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Since time is money and my PCB home making skills are not the sharpest, I would rather fit my PCB design with two unbalanced antennas (no balun, just impedance matching parts, for 315 / 433 MHz) for the one and only transmitter on the board and later chose between them by means of a 0 ohm resistor / jumper. Does this sound practical ? Any particular aspects I ought to take care with ?

EDIT: both antennas are supposed to work for both frequencies (properly tuned, of course). But I don't know... I currently have a design with differential loop antenna (unrelated to this question) - it works noticeably better at 315 than at 433 MHz.

These two in question, one will be an unbalanced loop antenna the other an unbalanced printed helical antenna. Only one of them will be used and on only one of the two frequencies at a time.

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  • $\begingroup$ Can you share a picture of the board? Are these chip antennas or all-printed? If chip, why not just put in footprints for both, solder on the one you need? $\endgroup$
    – tomnexus
    Sep 15 at 16:40
  • $\begingroup$ @tomnexus There is no board designed at the moment. These are pcb trace antennas $\endgroup$
    – kellogs
    Sep 15 at 16:43
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    $\begingroup$ Inverted F antennas? You should be OK as long as they're not touching, the wrong one won't cause much more trouble than the groundplane nearby. You might also find a topology that works for both frequencies, with one 0 ohm to extend the tail of the F down to 315 MHz (more difficult to tune both though). Or a single capacitor from the tip to ground, for 315? $\endgroup$
    – tomnexus
    Sep 15 at 16:47
  • $\begingroup$ You should edit the question to add the type of antennas, and to clarify what frequencies / when they'll be used. Do you mean you need 315+315 sometimes, and 433+433 other times, or 315+433, or just one radio that goes to 433 or 315 depending on which market you're shipping it to? $\endgroup$
    – tomnexus
    Sep 15 at 18:35
  • $\begingroup$ Hi, I'm sorry it's still not clear if you have a) two radios 433/315 active at almost the same time, b) one radio per finished board, but you want to have common stock of pcbs so want a 315 and a 433 so you can change country with just a jumper, or c) 433+433 active at the same time, etc. i.e. Would a single dual/band or adjustable antenna be good enough, or must you have two antennas in total. Also, how big is the board? Explain this better and you might get more answers. $\endgroup$
    – tomnexus
    Sep 20 at 3:33
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PCB trace antennas are in most cases compromises between a good antenna and an ok antenna that will fit on the board (a very common example is a meandered antenna). They are very sensitive in the near field because of everything being so close to them, including other PCBs or hardware. When tuning them the best solution is to tune them completely assembled in their final setup. Having a second antenna on the PCB even if it is left unconnected will most likely still affect the used antenna in some way (maybe in a good way although probability is a negative effect). You can do both antennas and scrape one of them off if this is a prototype to find which works the best. If this is a final PCB I would only do 1 antenna. If you still want to do both antennas you can, just know that there may be a negative effect that is hard to predict.

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  • $\begingroup$ In a professional environment, you could model the PCB in computer software and optimize the antennas so they will work well together, connected or not. In a home environment, the software and hardware to do this are likely cost prohibitive, so this answer is your best bet. $\endgroup$
    – user10489
    Sep 16 at 3:13
  • $\begingroup$ Yes, one can put in a lot of work to model the 2 antennas in such a way that they work together. A Yagi antenna could be thought of as 3 different antennas with only 1 of them connected, but they all work together as one antenna and the "unconnected" antennas significantly change the radiation pattern to make it directional. $\endgroup$
    – pgibbons
    Sep 17 at 13:26

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