , and the transmission coefficient is $S_{21} = 1/ K$ . The equivalent Pi-pad values, for the same $Z_{0}$ and $A_{d B}$ , are $R_{s} = Z_{0} (K^{2} - 1) / (2 K)$ for the series arm and $R_{p} = Z_{0} (K + 1) / (K - 1)$ for each shunt arm. The most common input mistake is applying the power-ratio exponent instead of the voltage-ratio one - using $K = 1 0^{A_{d B} /10}$ instead of $1 0^{A_{d B} /20}$ - which produces a pad with roughly double the intended attenuation.

A detail that doesn't show up on most T-pad tools: although both series resistors carry the identical calculated value of $R_{1}$ , they do not dissipate equal power once a signal is actually applied.

Current entering the pad is highest before it reaches the shunt node, so the input-side $R_{1}$ dissipates several times more heat than the output-side $R_{1}$ - at 10 dB into a matched 50 Ω pad with 1 W in, the input resistor dissipates roughly ten times what the output resistor does, even though their resistance is the same. Power-rating both series resistors identically, as many builders do, over-specifies the output resistor and under-specifies the input one.

Symmetrical T-Pad Topology

T-Pad Resistor Values for Standard 50 Ω Attenuation Steps

The values below are calculated with the formula above at $Z_{0} = 50 Ω$ , the industry-standard RF impedance, for the attenuation steps most commonly stocked as fixed pads.

Attenuation (dB)	K-Factor	R1 (Ω)	R2 (Ω)
1	1.122	2.88	433.34
3	1.413	8.55	141.95
6	1.995	16.62	66.93
10	3.162	25.97	35.14
20	10.000	40.91	10.10
30	31.623	46.93	3.17

T-Pad Attenuator Design: Common Questions

What does the K-factor represent in a T-pad design?

K is the voltage ratio between input and output, $K = 1 0^{A_{d B} /20}$ . It's the single value both resistor formulas are built from, which is why RF handbooks tabulate K before R1 and R2.

Can a T-pad be built with unequal source and load impedances?

Not with the symmetrical formula above. Matching unequal impedances needs a "taper pad" with two different series resistor values, derived from separate equations for the input and output arms.

Why choose a T-pad over a Pi-pad?

Both give identical attenuation and matching at the same $Z_{0}$ ; the choice mainly comes down to available resistor values and layout - T-pads suit lower shunt resistances, Pi-pads suit higher ones.

How is nepers different from decibels in the output?

The neper is a natural-log-based unit, $ln (K)$ , used in some transmission-line theory; decibels use $lo g_{10}$ . Both describe the same attenuation, just on different logarithmic scales.

Does resistor tolerance affect the actual attenuation achieved?

Yes. Since R1 and R2 come from exact irrational ratios, standard 1% or 5% resistors shift both the attenuation and the impedance match slightly; precision pads use custom or hand-selected values.