I'm having difficulty with the line input impedance of the circular planar antenna signal. Its impedance is 117 ohms and the correct is 50 ohms, I am not able to lower the value. I made modifications to the input increasing the line width and also made an impedance transformer. When I get it down, the whole process on the antenna I made is impaired. Example: reflection coefficient is lost, surface current loses efficiency and also its power and directivity. Please can someone help about this.
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An impedance of 117 Ohms sounds pretty reasonable for a particular point in the band, it translates to a reflection coefficient of -8 dB or so. I don't think you should be optimising it further yet.
UWB antennas are a compromise, intended to work over a very wide range of frequencies.
To judge its performance you need to analyse it over its full range of frequencies, 2 to 10 GHz or so, and look at the reflection coefficient over the whole band. Generally these shape of antennas work well enough directly at 50 Ohms without a matching network. Using a simple transformer like a quarter wave line of a higher impedance, will help one frequency but hurt others.
To tune the antenna, try adjusting the shape and size of the blob, especially its distance from the groundplane and its curvature near the feedpoint.
If you're able to change the shape - there are many design papers published about these, try searching for UWB in the IEEE APS transactions, or just on google.
For example, my first hit is this paper which shows the following antenna and its reflection coefficient:
Which indicates that the impedance in your simulation is pretty typical of UWB antennas of this type.
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\$\begingroup\$ Hi, thanks for the scientific article, so this part you mentioned, every 2 to 10 GHZ frequency path really has to be a 50 ohm impedance match across the board. But when I manage to reach that value, everything ends up being horrible, an example I gain directivity running from the surface. \$\endgroup\$– LUFERFeb 20, 2020 at 14:58
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\$\begingroup\$ I made two antennas, one that I made with my data and the other that is reproduced in the scientific article. Incredible as it may seem, the reproduction of the scientific article with all the data still remains a value close to 112 ohms. By logical frequency level in microwave, the impedance of 50 ohms. I really don't know what the mistake is? Please I need guidance. researchgate.net/publication/… \$\endgroup\$– LUFERFeb 20, 2020 at 14:59
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\$\begingroup\$ @lufer but it is matched to 50 Ohms, just not exactly 50. 117 is very close to 50 in this type of antenna, about -8 dB reflection coefficient. You are doing OK there, similar to the antenna in the paper. You should simulate over frequency, perhaps add your graph of reflection coefficient to the question so we're not just talking about one number at 7 GHz. \$\endgroup\$– tomnexusFeb 20, 2020 at 16:16
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\$\begingroup\$ Sorry, but I'm finding contradictory information, the more I try to understand what's going on, the more I get confused. Because when you handle with microwaves the input impedance has to be 50 ohms, which I understand that Zref refers to what I understand in the CST means that it is real impedance that is on the line at that moment. I will send the images. i.stack.imgur.com/uXJnn.png \$\endgroup\$– LUFERFeb 22, 2020 at 21:30
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\$\begingroup\$ It looks like you need to get some help driving CST. You are free to plot the real part of Zref, but it is a constant you choose, so not an interesting graph. You are also free to change Zref to any value. But why not leave it at 50 Ohms? There's no reason to change it, you just get strange reflection coefficient graphs. \$\endgroup\$– tomnexusFeb 23, 2020 at 1:38