4.3 Radiation Resistsance Flashcards
What is the equation for Radiation resistance?
R_rad = Average power radiated / < I^2 >
= (2pi/3) Z_0 (b/lambda)^2
How can you convert from the peak power to the average power?
Use I_RMS instead of I_0
What condition must be true for high efficiency of radiation resistance?
R_rad»_space; R_ohmic
What is an easy way to increase the radiation resistance?
Use a longer aerial/dipole
R_rad = = (2pi/3) Z_0 (b/lambda)^2
- increase b
What type of size of R_rad do Hertzian dipoles have?
Very low
- Can draw a large current
What is angular gain?
The angular distribution of radiated power
What is the equation for directivity?
Maximum radiated intensity / average radiated intensity
Or the same for the Poynting vector
What must we account for in the longer dipole antennae?
Retarded time as now b is no longer «_space;wavelength
What is the current at each end of the dipole anntenae?
0
I = I(0) cos(n pi / L)
Ends are at z = +L/2 and -L/2
What is a half wave dipole?
When the length of the dipole is wavelength / 2
How is the power radiated from a half wave dipole different to a Hertzian dipole?
Half wave dipole is slightly more focussed in the equitorial plane
- Two sideways infinities, but the Hertzian dipole is squished towards towards the z axis
Describe a radio aerial
A full height, half wave antenna
- Current fed at the mid point
How can half wave dipoles be made shorter?
Feed at the ground level using the ground as a conducting plane
- Half power and wavelength /4
How does short wave radio work?
Horizontal aerial
- Waves bounce off the ionosphere