Re: NEC-LIST: Inverted V & NEC

At 2:25 PM -0700 5/18/03, Jim Lux wrote:
>...I almost always put my feed points as a small standalone wire
>with one segment....

I, too, use a separate wire for the fed segment (or separate wires,
plural, in the case of plural feedpoints).

However, following advice that I believe I read in the NEC-4 user's
manual, I always excite the center segment of three identical (same
length and same radius) segments in a straight wire. So, for
example, to model an inverted V, I would place a short three-segment
horizontal wire at the top. Then, in the sloping wires on either
side, I would step ("taper") the segment lengths upward as I moved
away from the feedpoint, starting with the same segment length as in
the center-segment-fed wire.

IIRC, somewhere in NEC's modeling of the E-field around an excited
segment it is assumed that the adjacent segments have the same
geometry, meaning the same radius, the same length, and and also that
there's no _bend_ between these segments.

AFAIK, the only penalty for violating this rule/assumption is that
you'll get a less accurate feedpoint impedance. The antenna's gain
pattern will not be affected.

-Chuck, W1HIS

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