Just installed a Hustler mobile antenna on my van and while tuning it,
it got so hot that the coil started to melt ( the cover) . What would
cause this?

Any chance this is an old or defective antenna, with corrosion in the
winding?
Power dissipated in resistance rather than radiated, is what's making it get
hot.

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KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR