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I have a general question regarding the connection of an antenna to a RF module like GPS receiver or 433 MHz TXD:

Is a direct connection of an antenna (e.g. GPS ceramic patch) to a module preferred over the regular solution to use a 50 Ohm PCB trace? Or is there more signal reflection, attenuation or other negative impact?

Practically it would look like to have the RF module on the bottom of a PCB and the antenna wire would be fed from the top through a via directly to the antenna pin of the module.

Thanks!

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  • \$\begingroup\$ What do you mean by direct connection to the module? Basically you have to connect the antenna to a 50 ohms "thing" in the module (assuming the antenna impedance is 50 ohms), be it a PCB trace or the transceiver input/output, to avoid reflections. However, you may want to avoid coupling between the antenna and whatever other conductors in the module (which can affect the antenna performance) and so it may be a good idea to place it somewhat far away from other components. \$\endgroup\$
    – hcabral
    Aug 30, 2016 at 16:04

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The short answer is yes that could work provided you match the impedance. You essentially have a tree device connection (whether you use the transmission line or not) and all the impedances need to match (or be close).

$$Z_{antenna} \leftrightarrow Z_{connection} (via) \leftrightarrow Z_{TX/RX}$$

The longer answer is that there may be other engineering reasons to move the antenna away from the transmitter. Like @hcabral mentioned, there may be other practical things to consider when putting the antenna that close to the circuits e.g., electromagnetic interference (EMI), antenna loading, and unintentional radiation (maybe another circuit will couple to the antenna and produce unwanted radiation). Remember, with the antenna that close the whole board actually becomes the antenna. Another reason you may want a trace is for a balun. The antenna feed and TX/RX pin may not have the same impedance and therefore the trace may become a matching network.

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To give some practical advice: when you do something like that, make sure the via has a generously sized clear area around it because you will have a lot of capacitive coupling when crossing the board fabric. Specifically, don't forget to place keepout areas on the inner ground/vcc layers around your via.

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