How to use VNWA for designing a Matching network and how to use ZPlots VNWA to move measurement to the antenna feeding point

Transcription

1 How to use VNWA for designing a Matching network and how to use ZPlots VNWA to move measurement to the antenna feeding point Preface: To design a matching network for an antenna for matching it to a 50 ohm transmission line, is quite simple using the VNWA Matching Tool only based on a single sweep within, the frequency range of interest and measured at the feeding point of the antenna. This is made by calibrating the VNWA with a Short Open and Load calibration standard at the end of the feeding cable, connected to then antenna. However if you want to measure an antenna already mounted in the air and you have no idea what the cable length is, then a single measurement with the VNWA (at the TX end of the cable) we can determine the cable length very accurate, and by using the ZPlots program made by AC6LA, to be called from within the VNWA software, we can move the measurements to the feeding point of the antenna, and find the required impedances of the antenna feeding point for designing the matching network. ZPlots developed by AC6LA can be called directly from the VNWA software. ZPlots can as well be used as an independent program, and can read Touchstone files from other sources. After moving the measurement by the ZPlots to the antenna feeding point, the impedance can be saved into a Touchstone file (SnP), and subsequent import it into the VNWA software and use the VNWA Matching Tool, as explained in the following, and thus directly read the component values for the type of matching network selected. Both programs are free to download and use, but you need of course a VNWA to perform the measurements, or use other type of equipment able to produce a Touchstone file and subsequent import into the VNWA software followed by processing by ZPlots. That will be demonstrated for an antenna which I call the mystery antenna. The Mystery Antenna story: I had the parts for this antenna, and I had no idea of what it was designed for, or its past history, so I assemble it and did a single sweep from 1 to 30MHz.

2 The total assembled length is 3.75m and the metal frame structure used is only a mean to support it, and lag of proper ground plane is not relevant for doing this demonstration. As seen on next page it is a shorted antenna and with very low impedance (exposed to high power so a coil is burned?? time will show ). It resonates at 17MHz and thus can be used for the 18MHz band. However it should be adjusted in its length, which is quite simple, but let us just design a matching network as it is, and see below for marker 6 the R II is 14.48ohm and C II is -585,17pF thus inductive. The VNWA tool Matching Tool utilize R II and C II and thus selected for presentation of S11 traces. However before we do anything we transfer the S11 to Mem4 and present Mem4 as Smith Chart so we still can see the antenna plot in the smith chart for comparisons. As seen below the antenna plot is now red in the Smith chart as on top on the black S11 Smith chart trace. We use R II and CII traces/measurement as that is requested by the Matching Tool next to be used. The C II trace is with 500pF per division and reference 0 pf at division 5. R II is with 100 ohm/div and 0 ohm at division 0.

3 In the VNWA software we chose for Tools the Matching Tool and click Ok in the Prompt shown Enter the values ohm, pF and MHz for the frequency and select matching network 2 The matching network designed consist of Cp 418pF and Ls 93.6nH The matching network 1 could also have been used resulting in Lp 185nH and Cs 350pF Matching network 3 and 4 only applies if the antenna R II is above 50 ohm and the configuration depend whether the antenna is capacitive or inductive. Observe for marker 6 now the R II is 50 ohm and the C II is 0pF and a perfect power match exist. The loss resistance of the coil due to the Q of the coil is not accounted for so a small matching error may occur by measurements after the matching network build. That is the basic concept and a very simple method. Do a sweep and note the R II and C II at the frequency of interest enter these the three data elements into the Matching Toll and select matching network 1 or 2 and you are done. As mentioned the matching network 3 and 4 only relevant for R II above 50 ohm. If you only want to study the L and C values without the dynamic changes of the traces, then enter values for R II and C II into port 2 alternatively. For a capacitive and inductive antenna we thus have 4 different type of matching network to choose amongst.

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5 Please note! You can at any time select Restore Unmatched to revert back to the antenna measurements Now what happens when a cable is connected? Below a cable of 51.95m RG8A/U is inserted and obviously we cannot use this for any calculation as such. Of course we could design a matching network at the MHz for fitting between the TX and the feeding cable, as R II is ohm and C II is 25.9pF, but it is much better to move the measurement toward the antenna using the AC6LA program ZPlots called directly from the VNWA software. Visit the AC6LA homepage for ZPlots (just google it) and read how to install and setup the VNWA. In Tool section use Configure Tools and enter the path where you installed ZPlots.

6 In ZPlots select Smith and Click on Add/Subt T Line In the window appearing select Unit to meter and Cable type to RG-8/U or whatever the cable type you use.r You may define a Custom cable type yourself if parameters known.

7 By entering meter and by clicking Apply we see a sensible result. Next click on OK Now click on Save as S1P and save the touchstone file generated in a folder to be remembered, with a descriptive name.

8 Next in the VNWA import to S11 the just saved Touchstone file name and it is seen not exactly as the direct measurement stored in Mem4 so we next try to find the correct length as we know what the result shall be. The correct length is 51.8m as seen below, but if we do not have a direct measurement in the feeding point of the antenna to compare to what then??. Until now we have only seen that ZPlots is a fantastic program We simply find the correct length by direct measurement with the VNWA when in Time Domain Mode. The VNWA must be calibrated from lowest to highest frequency e.g. 1 to 1300MHz or even better from 0.1 to 1300MHz to use the Time domain mode, and a Tim trace selected here tim3 RealZ

9 Below is seen a measurement of the cable alone and UPS!!, we have a reflection at 6.24m and at 45.6m so the cable is not OK. At 6.24m it has probably been under pressure (37.93ohm) and at 45.6m probably been twisted/bend too sharp (60.54 ohm). At marker 12 (a time marker) the Z0 found to 49.96ohm. but most important the length found to be 51.8m HURRA. Below seen the same measurement (a single sweep) performed with the antenna connected and the result identical s expected. The marker must be placed at the rising edge of the reflection and not the peak, caused by the traveling wave running to the tip of the antenna and return. Thus the pulse width represent the antenna length

10 So before using the ZPlots, find the cable length accurately by the VNWA, it is far more accurate than a physical length measurement, which I actually did. Remember to use same VF value in the VNWA for time domain as the cable VF else measurements are way off. The R II is found to be ohm as opposed to ohm by direct measurement And C II pF as opposed to pF What is the impact of this difference? By Loading the touchstone file for the direct antenna measurement and use the ZPlots derived data we see the SWR raised to 1.2. However a cable length of 51.8m is extreme, and by a new non defective cable and a shorter length the difference will be marginal. The concept proven. Kind regards June Kurt Poulsen de OZ7OU