This is about impedance matching an antenna. The better the cable calibration (nulling), the easier it is. See Part 1 for information on connecting to an antenna and calibrating the VNA.
Matching
A typical matching network is a pi configuration, with “shunt” components on the transceiver and antenna sides connected by a “series” component.
- Set the VNA to S11 Smith Chart mode. Set the frequency sweep a little beyond the band of interest. So if you’re matching for the 902-928 MHz band, you might use 890-940 MHz. Set markers at the bottom, middle, and top of the band, e.g. 902, 915, and 928 MHz
- Start with a 0-ohm (substitute a cap at its SRF if necessary) in the series position, so that you can see the antenna without any matching. Note how far the markers are from the center of the Smith Chart. The closer they are to the 50-ohm center, the better the antenna performance. Get a mental notion of which way the entire curve would need to rotate to move all three markers toward the center. Save this and every reading your take to a thumb drive or a computer connected to your VNA.
If the antenna is starting with the markers near the outer circle, the first goal is just to get those points a little bit inward. Do this by trying an inductor or capacitor in the shunt position closest to the antenna. This chart from this website shows how components move the points on the Smith Chart. If you’re starting far from the center, though, do not worry about too much about moving in particular direction-- you just want to get away from the outer circle. (Keep saving every measurement taken with new matching value at every step.)- Replace the 0-ohm series component with an inductor or capacitor. This time, we are trying to get all three marker points to a location on the constant inductance curve that passes through the center of the chart. If the antenna started not far from the center and you chose a good value in the last step, you should be able to get to the center at this step.
- Add a cap or inductor in the series position nearest the VNA. Get as close as possible to the center.
- If you can’t get to the center, go back and try slightly different values from the last step. Look at all the past readings to get a feeling for how various values are moving the points. This will give you an idea of values to tweak. You may even go back and slightly tweak the component nearest the antenna. This is tricky because you don’t know how far that component moved the points. It may be necessary to start over. Record every measurement. You may try dozens of sets of values. Reviewing the screenshots of each reading will provide insight.
- Set the VNA to SWR mode. SWR gives you a figure of merit for how much power is reflected from the antenna. 1:1 is perfect. Below 2:1 is excellent. 3:1 means the VNA is receiving 25% of the power reflected back. This sounds bad, but it only works out to 1.25dB of performance, which may be acceptable.
- Turn off calibration and look at SWR plot. It may appear better because some of the reflected power is lost in the cable. The shape and values, though, should not be radically different.
Tips
- If the matching components do not have the effect of moving along the curves as shown in the diagram, it may mean the calibration was not done well. You can try calibrating again or you can get a feel for how values actually move the points with the calibration as it is. It won't follow the diagram, it may be rotated. As long as you get a feel for which way matching components move the points, you can get to the center. Even if calibration is very poor, if you get them to the center you will be very close to matched.
- Use matching components whose reactance works out somewhere in the 5-200 ohm range. Start on the high side for shunt components and with lower values for the series components.
- If the antenna is matched but not across the whole band, the antenna bandwidth may be improved by starting over and repeating each step keeping in mind the goal of getting all three markers to the center.
- If the S11 plot is closer to the center (or the SWR plot’s lowest point) at a frequency above or below the band, it may be possible to move this resonant frequency by modifying the antenna. This will make it easier to match.
- There is a saying “You can match to a rock.” It means that an antenna being matched means it will not reflect power back to the transmitter. It does not mean the antenna will radiate the power and work well.
Conclusion
Do not be reluctant to experiment. Even if it seems nothing you do matches an antenna, looking at dozens of saved plots may lead to a solution.
