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When I studied elementary antenna's theory, I saw that the resistance (for simplicity let's consider only the resistive terms) of an antenna Ra is made of:

1) a parasitic resistance Rl, which causes antenna losses.

2) a radiation resistance Rr which is in practice the free space resistance, and it is like a load resistance if for instance we think at a transmitting antenna.

Now I have two questions:

  • about 2), from its definition it seems to me that the radiation resistance should depend on the space conditions (rain, humidity, temperature, presence of turbulences etc). If it is true, how can we design a correct matching network between an antenna and a transmission (since it is not a fixed value)?

  • how can I physically evaluate the impedance of an antenna? For instance a stylus antenna (I must connect it to a coaxial cable and so I thought that its knowledge is important to choice the correct matching network)?

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  • \$\begingroup\$ Have you heard about Friis transmission equation? There are propagation path loss models. Some are not deterministic but emprical. \$\endgroup\$ – user16307 Sep 18 at 17:39
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    \$\begingroup\$ The radiation resistance does not vary enough over typical atmospheric conditions to significantly affect SWR, except for wavelengths approaching a small multiple of droplet size. \$\endgroup\$ – hotpaw2 Sep 18 at 18:32
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about 2), from its definition it seems to me that the radiation resistance should depend on the space conditions (rain, humidity, temperature, presence of turbulences etc). If it is true, how can we design a correct matching network between an antenna and a transmission (since it is not a fixed value)?

That is true, but in general such things don't have much effect. A microwave antenna in a heavy rain may see changes, but I'm not sure how much.

In general, if it's a concern, you'd establish the range of expected antenna impedances (because it won't be just the radiation resistances that change) and design a matching network that works well for anything in that range.

how can I physically evaluate the impedance of an antenna? For instance a stylus antenna (I must connect it to a coaxial cable and so I thought that its knowledge is important to choice the correct matching network)?

You connect it to an SWR meter, or an RF impedance analyzer, and you do some test transmissions.

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