<<< A couple more ideas for experiments. Maybe you or somebody else tried these already?
1. Place the rods "in series", creating an arbitrary long stick. Say, 4x140mm=560mm.
2. Leave spaces between the rods on the sleeve. The diameter stays the same with fewer rods, or the diameter can be increased with the same number of rods.
Combining the 2 ideas, one can make a long and wide ferrite antenna with fewer rods. >>>
Thanks for the experimental suggestions, Mike. Unfortunately all of these have been tried previously by myself or others, and never did result in any gain boost (or even equivalent performance) with the "standard" tightly packed ferrite rod cylindrical sleeve.
When the early FSL antenna experimentation was going on (2011-2012) all the major experimenters were trying every possible way to save money, since the cost of multiple ferrite rods was already extremely steep, even when purchased on eBay from eastern Europe. We tried every possible way to get equivalent gain by spending less money, including some wacky ideas which were never publicized. Unfortunately there were only two guaranteed ways to improve FSL antenna sensitivity-- use longer ferrite rods (or bars) in the sleeve, or increase the diameter of the ferrite sleeve by adding more rods (or bars) in the cylindrical assembly. The most sensitive FSL antennas turned out to be models which were tweaked in both ways-- although that started the infamous search for the Ultimate Gain Monster FSL (38 pound models costing over $1K to build).
Gary DeBock (in Puyallup, WA, USA)