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Why LNA input needs to be matched to 50 ohms? (Read 12199 times)

vp1953

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Re: Why LNA input needs to be matched to 50 ohms?
Reply #15 - Nov 06^th, 2010, 6:47pm

Hi Rfcooltools,

Quote:

This is problematic for two reasons first the signal sourced to you LNA will radiate at some frequency which may be at the desired or even at a harmonic and the FCC will deliver the cease and desist notice to the product.

Are you referring to re-radiation of the incoming signal (the fact that some of the incident signal is dissipated in the antenna source impedance, which shows up as re-radiation).

If it is just reradiation (even if the reradiation is frequency sensitive), it would not be considered an "intentional radiator" and hence no need to be certified under FCC Part 15 rules. Eg of such systems, passive RFID tags that backscatter the incoming radiation (at datarates from 40kbits to 600kbits) do not need FCC certification.

Could you elaborate a little more on the issue with the antenna - I am keen to know more about this as this is something that i am looking into currently.

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rfcooltools.com

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Re: Why LNA input needs to be matched to 50 ohms?
Reply #16 - Nov 9^th, 2010, 9:21am

vp1953,

For radiation I was not not thinking of the exact LNA application, but more in general for all types of RF signals. As for your argument on re-radiation I agree with your premise provided the gain is not excessive.

Unfortunately, I have not designed any antennas except accidentally. You may want to post this as a separate topic to get a better response.

http://rfcooltools.com

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