4.3 Radiation Resistsance Flashcards

1
Q

What is the equation for Radiation resistance?

A

R_rad = Average power radiated / < I^2 >

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

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2
Q

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

A

Use I_RMS instead of I_0

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3
Q

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

A

R_rad&raquo_space; R_ohmic

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4
Q

What is an easy way to increase the radiation resistance?

A

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

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5
Q

What type of size of R_rad do Hertzian dipoles have?

A

Very low

- Can draw a large current

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6
Q

What is angular gain?

A

The angular distribution of radiated power

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7
Q

What is the equation for directivity?

A

Maximum radiated intensity / average radiated intensity

Or the same for the Poynting vector

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8
Q

What must we account for in the longer dipole antennae?

A

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

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9
Q

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

A

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

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10
Q

What is a half wave dipole?

A

When the length of the dipole is wavelength / 2

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11
Q

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

A

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

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12
Q

Describe a radio aerial

A

A full height, half wave antenna

- Current fed at the mid point

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13
Q

How can half wave dipoles be made shorter?

A

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

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14
Q

How does short wave radio work?

A

Horizontal aerial

- Waves bounce off the ionosphere

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