Vectronics VEC-1290K - Manual (Page 23)

antenna to the low-impedance of the final RF stage transistor. Jumpers
are used to select the optimum amount of inductance for segments of the AM
band. Trimmer capacitor C11 allows for a precise match at the operating
frequency-or, maximum transfer of RF energy to the antenna.
The pi-
network is in essence is a low-pass filter configuration, and also offers some
degree of harmonic energy rejection.