Chapter 8 - Antennas

Question 1

Compare and contrast the Dipole and Monopole antennas.

Answer 1

1. Dipole Antenna:
   
   • Formed from an open-circuit transmission line with conductors bent so currents run in the same direction.
   • Resonant length is y/2 (half the wavelength).
   • Requires two conductors, with fields forming a toroidal (donut-shaped) radiation pattern.
2. Monopole Antenna:
   
   • Formed by replacing one conductor of a dipole with an earth plane or reflective surface.
   • Resonant length is y/4 (quarter the wavelength).
   • Uses the ground or a conductive surface as the second conductor, which effectively reduces the length by half compared to a dipole.

Question 2

Sketch both a monopole and dipole antenna. (Include V&I fields).

Question 3

Why is base tuner used?

Answer 3

To electrically alter the length of an antenna to match the desired transmission frequency and to match the antenna to its transmitter amplifier.

Question 4

At 300MHz what is the optimal length of a Monopole? – What about a Dipole at 200MHz?

Answer 4

Monopole -
λ = c/f
λ = 3x10^8 / 300x10^6 = 1m
L = λ/4. = L = 1/4 = 0.25m

Dipole -
λ = c/f
λ = 3x

Question 5

Which type of antenna would you use to propagate SHF? How does SHF propagate?

Question 6

If the monopole is 7.5m long what is its resonant frequency? (10 MHz)

Answer 6

Monopole - λ = L * 4

λ = 7.5 x 4 = 30m

f = c/λ

f = 3x10^8 / 30 = 10MHz