it's no big secret... just a ring that clamps on the bottom of the antenna and then the four-- 4 foot long ground plane radials screw into the ring.
I have no idea if my Antron 99 works better with the ground plane kit or not...
I bought the ground plane kit with the antenna for an extra $40.... for $40 bucks I wasn't going to risk spending all kinds of time and effort to install my antenna and find out later I needed the ground plane kit and have to redo everything......
today Amazon has it for $229.99.... so it went up $60 bucks in 2 years!
..... back in 2023 when I got my antenna... it was $129 for just the Antenna.. or $169.99 with the ground plane kit from Amazon.
I talk to lots of people that love their Antron 99 ( it's actually Solarcon A-99CK 17' Omni-Directional Fiberglass Base Station Antenna and Ground Plane Kit ) ... but most of the people I talk to on-line or on air... don't seem to have the contacts I get daily.... every time someone comes over my house and see's the big antenna standing up above my house they ask what it is.... I usually turn on my Anytone 5555 N II and turn it to 37 LSB and ask if anyone can hear me.... I always seem to get someone from another country... I have impressed a lot of people.. including myself!
WX2Watch was over two weeks ago and I asked for a come back on 37LSB to see how the conditions are and a guy from the West Indies came back loud and clear.... LOVE THAT A-99 antenna!
my 17 foot antenna is on the top of a 32 or 33 foot pole.... so it's 50 feet to the tip of the antenna... and my house is up on a hill..... I "personally" think the ground plane kits was worth the money.... but again i have never tried it with and without it to actually know.