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Even if the Slider loopstick loses a piece of its ferrite bar, as long as you can still peak the coil on the DX station's frequency, it will still operate at near 100% efficiency. This is because the coil peaking compensates for any loss of inductance due to the missing ferrite piece, resulting in near-optimum performance.
During the summer all three of us (John, Guy and I) experimented with huge loopsticks, (17", 22.5" and 27"), trying to get more sensitivity out of the Slider. The consensus was that the 7.5" model provided a lot of DX for its size, and that a lot more ferrite really didn't help as much as expected. So the loss of a piece of ferrite is no big deal for a Slider, as long as you can still peak the DX station's frequency.
73, Gary DeBock