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From: Taworri
Date: 09 Dec 2001
Time: 16:32:07
Remote Name: 210.8.232.5
I hope this question is sufficiently theoretical for this forum!
If you have an electrically very short vertical monopole, it has a very high capacitive reactance. It is possible to reduce this reactance to some degree by placing top hat loading at the top of the monopole. In my trusty "Langford-Smith", it shows that it is possible to determine the reactance of both the monopole and the top hat wires by use of the transmission line equation (although there must be more modern books with better methods, "Langford-Smith is the only one I have which even tackles the problem).
The method allows top hat calculation by treating each tophat wire in turn to find its individual reactive impedance, then determining the equivalent parallel impedance of all the tophat wires which then becomes the terminating impedance of the vertical monopole. So good so far!
My first question is 1. Is this "old method" reasonably accurate (say +/-20%)?
2. But my main question is - how can I calculate the top hat termination impedance for the monopole If the tophat is a thin metallic circular disc?
The parallel wire method must only be good up to a point!
Thanks for any illumination you can provide.