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[W9KY]

When I would mount a HF vertical above ground, I would "insulate" the base of the antenna from the "mast pipe" to keep it from becoming part of the "radiating" part of the antenna .  Likewise the "tuned" radials would also be insulated as well.  HOWEVER the plan is to install a Comet GP3 at a height of 25 feet and am just seeking some advice at to whether this VHF/UHF antenna needs to insulated from the 25 foot "mast pipe" or not NO big deal to insulate, just curious about the pro's and or con's of doing so !

Thanks up front your help, Mike W9KY
     

[K5LXP]

The coax shield will be grounded to the base of the antenna by default, so having a conducting mast along with it makes mast isolation a bit moot. For static/surge diversion you probably want the mast part of the mount.  The radials of the GP3 are there to isolate RF currents from the mount, mast and feedline.

Mark K5LXP
Albuquerque, NM

[WB6BYU]

If you have 1/4 wave radials connected at
the feedpoint, they should have a low enough
impedance that most of the RF takes that
path rather than down the mast.  But it does
make some difference how long the mast is.

And, unless you have a feedline choke at the
antenna, or some other sort of decoupling,
the outside of the coax will act in the same
way that the mast would, because it is also
connected to the base of the antenna.

You can try it both ways and see if it makes
a difference when listening to a distant
repeater, but usually it doesn’t make any
significant difference.