Re: NEC-LIST: Dipole reactance

Hi,

Doesn't the fringing capacitance on the ends of a wire dipole cause
the frequency where the reactance equals zero to be slightly less than
the frequency where its physical length is a half-wavelength? This
would result in a positive reactance at a frequency slightly above
resonance (the ".5 wavelength dipole"). Intuitively speaking, as you
make the dipole thinner there is less fringing capacitance which
pushes the series resonant frequency up, thereby making the reactance
at the half wavelength frequency lower. (Antenna is closer to
resonance defined by Za=Ra+j0.)

It may be easier to visualize if you think of the dipole getting
**fatter**, having more fringing capacity, pushing the resonant
frequency **down**, and **increasing** the reactance at the half- wave
frequency.

The slope of the reactance vs. frequency curve would be less for the
fatter dipole since the equivalent characteristic impedance is lower
(Zc~=60[ln(H/r)-1], H=wire length, r=wire radius, if memory serves me
correctly, and Xa=jZc/tan(theta), theta=2*pi*L/(lambda).

I forget--What is the thickness parameter OMEGA?

73, Paul, AA1LL
Greenville, NH
Received on Fri Mar 12 1999 - 19:02:25 EST