To measure the ripple, I inserted a 1 Ohm resistor into the power line and measured a voltage ripple of 105mVpp across it. An oscilloscope screen shot and its fourier transform are shown below. The fundamental frequency is 11MHz as expected, and several harmonics have about the same strength.

In order to suppress it, I came up with the schematic shown below, where the square stands for my oscillator and driver circuit. My reasoning as follows: The energy source is at left, but the ripple source is at right, so the LRC circuit should be read from right to left, and represents a 2. order lowpass. I calculated the values 2uH and 1uF for -40dB at 11MHz, but it doesn't work. So I tried several combinations from 1 to 2000nF and 1 to 220uH, but did not even achieve a factor of 2 in ripple reduction.

So I guess my concept is wrong. How would it be done right? By the way, I am mounting through-hole.

right? By the way, I am mounting through-hole.