Basic Antenna Theory...
A "perfect" whip (or dipole) antenna need to be 1/4 the wave length of the frequency you're receiving. 100Mhz is in the middle of the US FM frequency spectrum of 88Mhz to 108Mhz. Basically 31" is best length, so the closer you get to 31" the better. Obviously 31 is little short for the higher frequencies and a little long for the lower frequencies but it's a workable (cheap, simple) antenna for vehicles.
Quick note just to round this out; The ground of the car, truck, or bike is the other 1/4 length making a properly installed whip a 1/2 wave length antenna "system."
I'm guessing your latest EBay antenna allows you to get it out of the way but makes the antenna reception worse when you shorten it.
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