AntScope and AAplot
for RigExpert Antenna Analyzers
AntScope and AAplot are control software for RigExpert antenna analyzers. These programs run on an attached computer and add a number of useful features, including a graphical user interface that makes the analyzer accessible for vision-impaired amateur radio operators who can use a computer screen.
AntScope is on the CD supplied with RigExpert analyzers. It runs on Windows operating systems and under WINE on Mac OS X and Linux systems. AAplot by W7AY runs on Mac OS X computers.
RigExpert antenna analyzers are user-friendly vector impedance analyzers designed for amateur radio operators.
Available models are compared in this table. The model number indicates the top of the frequency range, for example, the AA-54 goes up to 54 MHz. Analyzers with higher model numbers have additional features, including higher resolution analog-to-digital conversion for increased accuracy.
Each analyzer has a keypad and screen for portable use without a computer. Most models have a 2.6" x 1.7" (128 x 64 pixel) monochrome LCD screen. Three models have a higher resolution (320 x 240 pixel) color TFT display.
Figures 1 and 2 show the resolution and relative sizes of text and graphics on the monochrome display. They were made using LCDclip, a program on the RigExpert CD. Visibility on the actual display is limited by low contrast against a grey-green background.
Figure 1. LCD SWR display.
Figure 2. LCD SWR graph.
QST reviews of an AA-54 (March, 2012) and AA-200 (August, 2008) include accuracy tests: For resistive loads, VSWR accuracy decreases notably when the VSWR is above 10:1. Impedance accuracy drops off at lower VSWRs. These specs are fine for most ham antenna work.
In an article on calibration, W7AY points out that the resistive bridge circuit used in these analyzers can have large errors at high VSWR's. In an email he added, "As you trim the antenna system (including a balun, etc.) and get closer to 50 + j 0 ohms, the analyzer should become quite accurate." (Quoted with permission.)
The CD supplied with the analyzer installs drivers and software, including AntScope. On my Windows 7 system it was necessary to install the USB driver manually, as explained in a troubleshooting note at the beginning of the software user manual.
Updates to AntScope and the firmware are available from the Web site.
AntScope User Interface
Figure 3. AntScope: VSWR vs. frequency.
Figure 4. AntScope: impedance, resistance, & reactance vs. frequency.
The other options in the View menu for AntScope graphs are return loss, phase, parallel impedance, and Smith charts.
The size of the AntScope window is adjustable by dragging the border or selecting View menu > Graph Size. View menu > Zoom-in Graph gives the option of a display with twice the default vertical scale size. Text in the user interface is small, so magnification software might be helpful.
AAplot is free Mac OS X software developed by Kok Chen, W7AY. A detailed user manual is available on the Web site.
AAplot User Interface
Figures 5 and 6 show AAplot graphs. Views of return loss, phase, and Smith chart format are also selectable.
Figure 5. AAplot: VSWR vs. frequency.
Figure 6. AAplot: Resistance and reactance vs. frequency.
The vertical scale for each graph is selectable to magnify vertically while the entire horizontal frequency scale remains in view. Clicking and dragging the graph up or down brings other areas into view.
Text is small, so the OS X Zoom magnifier might be helpful.
Calibration removes the effects of impedance and losses in a feeder system, so measurements can be displayed as if the analyzer were connected at the antenna feed point. This type of calibration is not intended to improve the accuracy of the analyzer.
The AntScope calibration algorithm is based on the widely used "Open-Short-Load" standard—three separate measurments on the feedline, disconnected from the antenna, with open, short-circuit, and 50 ohm terminations.
Open and short-circuit terminations of a transmission line cause high SWRs. W7AY points out that calibration would be improved if you could use terminations that keep the SWR below 6:1, where RigExpert analyzers are most accurate.
For that reason, AAplot allows you to use three different calibration terminations of your choice. W7AY suggests a resistor between 15 and 25 ohms instead of a short-circuit, between 150 and 300 ohms instead of an open line, and around 50 ohms for the third termination. He also describes an easy way to make terminations with minimal stray reactance.
RigExpert analyzers are useful for adjusting and comparing ham radio antennas and for understanding their electrical characteristics. AntScope and AAplot give additional options for displaying measurements from these instruments. A graphical user interface makes the analyzer accessible for vision-impaired hams who can use a computer screen.
Thanks to Chen, W7AY, for many helpful discussions.
Peter DeNeef, AE7PD, is an Extra Class amateur radio operator in the U.S. Email: HamRadioAndVision "at" gmail "dot" com.