The series capacitor in the antenna circuit balances the coupling of the antenna input with feedback from the L2 winding. That circuit with the antenna coupling capacitor is suitable for a short indoor antenna. If an external antenna is used it is necessary to provide a DC path to ground so as to discharge the static electricity that may build up.
But the use of a capacitor can also have a big impact upon the physical size of an antenna. If the goal is a compact, integrated antenna, the use of a capacitance ‘top hat’ on a Planar Inverted F antenna can reduce the antenna size by 30% or more. Radiation patterns and efficiencies can be optimized by this matching.
The capacitance of the antenna is fixed by antenna geometry. The coupling of the antenna to the tuned circuit is increased by increasing the coupling capacitance. That is why the coupling capacitor has a value adjustable from 2 pF to 18 pF.
That circuit with the antenna coupling capacitor is suitable for a short indoor antenna. If an external antenna is used it is necessary to provide a DC path to ground so as to discharge the static electricity that may build up. That would normally involve a tap or a third winding at the earthy end of the L1 coil.
So, yes, to tune a resonant antenna, you'll need an adjustable capacitor or inductor. You'd usually go the capacitor route, since adjustable caps are smaller, cheaper, and more exact, usually. Bonus: There's electronically variable capacitors!
Further, the use of a capacitance ‘top hat’ can match an antenna that’s even smaller than an ideal inverted F size for the band of interest. In this case, the capacitor can allow both reduced antenna size and system optimization. Care must be made in choosing the best capacitor for any type of antenna matching/loading application.
