Question E7C07:
Which describes a Pi-L network?  

Choices:  
A: A Phase Inverter Load network  
B: A Pi-network with an additional output series inductor  
C: A network with only three discrete parts  
D: A matching network in which all components are isolated from ground    

Correct Answer is  B: A Pi-network with an additional output series inductor  

To understand a Pi-L network:

1) Why the correct answer is correct
- A Pi network is a low-pass matching network with two shunt capacitors to ground and a series inductor between them (looks like the Greek letter π).
- A Pi-L network is a Pi network with an additional series inductor at the output side. That extra inductor forms an added L-section with the output capacitor and the load, increasing harmonic attenuation and improving load isolation.
- Therefore, “A Pi-network with an additional output series inductor” is correct.

2) Why the other choices are incorrect
- A: “A Phase Inverter Load network” — Not a standard RF matching term. Pi-L refers to topology (π followed by L), not phase inversion.
- C: “A network with only three discrete parts” — That describes a simple Pi network (2 capacitors + 1 inductor). A Pi-L has four reactive components (2 capacitors + 2 inductors).
- D: “A matching network in which all components are isolated from ground” — False. The Pi and Pi-L have shunt capacitors connected to ground.

3) Useful formulas and rules of thumb
- A Pi network is effectively two L-networks back-to-back. A Pi-L adds another series inductor, making the filter order higher.
- Each reactive order adds roll-off; approximate stopband slope:
  - 1st order: −20 dB/dec
  - 2nd order: −40 dB/dec
  - 3rd order (Pi): −60 dB/dec
  - 4th order (Pi-L): −80 dB/dec
  This is why Pi-L gives better harmonic suppression than a plain Pi.

- L-network design relationships (for impedance match between real resistances), with $R_\text{high}$ and $R_\text{low}$:
  $$Q = \sqrt{\frac{R_\text{high}}{R_\text{low}} - 1}$$
  Series reactance:
  $$X_\text{series} = Q \cdot R_\text{low}$$
  Shunt reactance:
  $$X_\text{shunt} = \frac{R_\text{high}}{Q}$$
  These are often used to choose starting values; a Pi network uses two such L-sections back-to-back. The Pi-L keeps the same idea but adds one more series inductor for extra low-pass action.

- Practical rule: Use Pi-L when you want better harmonic attenuation and improved isolation of the PA from load variations, while still matching (typically) a high plate/tank impedance to a 50 Ω load.