AAplot's Smith Chart interface displays the complex
impedance of a test load on a Smith Chart
A Smith Chart maps the entire right half of a complex plane
into a finite circle. AAplot can either plot the measure
value as impedance components (resistance and reactance) or
as admittance components (conductance and susceptance).
The following figure shows the Smith Chart interface with a
chart that is overlaid by a constant 1.5:1 SWR circle.
To get a larger or small chart, resize the AAplot window by
dragging the hashed triangle at the bottom right corner the
window. You can collapse the Toolbar of the window to
obtain a largest possible chart that fits on your computer
display. The Toolbar button is the translucent rounded
rectangular button at the top right hand corner of the
menu lets you switch
between impedance or admittance displays. The chart axes
labels will switch when you select between the two menu
items, for example:
The chart is normalized to the reference impedance that you
have chosen (see later). For a 50 ohm reference impedance
(Zo), each unit in the impedance scale corresponds to 50
ohms. For example, the point 1+j2 represents a impedance of
50 ohms and inductive reactance of 100 ohms. Similarly, for
the same reference impedance (Yo = 1/Zo = 20 milli
siemens), each unit of the admittance chart will correspond
to 20 milli siemens (milli mhos).
field lets you select the center frequency
of a sweep. The Width
field lets you select the
width of the sweep (from the lowest to the highest
frequency). Readings are taken from the antenna analyzer
when the Update
button is pressed. A progress bar
appears in the space between the Width
button while measurements are being
taken and downloaded.
Smith Chart options are selected in the Smith Chart box in
the Parameters window. The window is displayed with the
item in the Window menu of the main
The SWR Circle
checkbox and value let you choose
to include an SWR circle in the Smith Chart. You can plot
the complex conjugates of the data by selecting the
parameter in the parameters window sets the
Smith Chart's reference impedance, which can be complex.
All measured values are divided by Zo
drawn on the chart. For admittance plots, the reciprocal of
the normalized result is drawn on the chart. In addition,
the data points can be plotted as their complex conjugates
by using the Plot conjugate
The SWR Circle
checkbox determines if you want to
overlay a constant SWR circle on the chart. Loads with the
same VSWR (referenced to Zo) appear as points that are
equidistance from the center of the Smith Chart (i.e., VSWR
circles are concentric circles that are centered at the
center of the chart). Loads with smaller VSWR will be
closer to the center of the Smith Chart and loads that have
larger VSWR will be further from the center of the chart.
The SWR Circle
text field lets you define the size
of the SWR circle. (AAplot ignores SWR circles that are
less than 1.05.)
In addition to the parameters in the Post
box of the Parameters window, the
impedance/admittance menu also does cause the instrument to
re-measure data. Until the Update
button is again
pressed, AAplot reuses the previously measured impedances,
but transformed by the new parameters.
The following chart shows a load (a 3m long RG-58 coax
cable, terminated at the far end with load that is a 47 ohm
resistor in series with a 100 pF capacitor). The sweep is
centered at Fc
of 24 MHz with a Width
40 MHz (i.e., a sweep from 4 MHz to 44 MHz).
The curve that is drawn is an estimate of impedances for
different frequencies, obtained by quadratically
interpolating the measured data in the Bilinear transform
Notice that one of the plotted green points is a disc that
is not completely filled. This is the impedance at Fc and
its frequency and impedance are displayed near the bottom
right of the window (in this case, 105.43 + j 73.01 ohms).
You can display the frequency and impedance of any other
measured data point by clicking along the impedance curve.
AAPlot will choose the measurement that is closest to the
cursor when the mouse button is clicked, and move the green
donut to that measured position and update the data at the
The figure below shows the result when a location on the
spiral away from Fc is of is clicked.
As seen at the lower right corner of the above figure, the
frequency of this measured point is 9.652 MHz, with an
impedance of 4.82 - j17.42 ohms.
If you change from the impedance plot to an admittance plot
(below), notice that the displayed measurement shows up in
If you change the reference impedance (in the Parameters
window) Zo to 100 ohms, the displayed impedance remains the
same as the value for Zo of 50 ohms shown in the figure
before last. However, the chart is now centered at 100 ohms
and take on a different appearance:
You can ask AAplot to use a finer or rougher chart by
selecting Fine, Medium
Acquisition Resolution menu. Coarser measurements take less
measurement times and is also better on battery life, but
may also not be as precise.
You can measure at a single spot frequency either by
changing the resolution menu in the Parameters window to
, or by zeroing out the width
field in the Smith Chart window.
Menu in the Smith Chart Parameters
window lets you choose between taking a single "One
measurement, or taking continuous measurements.
If you choose either Slow
will periodically update the data after you click on the
button. In the Fast
will start taking the next set of data 0.5 seconds after
the last set has finished displaying. In the Slow
mode, it will pause for 5 seconds.
With the Fast
trigger modes, the
caption of the Update
button will change to
. Clicking on Stop
will break out of
the measurement loop after the next data set has finished