+ Post New Thread
Results 1 to 3 of 3
  1. #1
    Junior Member level 3
    Points: 342, Level: 3

    Join Date
    Sep 2018
    Posts
    26
    Helped
    1 / 1
    Points
    342
    Level
    3

    question about Friis equation with very large antenna gain

    Hi,

    in Friis equation, if the gain of transmit and receive antenna are very high, at least from this equation we may have Pr/Pt larger than 1. But since antenna is passive, it's not possible for this ratio to be larger than one. How can I understand this question? Mathematically what is limiting this ration?

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 261,317, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,664
    Helped
    13881 / 13881
    Points
    261,317
    Level
    100

    Re: question about Friis equation with very large antenna gain

    Friis equation is valid for far field. Most likely you are still in the near field range if your example achieves positive power gain. Or the assumed antenna parameters are simply unrealistic.

    How did you arrive at the antenna gain? Are you sure that it's valid for the transmission distance in your setup?



    •   AltAdvertisement

        
       

  3. #3
    Full Member level 4
    Points: 1,449, Level: 8

    Join Date
    Jan 2019
    Location
    Ireland
    Posts
    206
    Helped
    86 / 86
    Points
    1,449
    Level
    8

    Re: question about Friis equation with very large antenna gain

    Exactly as FvM mentioned. If Pr/Pt > 1, then it would also mean lambda/4piD >1 ==> D<lambda/4pi
    But by definition, far field extends upto infinity. Therefore the above conjecture is incorrect.



--[[ ]]--