Re: NEC-LIST: NEC2 versus NEC4

Hi John,

John Belrose wrote:
> We have numerically modelled 3-, 5-, 7-,12- and 54-wire fan antennas,
> and compared with impedance measured (a MF antenna scaled to VHF/UHF
> frequencies).
> Three wire fans (in particular) give you a very unexpected impedance
> vs frequency response --- very very sharp resonances --- and this is
> real since I find very good agreement between numerical simulation and
> experimental measurement [1].
>
> The problem with fan antennas with closed ends is that we have
> monopole type and transmission line like currents on the wires
> comprising fan. Transmission line currents are currents circulating
> around the triangles formed by closed end fans. These problems
> disappear if the ends of the fan wires are open.

I too have modeled and built fan dipoles. The very very sharp
resonances are internal current modes weakly coupled to the feed
point. With several wires the resonances change with open or closed
fan wire ends. You can remove them by resistively loading the
unwanted modes. e.g. For a two wire fan you get resonances due to
current unbalance between the two wires forming half a fan. This
unbalance current is small, and results since the two wires are never
identical, or equal capacitance to ground etc. The best way to remove
the resonances (if they bug you enough) is to put a load resistor (10
ohms is enough) in the end wire across the dipole ends. This ruins
the Q of the unwanted current modes, and since Hi Q, weakly coupled to
the feed point, is required for the sharp input impedance kinks.

When testing a fan, the slightest touch with a finger will erase the
kink on the network analyser. A dielctric rod just moves the kink.

73,

-- 
sringe_at_actrix.gen.nz-------------------------------------------------
Steve Inge, ZL2BDV, Engineering Analyst, Broadcast Communications Ltd.
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