RE:NEC-LIST: V- and Inverted V-Dipoles

Jack Belrose VE2CV <john.belrose_at_crc.ca> wrote:
> . . . If you want your dipole to behave like a dipole, and
> have a bit more gain, take care with the ends of your dipole ---
> keep them up.

and Chuck Counselman W1HIS <ccc_at_space.mit.edu> wrote:
> 1. For an HF antenna very near ground, power dissipation in the
> ground, due to ground resistivity, is a major part of the power
> budget.
>
> 2. The dipole having its ends near ground may induce more ground
> current and thus more dissipation, than the antenna with its center
> near ground and its ends higher.

Jack Belrose reported that a V-shaped dipole, with its middle near
ground and its ends elevated, has higher gain (in the zenith
direction) than a same-size, same-shape, inverted-V dipole, with its
middle elevated and its ends near ground. My NEC-4 simulations
confirm this.

Further, I find that the gain contrast between V- and inverted
V-dipoles is is increased (doubled, for the cases Jack first
described*) if the ground model is changed from "average" (epsr = 13,
sigma = 0.005 S/m) to "very poor" (epsr = 5, sigma = 0.001 S/m). This
finding tends to support the hypothesis that dipole ends near ground
cause more ground dissipation than dipole middle near ground.

So, as Jack says, try to keep the ends of your dipole away from the
ground. In my opinion the reason is not that the ends _radiate_ so
much; it's that their strong _near_ electric fields draw concentrated
bunches of charge to the very nearby ground surface; drawing this
charge through the volume-resistive ground dissipates power. A dipole
end is essentially a "point" attractor -- unlike the dipole middle,
which is more of a "line" attractor (and more of an H-field source).

73 -Chuck W1HIS.

___________________
*Heights of middle/ends = 1 m / 15 m; leg length about 19 m; resonant
at 3.8 MHz / 3.94 MHz; wire radius = 0.0015 m; Sommerfeld ground
modeling; no. of wire segments = 231; segment length uniform, about
0.16 m. At the center I put three horizontal segments with the source
in the middle. Doubling the wire radius had no substantial effect.
Increasing the no. of segs. from 77 to 231 did. With 231 segs., the
gain contrast between V and inverted-V configurations was 0.89 dB for
very poor ground, vs. 0.43 dB for average ground.
Received on Mon Feb 21 2000 - 21:10:44 EST