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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Sleeve dipoles
Date: Sat, 21 Dec 2024 20:34:04 -0000 (UTC)
Organization: A noiseless patient Spider
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Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
> 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?
> 
> 

I don’t think I follow your description, sounds like using the coax outer
has the lower vertical element?

The kind I know was made from a bunch of baked bean tins as the
sleeve/lower vertical element. Upper vertical radiator plugged into
connector on topmost can lid. Coax feed hung down inside the sleeve.
Support was insulating pole going up inside cans, so high voltage node at
the rim of lowest can was in the clear.


-- 
piglet