In the first picture we have two smith diagrams for two antennas that are in resonance. I wonder why one circle is bigger and one smaller? Also, in the second picture, if someone can explain to me the results of each quadrant? thank you
The size of the circle depends on the impedance that the antenna shows to the feed when it is resonant. The big circle crosses the axis at about 5 times Zo. This means that the antenna looks like about 250 ohms at resonance, and is not matched to the feed. The reflection coefficient is about 4/6 and about half the power will be reflected from the antenna.
The other one crosses the axis at about 1.6 Zo. This is reasonably well matched to the feed and most of the power will get into the antenna.
It depends how strongly the antenna is coupled to the feed. It can be over coupled or under coupled, or matched.
On the other plot, I am not sure what the top left one is. The bottom left is voltage standing wave ratio against frequency, and shows the magnitude of the complex impedance compared to the feed line impedance (Zo). The top right is probably phase against frequency, and the bottom left is a smith chart. All the others display something that is shown on this chart.
There are seven resonant frequencies to this component. Each corresponds to loop on the smith chart, a step on the phase chart or a minimum on the other two charts. near the middle of each resonance, the component is reasonably well matched to the feed line. In between. it isn't.
The loops on a Smith Chart can be understood as the impedance or admittance of a resonant circuit described simply as an LC series or parallel resonant circuit.
The Smith chart has a straight line on the horizontal axis. Any impedance here is purely resistive. There is zero resistance to the left and infinite resistance to the right. Each circle shows places with constant resistance R, but varying reactance X. The part circles show places with constant reactance, but with resistance varying along each part circle.
Each full circle on the Smith chart would be described by the formula z=R+jX in the document I've included below, for a different value of R. Changing the frequency take you around the circle by varying X.
If you reflect the Smith Chart left-right, it gives lines of constant conductance G and susceptance B, and is used for parallel circuit descriptions, y=G+jB.
Your two loops are aligned so they look like parallel resonant circuits, but the Smith chart is rotated for series resonance. You can change this by changing the display, I think, or by changing the reference distance for the measurement, which will rotate the whole measurement on the screen. The circles will then line up with the circles on the Smith Chart.
- Badges
- Science topic
- Similar topics
- Social Science
- Media
- Journalism