Re:Sleeve dipoles

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.

Cheers
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