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In tuning an antenna, there are multiple possible topologies that will take it to the middle of the smith chart.

Some differences that I can see between them are:

  • Component ESR (losses).
  • Effect of component tolerances.
  • Resulting antenna bandwidth.

For example:

Alternative 1 "LpCs"

enter image description here

Alternative 2 "LpLs"

enter image description here

Alternative 3 "LsCp"

enter image description here

Which one would you choose and why?

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  • \$\begingroup\$ Too broad - ask a more realistic question that doesn't require anyone answering to consider a million combinations of this and that in order to possibly satisfy the problem you have in mind. \$\endgroup\$
    – Andy aka
    Commented Aug 21, 2014 at 20:08
  • \$\begingroup\$ @Andyaka Oh my God, really Andy? This is a perfectly realistic question, and extremely narrow. In fact, it is partially answered in the beginning of it. If you need more information, just ask for it. Surely it isn't 'a million' things. \$\endgroup\$
    – apalopohapa
    Commented Aug 21, 2014 at 20:22
  • \$\begingroup\$ As you say in your question "there are multiple topologies" so don't act all innocent on me. If you want a question answered properly don't expect a thesis or an essay. \$\endgroup\$
    – Andy aka
    Commented Aug 21, 2014 at 20:36
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    \$\begingroup\$ @Andyaka The question is specific about choosing between exactly 3 different alternatives. I think it is useful and interesting to come up with criteria about how one goes about choosing one. An essay would be very generous, but not in the least expected. A few bullet points would suffice. An example based on experience would be great. \$\endgroup\$
    – apalopohapa
    Commented Aug 21, 2014 at 20:53
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    \$\begingroup\$ I am too very interested in the possible answers to this question. \$\endgroup\$
    – Guillermo Prandi
    Commented Aug 22, 2014 at 22:32

2 Answers 2

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Versions 2 and 3 offer a DC path to the antenna. If your antenna is a HF long wire, your protection from lightning strikes is not too good.
Smith charts imply you are talking UHF / microwave: antennas are smaller, lightning strikes are less common. If you have some form of masthead amplifier, you may actually want a DC feed.
Unless this is the case, I'd stick with version 1.

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So long since that question came up but I have discovered it recently and would like to add some thoughts:

Considering tolerances in antenna impedance (first point in these smith charts) I would probably prefer to choose the second approach. Thats because if the antenna impedance changes slightly there is a much higher deviation of the second point's location (when travelling long distances in the smith charts's outer area, like it is done in solution 1 and 3). Considering tolerances of the first matching element (inductance in this case), the third point (ideally at 50 Ohms) would deviate even more.

Another thought is to keep in mind, that the matching network can also be used as a filter for harmonics, so the choice of topology might play a role in filtering those out.

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