Re: Sleeve dipoles

Martin Rid <martin_riddle@verison.net> wrote:

liz@poppyrecords.invalid.invalid (Liz Tuddenham) Wrote in message:r

I an trying to get my brain around some aspects of vertical

sleevedipoles (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   Thequarter-wave piece of
feeder ascts as an isolattion stub so that thebottom 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 theexact physical centre of the dipole so as to
allow for the differentpropagation velocity of the waves in the feeder
from that in the dipoleelements, thus achieveing a better match.If the
bottom of the sleeve dipole is standing on the ground or a groundplane,
this makes sense - but what if it is mounted on top of aconductive metal
pole of unspecified length?  Won't the pole act as anumber of other
dipoles which, depending on its length, can distort theradiation pattern
in various ways?Worse still, what if the bottom element of the sleeve
dipole is simply acontinuation of the pole (or eletrically connected to
it) and the co-axis continued down inside the supporting pole to the
bottom?   Does theco-ax need to be bonded to the pole at the point where
the bottomelement should end?Is the pole length irerelevant because a
pole diectly below a verticaldipole is in the null zone, so anything
below an earthed bonding pointwill not be energised?-- ~ Liz Tuddenham
~(Remove the ".invalid"s and add ".co.uk" to
reply)www.poppyrecords.co.uk

 
The bottom  dipole half is never grounded. Another name is coaxial
 antenna. Usually you find them in marine application because of
 the counter poise.

Suppose the feed point had a 1:1 isolation transformer, there would be
no reason why the dipole couldn't be grounded at any point alog its
length.  The same isolatio could be achieved by a quarter-wave line.

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk