Michael Mannung wrote:
"What is the reason a capacitor is added to a shunt fed antenna?"

Shunt feeding a radiatir has advantages. It eliminates the base
insulator. Shunt feed acts as a dingle-turn loop. Field of the loop
magnetically induces voltage into the radiator. Attachment point on the
radiator is picked to nearly match the resistive part of the radiator
impedance to the Zo of the transmission line at the connection point.
There is always an inductive reactance at this point which can be tuned
out by using a variable capacitor connected in series with the shunt
feed connection.

The capacitor is simple to adjust, cheap, and nearly lossless as
compared with a coil.

Best regards, Richard Harrison, KB5WZI