# Power and Intensity of Radio Waves

1. Oct 28, 2007

### y_chen

1. The problem statement, all variables and given/known data

An AM radio station broadcasts with a power of 21.0 kW at a frequency of 910 kHz. Estimate the intensity of the radio wave at a point 18.0 km from the broadcast antenna.

2. Relevant equations

I= p/A I1/I2=r2^2/r1^2 w=2pi f

3. The attempt at a solution

From the given frequency, we can find omega (w). However, I am stuck on where to proceed afterwards. How would you find A? Thanks so much for helping.

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Last edited by a moderator: Mar 10, 2009
2. Oct 28, 2007

### GTrax

I cannot quite decode the relevant equation. Maybe its a fonts thing.
w (omega) = 2*pi*f is just expressing frequency in radians per second instead of cycles per second.

In free space, a radio wave intensity decreases as the area of the wavefront expanding sphere increases. In Earth atmosphere, it is also frequency-dependent because of propagation and absorption losses, ground losses, and ionospheric conditions. Not that we need be concerned..because
You have a given expression for the intensity, so we dodge all that hard stuff.

I can see $$I =\frac{p}{A}$$ which makes sense. Not sure what the rest (I1/I2) is set equal to $$r_2^2/r_1^2$$

I am unclear how $$\omega=2\pi f$$ while true, actually affects anything here.

Sorry I don't quite get it yet. Have another try about what this intensity expression is.

3. Mar 10, 2009

### flxwcc

just use the the amount of power and the distance. you dont need frequency.

I = P/(4*pi*r^2)