Coaxial Dynamics Model 81041
USB Wattmeter
This article is for vision-impaired amateur radio operators interested in the Coaxial Dynamics Model 81041 USB Wattmeter. This directional RF wattmeter has a built-in USB interface that can be used to display numeric, analog meter, and bar graph data on a computer screen.
Model 81041 Wattmeter
In many ways the 81041 wattmeter is similar to the Bird Model 43 wattmeter. Both instruments use the same high accuracy RF detection scheme. The 81041 has a larger, easier to read mirror-backed analog meter that is 2.3" high x 4.1" wide.
The 81041 is designed for CW and FM modes. It measures average power and is not designed for peak power measurements on SSB or pulsed/digital signals.
RF Power Detectors
The 81041 can use two detector elements ("slugs") that plug into the front panel at the same time—for example, a higher power element for measuring watts in the forward direction and a lower power element for the reflected component. A front panel switch selects forward or reflected power. If only a single detector element is installed, it can be rotated 180 degrees to measure in the opposite direction.
Various detector elements are available to measure from 100 mW to 10 kW over a frequency range of 2 MHz - 2.3 GHz. The slugs are interchangeable with those for a Bird Model 43 wattmeter.
VSWR
VSWR is not displayed directly on the analog meter. It can be calculated from the forward and reflected power or read from a supplied nomogram. If two detector elements are installed, the VSWR can also be displayed directly on a computer monitor as described below.
Accuracy
The stated accuracy is plus/minus 5% of the full scale power, so for a 100 watt detector element the accuracy is plus/minus 5 watts. These figures highlight a major advantage to using a second, lower power detector for the reflected component—higher accuracy in the measured power and calculated VSWR.
Graphic Display
The Model 81041 includes a USB interface for a graphic display on a computer screen. The supplied software runs with Windows Vista, XP, NT, or 2000. A version for Windows 7 is being developed.
The display window (Figure 1) shows three separate simulated analog meters plus digital readouts—forward watts, reflected watts, and VSWR. A fourth panel shows three bargraphs plus digital readouts—forward dBm, reflected dBm, and return loss in dBm. High contrast colors are used for visibility.
Figure 1. Graphic Display (courtesy of Coaxial Dynamics)
The forward power meter display is 4.9" wide x 2.3" high on a 19" monitor with standard 96 dpi resolution. The numbers on the scale are about 0.125" high. The size of the display window is not adjustable, but it can be enlarged with third-party magnification software.
Summary
The Coaxial Dynamics Model 81041 USB Wattmeter uses the same high accuracy RF detection scheme as the Bird Model 43. The stated accuracy of plus/minus 5% of full scale is higher than typical ham raido analog wattmeters.
The 81041 is designed to measure average power—not peak power. It can be used with a computer to show power and VSWR on a high visibility graphic interface that includes simulated analog meters, bar graphs, and digital readouts.
Acknowledgment
Thanks to Dave Kaltenborn, N8KBC, of Coaxial Dynamics for the figure and for answering my questions.
Author Information
Peter DeNeef, AE7PD, is an Extra Class amateur radio operator in the U.S. This Web site has no ads or conflicts of interest.
rev. 11/5/2011