1. Antennas
10/18/2017

2. Forward and Reflected Power
Feed line impedance should match load impedance
If it doesn’t, some power is reflected (reflected power)
The power traveling towards the load is called forward power
Reflected power and forward power interfere
They create a stationary wave pattern called a standing wave

3. SWR SWR Whichever of the following is greater than 1:
Load impedance to feed line impedance
Feed line impedance to load impedance
Lower is better
More efficient power transfer to antenna
Less is reflected
Power going through the feed line generates heat
Reflection can damage transmitter
1:1 is a perfect match

4. Polarization
Orientation relative to ground

5. Dipole antennas 2 conductors going out in either direction
Feed point in the center
Can move, but signal loss

6. Wi-Fi antennas are dipoles as shown by the omnidirectional radiation pattern.
As length of dipole increases, gain in peak direction (not all directions b/c Law of Conservation of Energy) increases up to about a dipole of 1 wavelength (when lobes in different directions become more numerous), but vertical coverage decreases.
Dipoles about ½ wavelength long are resonant.
Dipoles of different lengths are no longer resonant and have different feedpoint impedances, so a loading coil can provide impedance matching.

7. Monopole antennas Half a dipole above a ground plane
Ground plane can just be wires
Amount of ground plane needed inversely related with frequency

8. Yagi antennas Reflector(s) (larger than driven element) Driven element
Director(s) (smaller)
Horizontally polarized Yagis are often used for long-distance communication
Especially SSB and CW on VHF/UHF

10. Exam Questions

11. T9A03
Which of the following describes a simple dipole mounted so the conductor is parallel to the Earth's surface?
A ground wave antenna
A horizontally polarized antenna
A rhombic antenna
A vertically polarized antenna

12. T9A04
What is a disadvantage of the “rubber duck” antenna supplied with most handheld radio transceivers?
It does not transmit or receive as effectively as a full-sized antenna
It transmits a circularly polarized signal
If the rubber end cap is lost it will unravel very quickly
All of these choices are correct

13. T9A05
How would you change a dipole antenna to make it resonant on a higher frequency?
Lengthen it
Insert coils in series with radiating wires
Shorten it
Add capacitive loading to the ends of the radiating wires

14. T9A07
What is a good reason not to use a “rubber duck” antenna inside your car?
Signals can be significantly weaker than when it is outside of the vehicle
It might cause your radio to overheat
The SWR might decrease, decreasing the signal strength
All of these choices are correct

15. T9A08
What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz?
112 50 19 12

16. T9A09
What is the approximate length, in inches, of a 6 meter ½-wavelength wire dipole antenna? 6 50
112
236

17. T9A10
In which direction is the radiation strongest from a half-wave dipole antenna in free space?
Equally in all directions
Off the ends of the antenna
Broadside to the antenna
In the direction of the feed line

18. T3A03
What antenna polarization is normally used for long-distance weak-signal CW and SSB contacts using the VHF and UHF bands?
Right-hand circular
Left-hand circular
Horizontal
Vertical

19. T3A05
When using a directional antenna, how might your station be able to access a distant repeater if buildings or obstructions are blocking the direct line of sight path?
Change from vertical to horizontal polarization
Try to find a path that reflects signals to the repeater
Try the long path
Increase the antenna SWR

20. Sources ARRL Ham Radio License Manual from 2010 Dipoles:
Monopoles:
Yagi:
Exam questions: