Feeding the upper patch only (series feed): this is a classic technique to create a dual band antenna with a single feed. Typically, the lower frequency band uses the lower (and larger) patch while the upper frequency band uses the upper (and smaller) patch. At the lower frequency, the leaky cavity between the lower patch and the ground plane resonates while the leaky cavity between upper patch and lower patch is electrically short circuited because it is off-resonance (the off-resonance impedance of patch antennas is effectively zero). At the higher frequency, the leaky cavity between the upper patch and lower patch resonates while the leaky cavity between lower patch and the ground plane is electrically short circuited because it is off-resonance. This is illustrated in the impedance versus frequency plot in Figure 18 and the input reflection versus frequency plot in Figure 19. It is important that the upper patch is sufficiently smaller than the lower patch in order to avoid fringing field coupling between the two patches. This implies that the frequency bands should be separated by at least 5 to 10%.