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Comments about LEDs and Mr. Smith’s Chart

Readers write in about stories they read in RW Engineering Extra

We have a nice folder of email since our last issue of RWEE. The first came from retired broadcaster Jerry Arnold of Terre Haute, Ind., who writes in response to Cris Alexander’s article “Test Equipment Keeps Getting Smaller and More Capable” (RWEE, Feb. 10, 2016).

He writes, “Interesting article … But I still have my trusty GR1606! I sent it off a little while back to be calibrated. The guy at the cal place called back and told me he felt ‘guilty’ taking my money for calibrating my unit. When I asked why, he said because he did not have to touch anything. It perfectly matched all his NIS traceable standards!”


Hussein Dharma of Dewsbury, U.K., commented that he agrees the front-panel LEDs on Dana Puopolo’s modified audio device were way too bright (“Modding the Kentiger HY-502S Audio Unit, RWEE, Dec 9, 2015). Hussein said, “Thank you for the article. I was thinking of buying this, but those lights really put me off as well! Good to know there’s an easy fix for it.”

He continued, “The Bluetooth feature is what appeals to me, but I have a couple of questions: When you connect via Bluetooth, can you control the volume via the supplied remote? If you can, does it then control the volume from the source (e.g., the iPhone Bluetooth volume) or does it use its own source?”

Dana Puopolo has the answers. “Yes, you can control the volume of any source, including Bluetooth with the remote control. This means that when using Bluetooth, the volume can be controlled three different ways: from the source (phone, tablet, etc.), by the remote control or by the front-panel volume knob (which I usually just leave at around 1 o’clock).”


We also received a pleasant email from Doug Irwin, technical editor of one of our fine sister publications, Radio magazine. Doug enjoyed Mike Hendrickson’s article “Using a Smith Chart to Match Transmitter to Antenna Effectively” (RWEE, Feb. 20, 2016), and says he “read it with great focus.”

He wrote that the Smith chart is a tough topic to cover and to ask for clarification on parts of the text.

“I had a hard time finding the R=1.0 circle as described in the following paragraph: ‘Next, in step 6, follow the SWR circle counterclockwise, towards the load, until the circle intersects with the 1.0 resistance circle in the inductive susceptance (lower half of the chart) region of the chart.’ Having it highlighted in some way [on the Smith chart] would have been helpful.”

Mike Hendrickson responds, “A very good comment, Doug. I had assumed that the R=1.0 circle would be fairly easy to see since I had highlighted it Fig. 1. I can easily see that in the other figures that the R circles become difficult to find.”

Doug also commented that he wondered if he had missed a step because it wasn’t clear in the following paragraph how the admittance-point values changed from 0.642 to 0.2159: “Since the wavelength scale passed through the starting point, 0.5 λ needs to be added to the 0.142 λ just noted to give us 0.642 λ. The total wavelength or distance from the previous admittance point to the new admittance point is 0.2159 λ.”

Mike reviewed this and says he understood how it is confusing. He offers this updated text:

As part of step 5, note the value of λ on the wavelength scale (0.4261 λ). Because we will be locating a point closer to the load in the next step, we need to use the scale that is labeled “wavelengths toward load.”

Next, in step 6, follow the SWR circle counterclockwise from the intersection of the straight line drawn in step 3 with the SWR circle, towards the load, until the circle intersects with the 1.0 resistance circle in the inductive susceptance (lower half of the chart) region of the chart. Draw another straight line from the center of the SWR circle through the intersection of the SWR circle with the R=1.0 circle and continue the line to the wavelength scale. Note the value of the wavelength (0.142 λ). Since the wavelength scale passed through the starting point 0.5 λ needs to be added to the 0.142 λ just noted to give us 0.642 λ. Next subtract the wavelength value noted in step 5, 0.4261 λ from the value determined in step 6, 0.642 λ.

The resulting value of 0.2159 λ is the wavelength distance that the steatite slugs must be located in order to be placed at the R=1.0 location. This wavelength value needs to be converted back to inches. (0.2159 λ times 117.6 inches =25.39 inches) The center of the slugs will need to be located at approximately 25.4 inches from the input of the transmission line.

Mike also would like to have added the following paragraph:

The use of the Smith Chart in tuning a transmission line system can be broken into two parts. The first part is to find the location in the transmission line where the slugs must be placed in order to be located at the R=1.0 point. The second part is to determine how much opposite susceptance must be located at the R=1.0 point to bring the susceptance to 0.

Doug’s last question is about the measurement tools: “Another thing I wondered, as a reader, was how the impedance was measured in the first place. Was it a G-R bridge? A network analyzer?”

Mike responds: “The original measurements were all done using a network analyzer. I’ve used both Agilent and Anritsu analyzers in making these measurements. I have also spent some time using an economical vector network analyzer from Array Solutions. The cost of a VNA from Array Solution is around $1,000 to $1,400, a tenth of the cost of the Agilent and Anritsu analyzers. I did A/B comparisons between the Array Solutions VNA 2180 and the Agilent and Anritsu units. The measurements I did were within in the tolerance of the test gear. The Array Solutions unit does have a couple of handicaps, but these may not be important depending upon the system being measured: The first potential limiting factor is that the upper frequency range of the 2180 is 180 MHz; the other limiting factor is the RF test power output is a maximum of +7 dBm. This can be a very limiting factor if measurements are being made in a high RF field environment.” 

Thanks to all for writing. Send comments on any RWEE article to [email protected].