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What is wrong with my transmitted power calculation?

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YechielA

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Hello,
According to FCC the E field at 3 meters should not exceed 500uV/m.
So let us calculate the total radiateted power assuming isotropic radiator.

[1] Pt=(|E|^2/ηo)*(4*Π*D^2) WHERE E IS EQUEL 500uV/m and D=3 meters.

on the other hand, according to FRIIS equation, the free space path loss is given by: (λ/4*Π*D)^2 so that,

[2] Pt= (|E|^2/ηo)*[1/(λ/4*Π*D)^2]=(|E|^2/ηo)*(4*Π*D/λ)^2.

But both equation contradict and give different results.

Any idea ???
 

When calculating received power from field-strength respectively power density using Friis formula, there's an effective antenna height plugged-in. It's not correctly applied in [2], I think.

The first expression which simply determines distant power density from radiated power and calculates field strength referring to field impedance is the correct one.
 

Suggestion? Break down (or separate out) the individual component parts of the equations and plug in the numbers to find out which terms/calculations are contributing the differences.

I usually use MathCad (WYSIWYG math app) to do this, since I like to see the maths as I was taught (versus Excel spreadsheet, but spreadsheet can be used too.)

Jim
 

FvM said:
When calculating received power from field-strength respectively power density using Friis formula, there's an effective antenna height plugged-in. It's not correctly applied in [2], I think.

The first expression which simply determines distant power density from radiated power and calculates field strength referring to field impedance is the correct one.
Click to expand...

Hi FvM,
Yes I know the first equation is right!
I'll investigate what you said.
 

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