When should you use a Pi or T matching network?
A Pi (shunt–series–shunt) or T (series–shunt–series) network is two back-to-back L-networks through a virtual intermediate resistance. Choosing that resistance sets the network’s loaded Q, which must exceed the minimum Q of a single L-network for the same ratio. Pi networks are classic PA output tanks because they also low-pass filter harmonics.[1]
How is a Pi network designed?
Pick a desired Q. The virtual resistance is Rv = Rhigh/(Q² + 1), lower than both terminations. Design an L-network from each termination down to Rv; the two series elements combine into the center element. For a T network, Rv is higher than both terminations: Rv = Rlow(Q² + 1).[1]
When is a higher Q useful?
Higher Q narrows bandwidth and improves rejection of harmonics and out-of-band signals; lower Q widens bandwidth and reduces loss and tolerance sensitivity. For broadband matching, use multiple low-Q L-sections in cascade with geometrically spaced intermediate resistances.
Where are Pi networks used?
Tube and transistor transmitter tank circuits, harmonic filters at PA outputs, and antenna tuners. The Pi topology absorbs device output capacitance into the first shunt capacitor.[2]
Frequently asked questions
Is a Pi network a filter?
In its low-pass form it acts as a matching network and a low-pass filter.
Can a T network match equal resistances?
Yes; with equal terminations it acts as a phase-shifting or filtering network.
What is the minimum Q?
That of a single L-network for the same impedance ratio.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “RF Circuit Design, 2nd ed.”. C. Bowick, Newnes. 2008.
- “The ARRL Handbook for Radio Communications”. ARRL. annual editions.