4.3 Radiation Resistsance

Question 1

What is the equation for Radiation resistance?

Answer 1

R_rad = Average power radiated / < I^2 >

= (2pi/3) Z_0 (b/lambda)^2

Question 2

How can you convert from the peak power to the average power?

Answer 2

Use I_RMS instead of I_0

Question 3

What condition must be true for high efficiency of radiation resistance?

Answer 3

R_rad&raquo_space; R_ohmic

Question 4

What is an easy way to increase the radiation resistance?

Answer 4

Use a longer aerial/dipole
R_rad = = (2pi/3) Z_0 (b/lambda)^2
- increase b

Question 5

What type of size of R_rad do Hertzian dipoles have?

Answer 5

Very low

- Can draw a large current

Question 6

What is angular gain?

Answer 6

The angular distribution of radiated power

Question 7

What is the equation for directivity?

Answer 7

Maximum radiated intensity / average radiated intensity

Or the same for the Poynting vector

Question 8

What must we account for in the longer dipole antennae?

Answer 8

Retarded time as now b is no longer &laquo_space;wavelength

Question 9

What is the current at each end of the dipole anntenae?

Answer 9

0
I = I(0) cos(n pi / L)
Ends are at z = +L/2 and -L/2

Question 10

What is a half wave dipole?

Answer 10

When the length of the dipole is wavelength / 2

Question 11

How is the power radiated from a half wave dipole different to a Hertzian dipole?

Answer 11

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

Question 12

Describe a radio aerial

Answer 12

A full height, half wave antenna

- Current fed at the mid point

Question 13

How can half wave dipoles be made shorter?

Answer 13

Feed at the ground level using the ground as a conducting plane
- Half power and wavelength /4

Question 14

How does short wave radio work?

Answer 14

Horizontal aerial

- Waves bounce off the ionosphere