Reg Edwards wrote:
There is a 3-phase transmitter feeding a 3-phase antenna via a 3-wire
transmission line.

The antenna consists of three 1/4-wave horizontal radiators spaced at
120 degee intervals.

Is the radiation pattern in the horizontal plane perfectly
omni-directional?
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Reg.

Hi Reg, Is the 3-phase transmitter balanced with the output current
equal magnitude, and the phase angle exactly 120 degrees? Is the
3-phase antenna fed via a 3-wire transmission line, or three equal
length transmission lines? Us Amateurs don't have a lot of experience
with 3-wire coax. Are the horizontal radiators 1/4-wave, or 1/2 wave?
If they are 1/4 wave, or 1/2 wave and spaced 120 degrees in the
horizontal plane, doesn't that form an equilateral triangle? So, you
are feeding a triangular antenna whose sides are 1/4-wave in the centre
of each side with balanced 3-phase rf energy?
Even if you clarify this is I still won't know the answer to your
question until I model the array.
Gary N4AST