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If an omni-directional antenna (transmitting antenna) transmits to a directional antenna (receiving antenna), how will the SNR be affected compared to when both antennas are omni?

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Assuming your system is limited by atmospheric noise and not electronic noise, the SNR will be increased by the gain of the directional antenna. This gain represents the reduction of atmospheric noise (assumed isotropic) over an omni-directional antenna.

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  • \$\begingroup\$ Maybe you mean thermal noise rather than atmospheric noise. \$\endgroup\$ – Andy aka May 1 '18 at 21:04
  • \$\begingroup\$ I think you can assume all white noise, outside or in LNA. However not strong co-channel noise that is external or internal unless the co-channel radiator is rejected off-axis by the narrow beamwidth.e.g block CN tower yet pickup Buffalo north of Toronto > 150km away on HDTV. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 1 '18 at 21:08
  • \$\begingroup\$ thus by having stronger tower deep in skirts of antenna pattern you can boost SNR by more than antenna gain since reducing the interference prevent saturation or CNR interference while gain boosts desired signal, so one can benefit both gain of desired signal and attenuation undesired signals by diversity tradeoffs. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 1 '18 at 21:15
  • \$\begingroup\$ @Andy Atmospheric noise is any noise that is received externally by the antenna. This is the noise that is reduced by using a directional antenna. Thermal noise is the lower limit of atmospheric noise (after all manmade sources are eliminated). \$\endgroup\$ – Barry May 1 '18 at 23:47

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