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I have a question regarding an integrated PCB Antenna that I would like to try out, however, if it fails I want to be able to use another SMA antenna. The PCB Antenna is an inverted F-antenna meaning that it will be "shorted" with one of its ends connected to GND, as seen in the attached picture.

I have added two 0 ohm resistors that I was planning to desolder depending on which antenna I am going to use. Now my question is if the PCB antenna only is connected to the GND (far left) and disconnected everywhere else (i.e. R6 is removed and only R4 remains), could this affect the performance of my SMA antenna?

PCB & SMA Antenna

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  • \$\begingroup\$ Yes of course it will affect it by a small gap capacitance and resonant track, how much is hard to tell \$\endgroup\$
    – Tony Stewart EE75
    Sep 16, 2018 at 14:23
  • \$\begingroup\$ What is the frequency? stubs shorter than 10 or 20 degrees at the carrier frequency may be don't-care effects. \$\endgroup\$
    – analogsystemsrf
    Sep 16, 2018 at 14:29
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    \$\begingroup\$ Can't you share the pads of R6 and R4? (ie place one pad on top of the other) \$\endgroup\$
    – Mike
    Sep 16, 2018 at 16:56
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    \$\begingroup\$ This is not an SMA connector. \$\endgroup\$
    – Lior Bilia
    Jun 18, 2019 at 20:29
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    \$\begingroup\$ U.FL connector ? \$\endgroup\$
    – Blup1980
    Aug 24, 2019 at 11:57

4 Answers 4

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There is a similar connector that can connect the onboard antenna when there is no coax and disconnect the onboard antenna when the coax is connected. That would solve your problem without any soldering to change the configuration.

See What connector is similar to U.FL but has no pin? for a discussion on the topic.

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  • \$\begingroup\$ Are you sure those connectors will perform well in the field? I understand that they are used for production testing purposes, so they only work fine in a controlled environment (no shock / vibration, stable temperature) and their useful lifetime may be much shorter that the lifetime of the actual antenna. \$\endgroup\$
    – Dmitry Grigoryev
    Jun 29, 2020 at 10:54
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The theoretical answer: Yes, it will affect your SMA antenna. The practical answer: No, the effect is tiny so you should not notice it.

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When doing RF layout you should always be on the lookout for elongated shapes in the ground plane. When using the U.FL connector your ground plane definitely has an "antenna shaped" stub/elongation of the ground plane. If you don't have expensive RF debug equipment I would recommend you stick closely to any RF rule-of-thumb:

  • Put a placeholder 0-ohm resistor on the antenna ground stub. This will eliminate any weird emissions from that part of the ground plane.
  • Just to exemplify how detail-oriented RF design normally is: When you use your U.FL connector you have a small amount of trace from R4 to the R6 pad. Even this should be eliminated because it represents an impedance discontinuity.
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You can implement this with only one resistor, or more exactly, a 3 pad layout, where the resistor can be installed horizontally or vertically. This way, you save about 2pF of parasitic capacitance, which is critical for a 2.4GHz design.

Keep the trace to U3 as straight as possible, you want to minimize the number of bends on an RF trace.

Keep the trace to the connector as straight and as short as possible. you don't need the bend if you rotate the connector by 90 degrees.

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