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I m very confused about something and I wanna ask it by giving example. Let’s assume that an antenna has a frequency band of 1000 MHz to 1010 MHz with VSWR level 1.5 and impedance of 50 Ω, and uses a LNA at the backend whose frequency range is 950 MHz to 1100 MHz, same impedance of 50 Ω (I just make up the numbers).

Does that mean the LNA operates at the antenna’s out-of-band with 50 Ω however the antenna is not 50 Ω anymore at the out-of-band, so the mismatch creates oscillation in LNA. Is that possible problem?

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  • \$\begingroup\$ It seems unlikely this would happen. \$\endgroup\$
    – Andy aka
    Commented Sep 25, 2023 at 12:58
  • \$\begingroup\$ antfellow - Hi, This is the 4th question about LNA behaviour you've posted, but you deleted the previous 3 and this has been noticed by other site members. As general guidance, deleting a question should be a last resort. In some cases, it deletes effort from other site members (e.g. an answer which hadn't yet been upvoted and so didn't prevent the question being deleted, or comments which asked for more details). That can result in wasted time e.g. if a related question is asked again & people have to repeat the same analysis. Please consider these effects before deleting a question, Thanks. \$\endgroup\$
    – SamGibson
    Commented Sep 25, 2023 at 16:27
  • \$\begingroup\$ @SamGibson Hi, frankly, I do not delete them. Someone closes my questions without giving me any detail how I can make it detailed. So, I had to delete them. \$\endgroup\$
    – Xant
    Commented Sep 25, 2023 at 19:15
  • \$\begingroup\$ @antfellow - Hi, "frankly, I do not delete them {...} So, I had to delete them." Right, so you did delete them :) "Someone closes my questions" Your 1st question was not closed before you deleted it; three site members (FYI, not a single "someone") voted to close the 2nd & 3rd. If you disagree with closure, the correct approach is explained on this page in the help center. I know it can be frustrating when a question is voted closed, but deleting & reposting gets noticed and is often viewed as wasting people's time, so please avoid doing that. Thanks. \$\endgroup\$
    – SamGibson
    Commented Sep 26, 2023 at 2:40

1 Answer 1

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That means that at the out of band frequencies, the input to the LNA and the output of the antenna are not going to be matched. So you will get some reflections back from the LNA to the antenna - an increase in return loss.

In the digital world, this might show up as some minor ringing on a signal edge. But in your example, no oscillation, anyplace. Worst case might be a bit of amplitude ripple, depending on the amount of mismatch.

Note that I am ignoring any cable that might be between the antenna and the LNA.

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