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T-network tuner calculator

What to know before you calculate

A T-network covers a very wide range of impedances, which is why it sits inside most amateur tuners. But it is a high-pass filter: it passes harmonics better than the wanted signal. And one detail that is rarely mentioned: at a high Q it simply heats the coil.

Rin 50 ΩL1L2CRout 50 Ω
The T network is a high-pass: it does nothing about harmonics
Frequency (MHz)
Input impedance (Ω)
Output impedance (Ω)
Network Q
Coil Q
Values
L1 — input side
C — to ground
L2 — output side
Check
Virtual impedance
Network bandwidth
Network efficiency

📐 The formulas​

R_virt = R_in × (Q² + 1)
X_L1   = Q × R_in
Q2     = √( R_virt / R_out − 1 )
X_L2   = Q2 × R_out
X_C    = R_virt / (Q + Q2)

Network efficiency:

η ≈ 1 − Q_network / Q_coil

Symbols:​

  • Q — the loaded Q of the network, which you choose
  • Q_coil — the coil's own Q: an air-cored one on HF gives 150–300
Where this calculator stops:
  • A T-network is a high-pass filter: it does not suppress harmonics. A transmitter needs a separate low-pass filter
  • Estimating efficiency from the ratio of Qs is a first approximation; capacitor losses are not accounted for
  • The L–C–L arrangement is calculated; in a real tuner the capacitors are the variable parts (C–L–C). The numbers are the same, what you turn is not
  • A large impedance ratio demands a high Q, and then losses become the main limit

🎯 In practice​

  • An antenna tuner — the classic circuit.
  • Estimating the losses in a tuner you own: how much power stays in the coil instead of reaching the antenna.
  • Deciding whether tuning this antenna is worth it at all, or whether changing the feedline is simpler.
Practical advice:
  1. Use the lowest Q that still achieves a match: a higher Q means higher losses.
  2. A low-pass filter after the tuner is needed if you are transmitting.
  3. If the coil gets hot, the Q is too high. That is visible without any instruments.
  4. A tuner matches the rig to the feedline, not to the antenna: the SWR in the feedline itself is unchanged.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «T-network tuner calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/t_network (licence CC BY-NC-SA 4.0).

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