Rearranging formula's

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Cabdullahi

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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?
 
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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
 
:hiding: i have a headache..... * nightmares*
 
Abdullahii said:
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
Click to expand...

If Pr is 0, I think your equation would give you 0 for Pt always.
 
Abdullahii said:
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?
Click to expand...

You've rearranged it perfectly...so you must have made some other error somewhere in your calculation.
 
Alpha Dude said:
If Pr is 0, I think your equation would give you 0 for Pt always.
Click to expand...

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
 
Abdullahii said:
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
Click to expand...

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.
 
Nathaniel said:
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.
Click to expand...

i made the mistake of thinking that unknown equates to 0 :(
 
Abdullahii said:
i made the mistake of thinking that unknown equates to 0 :(
Click to expand...

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.
 
Nathaniel said:
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.
Click to expand...

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
 
Abdullahii said:
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
Click to expand...

Can you measure the power received? I'm afraid I know very little about yagi-uda antennae... :hmm:
 
Nathaniel said:
Can you measure the power received? I'm afraid I know very little about yagi-uda antennae... :hmm:
Click to expand...

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 :(
 
Abdullahii said:
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 :(
Click to expand...

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.
 
Nathaniel said:
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.
Click to expand...

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
 
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 :hmm:

Is that correct?
 
Nathaniel said:
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 :hmm:

Is that correct?
Click to expand...

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
 
this is what i got

Pr = 1520417.374 / 1336581.306 =1.137 ?
 
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).
 
Nathaniel said:
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).
Click to expand...

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!!!!! :D:D:D:D:D:D
 
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