Henry Kolesnik wrote:

I know that a shorted 1/4 wave stub exhibits a very high impedance. But
for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance
or a short. There are claims that this can be used to filter the even
harmonics. Shorts can't diisipate power and must reflect, so how does a
stub work?


It works exactly as you stated: "It reflects the second and all even
harmonic energy back to the source."