It is a very common practice that stacking the patch antenna and improving the gain. Patch antenna , Di-poles are mostly resonant antenna. But is it possible to follow the same approach for travelling antenna (spiral antenna or log periodic tooth antenna)
It is possible, but usually unsatisfactory, because the separation (in wavelengths) changes across the broad bandwidth, so the pattern changes a lot.
Perhaps using a fractal or pseudo-random array might be a good approach, so that the spacing has no scale dependence.
Narrow-band travelling wave antennas can be stacked into an array. The separation can be bigger than usual because each antenna has an effective aperture that is larger than its physical width.
Malcom is correct. The problem with ultra wideband antennas when going for arrays is the performance throughout the band. The reason is the separation between the elements, similarly in stacking also the distance will matter.
In some case you can stack log periodic arrays if they point at the common centre. This turns two flat log periodics into a pyramidal log periodic, for instance.
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