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I want to extend WiFi signal using a passive repeater between two isolated boxes (have to make both box wireless).

  • Transmitter antenna from box A is placed in the gap between box A and B.
  • In the same gap at distance of about 10cm receiving antenna for box B which will be connected through coax to a computer in box B.

For link budget calculation:

  • Receiver antenna gain 0db.
  • Max Transmitter power for wifi 20dBm
  • Propagation loss in free space for 10 cm is -16db
  • Wire loss is 0.3dbi/m for 3m - 0.9db.
  • Prx = 20dbm - 16db -0.9db = 3.1dbm.

Is this calcuation correct?

Also, if free space is very less, can I couple all signal to receiving antenna?

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  • 1
    $\begingroup$ Welcome to ham.stackexchange.com! $\endgroup$ – rclocher3 Sep 12 '19 at 13:39
  • $\begingroup$ Related posts here and here. $\endgroup$ – Mike Waters Sep 12 '19 at 17:04
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Your math seems correct, but you're making some assumptions that might not work out in practice. First, the Friis transmission equation assumes that the transmitting and receiving antennas are in each others' far fields, but the 10 cm distance you specify is less than a wavelength (12.2 cm at a typical WiFi frequency of 2.45 GHz), which puts you in the "transition zone" between the near field and the far field, which means that your path loss may be different.

Second, your antenna gain of 0 dB basically assumes that your antennas are isotropic, which means that they radiate and receive equally in all directions. There is no such thing as an isotropic antenna in the real world, although there are real antennas that have 0 dB gain in specific directions. A "lossless" dipole antenna (another antenna that doesn't exist) has a gain of about 2.15 dB. So if you were using two dipoles in free space that were broadside to each other, far enough that they are in each others' far fields, then you would have antenna gain of about 4.3 dB.

So it's not possible to give an exact estimate of the gain you would see in real life from the information that you have given us. I'm not an electrical engineer, but I think that you would have better coupling (more gain) than your calculations say.

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