# Homework Help: Radio Frequency Power

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1. Nov 25, 2014

### SiberianS;pth

1. The problem statement, all variables and given/known data
A coaxial transmission line consists of an inner cylindrical conductor of radius 1mm and a cylindrical outer conductor chosen to make the characteristic impedance 75 ohms. The space between the conductors is filled with a gas that can stand a maximum field of 10^5 Vm^-1 without dielectric breakdown. Estimate the maximum mean radio-frequency power that can be transmitted along this line into a matching load.

2. Relevant equations
The impedance of a co axial cable is Z=Zo*ln(b/a)/n*2*pi where b is outer radius, a inner radius and n is sqrt(permittivity).

3. The attempt at a solution
So for the empty coaxial cable I had Z=Zo*ln(b/a)/2*pi

I rearranged this to work out the value of the outer radius which I got as 3.49mm.

I used the distance between inner and outer radius and the max electric field to work out potential difference. I then tried to the use the standard P=Vi formula to work out power but my answer is about 8 times too big. I assumed all the power is transmitted due to impedance matching.

2. Nov 25, 2014

### Staff: Mentor

Welcome to the PF.

The breakdown voltage is based on the peak voltage, while the power is based on the RMS voltage. Still, that should only give you a difference of √2, not 8. Can you post your work so we can check the numbers?