Kok Chen, W7AY [w7ay (at) arrl (dot) net]
Last updated: April 27, 2011


AAplot is a Mac OS X application that works with Rig Expert antenna analyzers (vector impedance meters).

AAplot requires minimally Mac OS X 10.6 (Snow Leopard) to run in, and a Macintosh with at least one USB port. The application can be downloaded from here.

Both the AAplot application and the sources are completely free. AAplot is not a supported program, and it is not affiliated with Rig Expert. I wrote it for myself so I can tether to an AA-230PRO with a native Mac OS X application.

If you need other functions than the ones already implemented in AAplot, please download the Xcode project and implement it yourself. Although I have only tested AAplot with the AA-230PRO, the application should work with other Rig Expert analyzers that comply with their published data exchange protocol. The instructions for finding the Xcode project is found in the download page.

It should also be relatively easy to adapt AAplot to other antenna analyzers, as long as each measurement consists of a triplet of a frequency, a resistance and a reactance value.

AAplot saves its property list (plist) file as w7ay.AAplot.plist in the Preferences folder of the Library folder that is in your home directory. Calibrations are saved into a folder named AAplot in the Application Support folder of your home directory's Library folder.


The following figure shows AAplot opened to the Smith Chart view.

Screen shot 2011-03-01 at 11.59.57 PM

The segmented control in the left center of the window's Toolbar selects the various tools that occupy the content of the window.

Selecting Antenna Analyzer Port

The Connect button on the right of the Toolbar is used to manually connect through the USB port to the analyzer. The indicator to the right of the button shows the connection state -- green if connected and red if not connected. When connected, the button's caption will also change to Disconnect, allowing you to manually disconnect from the analyzer.

The USB port is selected in the Connection window, shown in the figure below:


The Connection window is opened by selecting the Connection item in the Window menu of the main Menu Bar, seen below:

Screen shot 2011-04-27 at 6.54.30 PM

The Serial Port menu in the Connection window lists all serial ports that AAplot finds. The USB port of the Rig Expert antenna analyzer requires an FTDI driver (kernel extension). Be sure to install the FTDI driver before you run AAplot.

Auto Detecting Antenna Analyzer Port

If you are not sure which serial port is the one to choose from the menu, click on the Detect Device button that is to the right of the Serial Port menu in the Connection window. When you click on the Detect Device button, AAplot will display this window:


Follow the two instructions precisely, and click OK after each step. After the second step, the serial port menu should automatically point to the antenna analyzer 's USB port.

Connecting to the Antenna Analyzer Port

After selecting the serial port that belongs to the antenna analyzer, you can connect to it. But first confirm that the power of the antenna analyzer is turned on, and the analyzer has been placed in PC Mode.

Click on the Connect button at right of the main window Toolbar and the indicator should turn from red to green, and the Connect button caption should turn to Disconnect.

If you quit AAplot while it is connected, AAplot will try to select the same port the next time you launch AAplot, and automatically connect to it. Remember to turn the analyzer on and set to PC Mode before launching AAplot. If not, AAplot will not be able auto connect, and you will have to manually connect to the analyzer.

If you disconnect from the antenna analyzer before quitting AAplot, it will not perform an auto connection the next time AAplot launched.


Currently, AAplot implements the following tools. Details can be found by clicking on the following links (or use the tabs on top of this page).

  • Smith Chart interface. Measures the impedance of the device under test for a range of frequencies and plot them on a Smith Chart.

  • Scalar Chart interface. Measures the impedance of the device under test for a range of frequencies and plot the impedance, impedance magnitude, SWR or Return Loss.

  • Transmission Line calculator (TL). Determines the frequency for a desired wavelength transmission line, or conversely, determines the electrical wavelength of a transmission line for given the frequency.

Parameters window

Certain plot parameters can be set in the Parameters window. The window is opened with the Parameters item in the Window menu.


The Post Processing box contains parameters that are used in the plots. When a parameter in this box is changed, the associated plots are updated without the need to take new measurements from the instrument. Zo, for example will change the Smith Chart interface and the SWR and Return Loss plots in the Scalar interface.

The intrinsic impedance of a transmission line and the reflection coefficients are determined by Zo.

The Data Smoothing checkbox smooths out minor noisy measurements. The smoothing function changes the value of a sample by using four neighboring samples using a least square quadratic fit. The following two figures show the "before" and "after" plots:

Noisy Smoothed

The Acquisition box determines the parameters that are used when a measurement update is requested.

The Resolution Menu lets you choose how many discrete impedance points to plot. The measurement resolution can be set to Fine, Medium or Coarse. Fine resolution takes more time to measure. Most of the time, a medium resolution is adequate. In addition, the resolution can be set to Single Frequency. A single point, measured at frequency Fc, is plotted when you select Signal Frequency, otherwise points will span frequencies from Fc-Width/2 and Fc + Width/2.

The "Timebase" menu can be set to One Shot or to periodically repeat (Slow or Fast). When set to One Shot, a single set of measurements is taken each time AAplot is asked to update data.

If you choose either Slow or Fast, AAplot will periodically update the data after you click on the Update button. In the Fast mode, AAplot 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 or Slow trigger modes, the caption of the Update button will change to Stop. Clicking on Stop will break out of the measurement loop after the next data set has finished downloading.

The AA-230 PRO (firmware 308) has a problem with some frequencies that are multiples of 1.25 MHz. You can avoid these frequencies by selecting the Avoid critical frequencies checkbox. When the checkbox is selected, AAplot will offset the measurements by 1 kHz if it finds that a measurement sample will land on a multiple of 1.25 MHz.

The following figure (left) shows a measurement that includes a bad impedance sample at 6.25 MHz and a measurement (right) when the sample at 6.25 MHz is moved to 6.251 MHz:

critical noncritical


You can estimate the impedance of a load that is connected at the far end of an arbitrary transmission line after calibrating the transmission line. The calibration process is described here.

The calibration process can also used with any fixture or even the analyzer's SO-239 connector to "calibrate" away any constant error of the fixture.

Reading Data that are saved to the Analyzer's Memory

The RigExpert Analyzer (except for the AA-30) is able to save data that was collected for its SWR or (R,X) graphical display.

Memory sets in the analyzer are numbered from 0 to 89, and each one can also have a user defined name (please see the RigExpert's User's Manual on how to add a name to a memory set).

AAplot can read back any of these memory sets and plot each set into either the Smith Chart display or the Scalar plot display. In addition, AAplot treats the analyzer's memory data as if it was collected by AAplot in real time -- each set that is read by AAplot can thus be saved to your computer with the Save As... menu item in AAplot's File Menu.

AAplot has to be connected to the Analyzer (the green indicator at the top right corner of the main AAplot window).

The Memory Manager menu in AAplot's Window menu opens the memory manager.

Screen shot 2012-05-15 at 10.01.36 PM

The Memory Manager window contains a popup menu that addresses the active memory sets in the analyzer. The title of each menu item includes the memory number and the name that is assigned to that memory in the analyzer. The list of active memory names is downloaded from the analyzer when the Memory Manager window is opened . Depending on the active memory, this can take a little while.

Screen shot 2012-05-15 at 3.48.38 PM

If the memory on the analyzer changes, you can refresh the popup menu by closing and reopening the Memory Manager window, or by pressing the Refresh button.

After selecting the popup menu item, press on Chart Data in Memory and the data is downloaded and plotted to the Smith Chart and the Scalar charts.


You can collapse the Toolbar (with the translucent button at the top right of the AAplot window) to maximize the user interface size once you have connected to the Analyzer. With the Toolbar collapsed, you can still select which interface to use by using the menu items in the View menu of the main Menu Bar.

The Check for Updates... menu item in the AAplot menu (next to the Apple menu) checks with a file on my web site to see if there is a newer version of AAplot.

The Save As... menu item in the File menu of the main Menu Bar lets you have data sets that can later be opened by the Open item in the File menu. You can Open a file (or select a file from Open Recent...) without the need to connect to an antenna analyzer. The files that are written by Save As... are XML formatted text files that can be edited with standard Text Editors, or by using Apple's Property List Editor (in the Utilities folder of the Developer volume). Hint: you can edit the data values of a file that is saved from the Smith Chart interface to create custom Smith Charts of your own.

You can open this web page by selecting Open AAplot web page in the Help menu of the main Menu bar.