Practical_Antenna_Handbook_0071639586

550 P a r t V I I : T u n i n g , T r o u b l e s h o o t i n g , a n d D e s i g n A i d
field of the antenna as a function of azimuth (compass heading) for a given radiation
(elevation) angle, or as a function of radiation angle for a user-Âselected azimuth (heading).
Alternatively, the user can select a 3D presentation covering all elevations and
azimuths in a single plot. Clicking on the “SWR” button brings up a new window for
the user to define a frequency range and sample frequency intervals prior to running an
SWR plot.
The wire and source tables used to create this antenna are shown in Fig. 25.3B. Note
that the number of segments chosen for each of the three wires that make up the antenna
keep the segment lengths as similar as possible. The horizontal wire in this example
is divided into an odd number of segments to allow placement of a source at the
exact midpoint of the center segment and, hence, at the midpoint of the antenna, in case
it is to be a center-Âfed dipole. As can be seen from the rightmost two columns of the wire
table, this antenna is being modeled with insulated wire.
The “View Antenna” window (Fig. 25.4) displays the x,y,z coordinate system and
places all the wires specified in the Wires table according to coordinates entered there
for each wire end. This window includes its own set of tools, including scroll bars to the
left of the main window for zooming and repositioning the image within the window.
Other options available from the drop-Âdown menus include printer selection and setup,
and a list of useful related objects (legends, currents, etc.) that can be displayed or not,
as the user chooses. Once a set of calculations has been run, this window graphically
shows (in a separate color) the magnitude of the current at all points on the antenna. A
separate tabular listing of the current and its phase in each segment is also available for
viewing, printing, or saving to a stand-Âalone file.
Once the user is satisfied by examination of the View Antenna display that all antenna
data have been entered properly, pressing the “FF Plot” button in the main window
opens a new window (Fig. 25.5) displaying the antenna’s far-Âfield radiation pattern
in the format previously determined by the choice of options in the main window. The
plot shown here happens to be the radiated field versus azimuth (compass heading) for
a bent dipole in free space (so the elevation angle is immaterial). Data entries beneath
the plot display supporting information such as the elevation angle used for the calculation,
the gain of the strongest lobe (wherever the pattern touches the 0-ÂdB outer circle)
over that of an isotropic radiator, front-Âto-Âback (F/B) ratio, beamwidth, and the azimuthal
angle (listed as “Cursor Az”) for the strongest lobe. Alternatively, the user can
opt to run an Elevation plot for a specific heading by making those selections and clicking
on the “FF Plot” button again. The pattern will look different, of course, but the
supporting data beneath it will be consistent with the first run.
Note Unless the antenna model is specified as being in free space, azimuth plots for a 0-Âdegree
elevation angle will return an error message for either of two reasons:
1. As discussed in Chap. 3, the presence of a perfectly conducting ground beneath a
horizontally polarized antenna will result in zero radiated field at zero elevation angle
because a voltage difference (an E-Âfield, in other words) cannot exist between any two points
on a lossless ground plane. When running azimuthal patterns for a horizontal dipole or Yagi
over any kind of ground other than “free space”, choose a nonzero elevation angle (somewhere
between 10 and 20 degrees, say) to see the low-Âangle azimuthal radiation pattern of the