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
Shorts can't diisipate power and must reflect, so how does a
stub work?
Hi Hank, A 1/2 wave shorted stub looks like a series resonant circuit. It
makes a good notch filter, which is one of the Amateur applications that I have
used. You are right, they don't dissipate any power since Power=I**2*R and R
equals zero.

73 Gary N4AST