Not sized properly. Transmitter is sitting on a pretty tall pole right now and getting it off is a bit of a hassle. And I also already covered cap with a glued heatshrink to protect against elements, so accurate measurements are not possible without destroying it... But I hope it can give an idea what I came up with.
Sounds like you might have inadvertently created a wave guide.
If you are feeling adventurous you might want to try dropping your creation into an Antenna Simulation software like NEC or something similar.
https://www.xnec2c.org/