RF bridge attenuation calculator
Bridged-Tee attenuator calculator
Calculate the fixed series arms, bridge resistor and shunt resistor for a symmetrical matched Bridged-Tee attenuator. Inspect preferred-value error, VSWR and total ideal power loss before implementation.
Calculated bridged network
Each fixed series arm
50 Ω
Equal to Z₀
Bridge resistor
108.1 Ω
Across input and output
Shunt resistor
23.12 Ω
Center node to ground
Nearest E24 starting point
Each series arm: 51 Ω
Bridge / shunt: 110 Ω / 24 Ω
Estimated attenuation: 9.942 dB
VSWR at design impedance: 1.021:1
If Z₀ is not an E24 value, rounding the fixed arms also changes the match. Use tighter series values or combinations where needed.
Ideal load power
100 mW
1:10 power ratio
Ideal pad dissipation
900 mW
Use as a lumped starting point
At RF, package parasitics, layout symmetry, ground inductance and resistor temperature coefficient can change attenuation and return loss.
Bridged-Tee network
Fixed series arms equal Z₀; the bridge and shunt set the ideal loss.
Z₀ = 50 Ω · R bridge × R shunt = Z₀²
How to design a Bridged-Tee pad
The symmetrical network uses two fixed series arms equal to Z₀ while the complementary bridge and shunt resistors set attenuation.
- 01
Choose the equal port impedance
The source and load must share the same real reference impedance for this closed-form design.
- 02
Convert attenuation to K
The requested positive dB loss is converted to the linear voltage ratio K = 10^(A/20).
- 03
Calculate the bridge and shunt pair
The bridge increases with attenuation while the shunt decreases; their product remains Z₀² in the ideal network.
- 04
Verify available resistor values
Round all four resistors deliberately and review the resulting loss and match rather than rounding the variable pair alone.
Symmetrical Bridged-Tee equations
Two series arms remain equal to Z₀. A resistor bridges the input and output, and another connects the center node to ground.
Voltage ratio
K = 10^(A dB / 20)
Each series arm
Rarm = Z₀
Bridge resistor
Rbridge = Z₀ × (K − 1)
Center shunt
Rshunt = Z₀ / (K − 1)
Complementary relationship
Rbridge × Rshunt = Z₀²
The bridge and shunt form a complementary pair around the fixed Z₀ arms. In the ideal matched model, changing both according to the equations changes attenuation without changing the nominal port impedance. Independent rounding breaks that exact relationship, which is why the calculator reports the realized E24 two-port result.
Do not treat the fixed-arm value or total pad dissipation as a complete component specification. RF layout, individual resistor stress, parasitics and tolerance tracking still require circuit-level verification.
Practical Bridged-Tee checks
The topology is attractive for variable or switched loss, but its match depends on coordinated resistor values and a physically symmetrical layout.
Realize Z₀ accurately
If the system impedance is not a preferred resistor value, use a tighter series or parallel realization for each fixed arm.
Keep bridge and shunt changes coordinated
The two variable elements must preserve their complementary relationship to maintain the nominal match.
Evaluate each resistor separately
Bridge, shunt and fixed arms can see different voltage and current stress even though the total pad loss is known.
Minimize shunt-path inductance
A long via or return path adds impedance and degrades high-frequency attenuation and return loss.
Verify with a VNA or appropriate fixture
Measure insertion loss and both port return losses across the intended band after layout and assembly.
Bridged-Tee attenuator questions
What makes a Tee attenuator bridged?
A fourth resistor connects across the input and output nodes above the underlying Tee network.
Why do the two series arms equal Z₀?
That fixed-arm condition, together with the complementary bridge and shunt pair, produces the symmetrical matched form used by this calculator.
Why does one resistor rise while the other falls?
Their product stays equal to Z₀². Increasing attenuation raises the bridge resistance and lowers the center shunt resistance.
Is a Bridged-Tee suitable for variable attenuation?
The topology is commonly used where two resistive elements can track together, but the control law, device parasitics and linearity must be verified in the real implementation.
Why can the E24 result have noticeable VSWR?
Rounding the fixed arms, bridge and shunt independently breaks the exact matched ratios. Use closer values or precision networks when return loss matters.
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