My understanding is that there is also a problem at the other frequency end.
For example, if it actually works on 6 meters, then, at 432 MHz you will
have very high angle radiation; i.e. going up, instead of along the horizon.

Tam/WB2TT

Hi Tam,

If its the design I've worked with, it should be OK on the horizon on 432.

When the disc section no longer functions as a counterpoise, then multiple
resonances happen across the cone and you get a lobe on the horizon. That is
probably what you are describing.

73,
Chip N1IR