The tuning screws in a helical resonator casting are not really
slugs, they are capacitors.
'Pulling', as engineering handbooks refer to it, is the tuning of a helical resonator to a desired frequency by creating a capacitance at the free end of the resonator. This, of course, is very different to the effect of iron, aluminum or brass cores in a conventional coil. I suspect the effect is called this because the free air resonance of a helical resonator starts high and then is pulled lower by the capacitance exhibited by the proximity of the tuning screw on the open end of the helical resonator coil.
The percentage of frequency that a resonator can be pulled is
somewhat limited. Why? The capacitance imposed upon the free
end of the coil produces a lower operating "Q" which increases pass bandwidth.
These effects get worse the lower the coil is pulled.
The increasing capacitance also results in greater insertion loss through the filter. Why? As capacitance is increased on the free end of the coil, it resonates lower in frequency, but this increased capacitance dissipates some of the energy propagating through the casting, raising its insertion loss.
Wider pass bandwidth and greater insertion loss through the filter are two things that are undesirable in front-end preselection, especially on two meters with a 600 kilohertz split. Manufacturers purposly limit the amount of pulling mechanically so these ill effects won't (can't) happen. This usually results in needing several different coils to tune across an entire band - like is the case with the Motorola MICOR and GE MASTR II.
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