http://www.icom.co.jp/world/support/download/brochure/pdf/AH-4.pdf
I am using an automatic antenna tuner, ICOM AH-4, for my dipole antenna. Its circuit diagram is not disclosed by ICOM, but JJ2PNX traced the circuit and gives us the following information:
http://www.momose.com/hirofumi/jj2pnx/ah4/index.html
It seems AH-4 can offer two types of L-section networks depending on how the switches RL3 and RL4 (+RL14) are controlled, namely:
In the case RL3 is closed, and RL4 being open.
In the case RL3 is open, and RL4 being closed.