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Rearranging formula's

  1. #1
    Cabdullahi's Avatar Full Member
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    Rearranging formula's

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    I just want someone to check if i rearranged this formula correctly



    .............Pt.Gt.Gr.lambda^2
    Pr =--------------------
    ............16pi^2 d^2

    I made Pt the subject because thats what i want to calculate but im getting weird answers..this is how i rearranged the formula to make Pt the subject


    Pr.16pi^2.d^2
    -------------------- = pt
    Gt.Gr.lambda^2

    Where have i gone wrong?
    Last edited by Cabdullahi; 01-14-2010 at 10:44 PM.

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    Alpha Dude's Avatar Full Member
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    Re: Rearranging formula's

    Pr.16^2.d^2
    -------------------- = pt
    Gt.Gr.lambda^2
    Where's pi gone?

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    Cabdullahi's Avatar Full Member
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    Re: Rearranging formula's

    sorry pi is there i forgot to write it

    also in the second equation the Power received(Pr) is not needed because its 0 when you want to know the power transmitted

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    Re: Rearranging formula's

    i have a headache..... * nightmares*
    Rearranging formula's

    Know that Allah sees you.

    Know that Allah hears you.
    Wherever you are.

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    Re: Rearranging formula's

    format_quote Originally Posted by Abdullahii View Post
    sorry pi is there i forgot to write it

    also in the second equation the Power received(Pr) is not needed because its 0 when you want to know the power transmitted
    If Pr is 0, I think your equation would give you 0 for Pt always.

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    Nathaniel's Avatar Full Member
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    Re: Rearranging formula's

    format_quote Originally Posted by Abdullahii View Post
    I just want someone to check if i rearranged this formula correctly

    .............Pt.Gt.Gr.lambda^2
    Pr =--------------------
    ............16pi^2 d^2

    I made Pt the subject because thats what i want to calculate but im getting weird answers..this is how i rearranged the formula to make Pt the subject


    Pr.16pi^2.d^2
    -------------------- = pt
    Gt.Gr.lambda^2

    Where have i gone wrong?
    You've rearranged it perfectly...so you must have made some other error somewhere in your calculation.

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    Cabdullahi's Avatar Full Member
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    Re: Rearranging formula's

    format_quote Originally Posted by Alpha Dude View Post
    If Pr is 0, I think your equation would give you 0 for Pt always.
    Consider this Pr is unknown but all the other components are known...i want to know what power an antenna will transmit by rearranging the formula Pr .....and Pr at this stage is of no importance only when i find what my antenna is transmitting using the other components will Pr be important for finding what the power i've received

    so its 0 and i omitted it from the Pt equation

  10. #8
    Nathaniel's Avatar Full Member
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    Re: Rearranging formula's

    format_quote Originally Posted by Abdullahii View Post
    Consider this Pr is unknown but all the other components are known...i want to know what power an antenna will transmit by rearranging the formula Pr .....and Pr at this stage is of no importance only when i find what my antenna is transmitting using the other components will Pr be important for finding what the power i've received

    so its 0 and i omitted it from the Pt equation
    Setting a parameter to 0 is not the same as saying it's unknown - in fact it's saying that you know its value is 0! Alpha Dude is correct, if you set Pr to 0, Pt will also be 0. If Pr is unknown, then you can only find Pt as a function of Pr.

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    Cabdullahi's Avatar Full Member
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    Re: Rearranging formula's

    format_quote Originally Posted by Nathaniel View Post
    Setting a parameter to 0 is not the same as saying it's unknown - in fact it's saying that you know its value is 0! Alpha Dude is correct, if you set Pr to 0, Pt will also be 0. If Pr is unknown, then you can only find Pt as a function of Pr.
    i made the mistake of thinking that unknown equates to 0

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    Nathaniel's Avatar Full Member
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    Re: Rearranging formula's

    format_quote Originally Posted by Abdullahii View Post
    i made the mistake of thinking that unknown equates to 0
    OK, so if Pr is unknown, you can write down Pt in terms of Pr. Are you being asked to evaluate Pt or just find an expression? If you have to evaluate, perhaps they have given you some more information to help you find Pr, or you can make a good approximation.

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    Re: Rearranging formula's

    format_quote Originally Posted by Nathaniel View Post
    OK, so if Pr is unknown, you can write down Pt in terms of Pr. Are you being asked to evaluate Pt or just find an expression? If you have to evaluate, perhaps they have given you some more information to help you find Pr, or you can make a good approximation.
    I have to design a yagi-uda antenna...i've calculated the gain, wavelength(lambda) and i was given the distance (d)...using lambda i constructed the spacings between the elements on the yagi uda and so by using the Pt formula i want to know what sort of power will i be transmitting using the components given to me already

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    Nathaniel's Avatar Full Member
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    Re: Rearranging formula's

    format_quote Originally Posted by Abdullahii View Post
    I have to design a yagi-uda antenna...i've calculated the gain, wavelength(lambda) and i was given the distance (d)...using lambda i constructed the spacings between the elements on the yagi uda and so by using the Pt formula i want to know what sort of power will i be transmitting using the components given to me already
    Can you measure the power received? I'm afraid I know very little about yagi-uda antennae...

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    Re: Rearranging formula's

    format_quote Originally Posted by Nathaniel View Post
    Can you measure the power received? I'm afraid I know very little about yagi-uda antennae...
    if i measure the power transmitted i can calculate the power received...im getting something like

    Pt = 1.9378 watts

    and for Pr = 1.137 watts

    Looking at that my antenna is somewhat efficient but i honestly don't know there is loss but im skeptical about my calculations

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    Nathaniel's Avatar Full Member
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    Re: Rearranging formula's

    format_quote Originally Posted by Abdullahii View Post
    if i measure the power transmitted i can calculate the power received...im getting something like

    Pt = 1.9378 watts

    and for Pr = 1.137 watts

    Looking at that my antenna is somewhat efficient but i honestly don't know there is loss but im skeptical about my calculations
    What kind of value were you expecting? If it helps, I can go over the calculations you did and check you haven't made a small error somewhere.

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    Cabdullahi's Avatar Full Member
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    Re: Rearranging formula's

    format_quote Originally Posted by Nathaniel View Post
    What kind of value were you expecting? If it helps, I can go over the calculations you did and check you haven't made a small error somewhere.
    thanks sir

    to find Pt you'll need
    these components :

    Gt = 11.62* in ration not db

    Gr = 11.62* in ration not db

    d = 23 meters

    lambda = 61

    to find Pr you'll need

    Pt = You'll get this by doing the Pt calculation

    Gt = 11.62* in ration not db

    Gr = 11.62* in ration not db

    d = 23 meters

    lambda = 61


    * = 11.62 in ratio is 14.52

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    Nathaniel's Avatar Full Member
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    Re: Rearranging formula's

    So to calculate Pr you're doing:

    Pr = (1.9378 . 14.52^2 . 61^2)/(16 . pi^2 . 23^2) = 18.1981 (4dp) <- much bigger than your answer

    Is that correct?

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    Re: Rearranging formula's

    format_quote Originally Posted by Nathaniel View Post
    So to calculate Pr you're doing:

    Pr = (1.9378 . 14.52^2 . 61^2)/(16 . pi^2 . 23^2) = 18.1981 (4dp) <- much bigger than your answer

    Is that correct?
    its wayyyy too big i cannot receive more power than i transmitted this never has been achieved in science

    try to calculate one component at a time for example write 16 .pi = 50.2 ^2 =2526.61 and note it down somewhere and so on until you get all the component values this way then multipy them just like in the equation and then divide the top by the bottom

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    Re: Rearranging formula's

    this is what i got

    Pr = 1520417.374 / 1336581.306 =1.137 ?

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    Nathaniel's Avatar Full Member
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    Re: Rearranging formula's

    I get the bottom half of the fraction to be 83536.3317...is the formula meant to be (16.pi)^2 or 16.(pi^2)? I did the latter (which is what your formula implied).

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    Re: Rearranging formula's

    format_quote Originally Posted by Nathaniel View Post
    I get the bottom half of the fraction to be 83536.3317...is the formula meant to be (16.pi)^2 or 16.(pi^2)? I did the latter (which is what your formula implied).
    in the course notes it says 16π2 i think i made a mistake instead of (16.pi)^2 it should be 16.(pi^2)

    i calculated Pt correcting the 16 pi squared issue and i got :

    pt = 0.11 watts two d.p


    and

    pr = 0.10 watts to two d.p

    which means my antenna has a loss of 0.01 i think ? which is absolutely sweet...the antenna looks efficient according to the calculations

    cheers nathan for clearing things up!!!!!
    Last edited by Cabdullahi; 01-15-2010 at 02:22 AM.


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