[Amps] VHF amplifier input network

[Han Higasa]

Hello OMs.

I am using a home brew made six-meter linear amplifier with
a 3CX1200A7, pi input and pi-L output network.

Now I have a difficulty to find a design/calculation method for
the very popular input matching network:

input - VC1 - L1 - VC2 - L2 - filament (cathode) + Cin//R 

Where VC1 is in series to the circuit, and VC2 is connected
in parallel to the junction of  L1-L2 and the ground.


This type of input matching networks have been used on
various VHF amplifiers including the publication of the ARRL 
handbook, GS35B and 8877 amplifiers seen on the web.

With a spectrum analyzer, tracking generator and an old Anzac
return-loss bridge I found that VC1 adjusts the tuning frequency
and VC2 adjusts SWR by cancelling any reactance on the frequency.

Would you please tell me any basic theory or references for the 
circuit? 

thanks reading, de Han JE1BMJ