Antenna Types

Question 1

Answer 1

Dipole Antenna

Question 2

Answer 2

Half-wave dipole

Question 3

Why is the actual length of a half wave dipole slightly shorter than half?

Answer 3

With the nominal length, there will be an inductive reactance. We want to match the impedence between the antenna and the transmission line so we cancel the inductive reactance by shortening the antenna.

Question 4

Antenna Impedence is largely comprised of the real resistance and the reactance. Where is the reactance energy stored?

Answer 4

Stored in the near field of the antenna.

Question 5

What is antenna impedence?

Answer 5

Opposes flow to AC. Comprised of Reactance and Resistance.

Question 6

Why is impedence important to antennas?

Answer 6

Impedence between the antenna and the transmission line must be matched for maximizing power transfer and minimizing signal loss.

Question 7

What is reactance?

Answer 7

Represents the imaginary part of impedence.

Reactance represents the storage and release electromagnetic waves due to inductive and capactivie effects

Question 8

What is Capacitive Reactance?

Answer 8

Capacitors will oppose the flow of AC current by storing and discharging energy in sync with the alternating voltages.

Inversely proportional to the frequency.

Fascilitates transfer of energy between electrical current and electric field.

Question 9

What is Inductive Reactance?

Answer 9

When AC flows through an inductor, the changing magnetic field around the inductor induces a current which opposes the change in applied current, thereby opposing the flow of AC current.

Directly proportional to the frequency.

Fascilitates transfer of energy between electrical current and magnetic field.

Question 10

In a half-wave dipole, what is the direction of polarization wrt to the wire orientation.

Answer 10

Polarization follows the wire orientation.

Question 11

Answer 11

Broadside Array.

Bi-directional above and below the plane of the antenna.

Question 12

Answer 12

End-fire Array.

There is a progressive 90 degree phase shift between elements.

This creates a unidirectional beam pattern in the plane of the array.

Question 13

Answer 13

Yagi-Uda Antenna.

Parasitic elements reduce the radiation resistance of the driven element.

Question 14

Answer 14

Folded Dipole

Question 15

Answer 15

Phased Array

Question 16

How does a Monopole (or Marconi) antenna work?

Answer 16

The antenna is mounted perpendicular to a highly ocnducting surface. The reflection on the surface produces the other half of a virtual dipole.

Question 17

What are three ways to direct the beam of a phased array?

Answer 17

Phase Delay
Element Amplitudes
Element Spacing

Question 18

“The time difference between when each element must be activated is very small. However, the same effect can be achieved by adjusting the phase of the singals applied to each element.”

Answer 18

Phase Delay

Question 19

Adjusting the amplitudes of the signals applied to each element.

Answer 19

Amplitude Adjustment

Question 20

Changing the element spacing to alter the interference pattern and phase relationship.

Answer 20

Element Spacing

Question 21

What are two types of parasitic elements?

Answer 21

Reflector
Director

Question 22

What is a reflector?

Answer 22

A parasitic element placed behind the driven element to enhance directivity in the forward direction.

Question 23

What is a Director parasitic element?

Answer 23

Placed in front of the driven element, difracting the EM waves to enhance directivity.

Question 24

What is an advantage of a folded dipole over another dipole?

Answer 24

Higher radiation resistance

Reactance vaires less with frequency

Question 25

Define Radiation Resistance

Answer 25

How efficiently the antenna converts electrical energy into radiated EM energy.

Question 26

Do we want Radiation Resistance to be high or low?

Answer 26

High -> more current is converted to radiation

Question 27

A folded dipole has a longer current path than other dipoles, what effects does this have?

Answer 27

Uniform current distribution -> Reduce loss due to Skin Effect and Proximity Effect -> higher Radiation Resistance

Reduced Current Density -> mimizes inductive effects -> reduced inductive reactance -> reactance vaires less as frequency is shifted from resonance -> greater Bandwidth.

Question 28

Draw the Voltage Distribution and Current Distribution on a a halfwave dipole.

Answer 28