Sleeve dipoles

I an trying to get my brain around some aspects of vertical sleeve
dipoles (in particular for 2-metres wavelength).

I understand the principle that the feeder (assumed 75 or 50-ohm co-ax)
is threaded up through the bottom quarter-wave element   The
quarter-wave piece of feeder ascts as an isolattion stub so that the
bottom of the element can be earthed and the feed point is 'half-hot',
with the top of the upper element 'fully-hot'.

A further refinement is to offset the feed point slightly lower than the
exact physical centre of the dipole so as to allow for the different
propagation velocity of the waves in the feeder from that in the dipole
elements, thus achieveing a better match.

If the bottom of the sleeve dipole is standing on the ground or a ground
plane, this makes sense - but what if it is mounted on top of a
conductive metal pole of unspecified length?  Won't the pole act as a
number of other dipoles which, depending on its length, can distort the
radiation pattern in various ways?

Worse still, what if the bottom element of the sleeve dipole is simply a
continuation of the pole (or eletrically connected to it) and the co-ax
is continued down inside the supporting pole to the bottom?   Does the
co-ax need to be bonded to the pole at the point where the bottom
element should end?

Is the pole length irerelevant because a pole diectly below a vertical
dipole is in the null zone, so anything below an earthed bonding point
will not be energised?


--
~ Liz Tuddenham ~
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