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colinear representation in NEC
a) | b) | | | | | | | | | | | | | | | | | -------| | -------| ||| | ||| | ||| | ||| | ||| | ||| | --- S S --------------- ----------------- Fig a) above is an attempt to portray a colinear vertical over infinite ground with a source at "S". The configuration is easy enough to model in NEC with sensible results. The common explanation for operation of a) is that the U shaped section is a quarter wave s/c stub, that it is responsible for delivering direct in-phase drive to the upper section, and that it plays no part itself in radiation ie, that the common mode current on the pair of conductors is zero at all points. Notwithstanding the conventional wisdom, it seems unlikely that there is no common mode current on that section, and NEC models suggest that there is, and that it accounts for some small asymmetric distortion of the pattern. Fig b) above is an attempt to represent a coaxial arrangement of tubes where the lower end of the tubes are connected together, and that is fed at S against an infinite ground. My questions a 1. To what extent is b) equivalent to a)? 2. How is b) modelled in NEC? Thanks Owen |
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