[BC] Assistance with a tall tower

[Tom Osenkowsky]

I am not being intentionally vague, but it is best to measure the base Z and (re)design a matching network that works best for your particular installation (transmitter). The base Z will be affected by many things including base insulator, feedpipe, isocouplers (if any), lighting choke, etc.
 
I may use a pi, Tee or even L network depending on the length of transmission line and transmitter type. Every transmitter has a preferred sideband impedance rotation at the antenna trminals which should result in symmetrical sidebands at the RF generation point (i.e. plate of the final RF amplifier tube). Use what works best fo you. Without knowing all the specifics, it is not posible to suggest a final design. The transmitter manufacturer should be able to provide you with the preferred impedance rotation at the antenna terminals. Be sure to measure there. In a DA, the phasor/switching hardware can have a significant infludence on the impedances.
 
Tom Osenkowsky, CPBE