The images that follow show the near field Z as a measure of mismatch to 377 Ohms of free space. The coloring shows the contours of SWR averaged through the depth of a one wavelength sided cube. The degree of mismatch has been limited to display the values of 1:3:1 SWR. In other words, Z values outside the range of roughly 120 Ohms to 1200 Ohms are clipped so that extreme values did not drive the graphics presentation. The Blue coloring below corresponds to a SWR of 1.25.
The height of the Dipole, at a quarter wave, is not atypical, especially at 80M. It is interesting to observe the central region of mismatch, and the flares off the ends of the Dipole. The middle region reveals a formation that could be called the "deadly embrace" of ground. One would think that raising the Dipole would loosen this grip and the second set of images that follow this will examine that.
download the mathcad file used to generate the images.
download the eznec file used to generate the data.
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This next block shows the variation of the fields as the antenna is raised from its quarterwave elevation to halfwave elevation.. It is quite obvious that the middle region has pulled free of ground, but the Dipole end flares show a marked propensity to flow toward ground. And even more, ground appears to remain a significant source of mismatch. The set of images that follow these will study how a ground shield changes this observation.
download the eznec file used to generate the data.
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The interesting development of downward trending flares of SWR from the tips of the Dipole caused me to consider how much ground affected the situation. The side views of the Dipole reveal considerable ground fields remaining. As a notion, I thought I would add a ground screen. The results that follow are quite a departure from ALL of the near field work I have been performing lately. The Dipole almost seems to have disappeared as it is presenting less disturbance to the general field.
download the eznec file used to generate the data.
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