I followed an article Antenna Here is a Dipole [PDF] by James Hearly, NJ2L to build a dual band 40/15m dipole:
The antenna works more or less as expected. My 40m dipole was resonant at 7.030 MHz and 21.600 MHz. Adding capacitance hats according to the scheme almost doesn't affect 40m and shifts the second resonance to 21.300 MHz.
Although I don't understand how it works. Why capacitance hats make the dipole electically longer on 15m, but don't affect 40m? Also why the third harmonic resonance turns out to be ~2.5% higher than 7.030 x 3?
I realize this might be a complicated question. In this case maybe there is a book on this subject you could recommend? "The ARRL Antenna Book" doesn't give such details. Should I try "Antenna Physics: An Introduction" also by ARRL?