Aside from matching the impedance of the antenna/s, decoupling stubs also keep stray R.F. off of the feedline coax running into the antenna.

It reduces the felt R.F. on the shielding/braid of the feed cable and also provides a groundplane circuit as well.

Since the antenna in this thread is a collinear design, the feed is in the base only and is NOT of the "corporate" feed design used by Decibel, Celwave, Kathrein for their products.

Corporate feed structure positions the actual feed point near the physical center of the radiating elements, and this method allows the R.F. to be delivered to every integral element equally. This also gives a better radiating pattern, tilt and E/H plane patterns as well. It also maintains a better 50 Ohm impedance match to the transmitter which keeps
intermod products from being generated by corroded/exposed connectors, matching stubs/tuners and the like. Internal matching/feeds keep this "important" portion of an antenna out of the elements and provides a longer lasting antenna service life(theoretically, that is).