Lesson 8:
Antennas
Objectives:
When the students have finished this chapter they should be able to:
- describe the features of the most common antennas used in Amateur
Radio
- calculate the length of radiator required
for different types of antennas
- use simple antenna formulas
Apparatus:
- Samples or photographs of various type of antennas
(dipoles,
verticals, Yagi-Udas)
- Various types of baluns
- Insulators
- The run of RG58 with a coax connector at one end from Chapter
7
- 300 Ohm twin-lead
- Soldering irons and solder
- Measuring tape
- Antenna tuner
- SWR Bridge or Antenna Analyser
- Dummy Load
Handouts:
- Sample antenna patterns for various classes of
antennas:
These antenna patterns were produced using EZ-NEC, an
antenna design programme for Amateur Radio operators. Note: the
trap yagi plot uses different reference coordinates (2.15 dbi
reference and real ground) and is rotated 90°
counterclockwise.
- Plans for making a J-pole antenna based on 300 Ohm twin
lead.
Below are URL's for four sites that have plans - numbers 1 and 2 are
for the model based on 300 Ohm TV twin-lead. If you want to be really
ambitious then you could have them build a more permanent version using
copper pipe. URL's 3 and 4 use copper pipe. However to build these
you will have to teach your class how to solder using a gas-fired torch;
not recommended if you are located in a school! A search using "J-pole
antenna" will bring up lots of other possibilities.
- http://cgi.fiu.edu/orgs/w4ehw/j-pole-plans.html
- http://hamgate1.sunyerie.edu/races/antenna/pocket.htm
- http://www.qsl.net/wb8erj/146jpole.htm
-
http://www.packetradio.com/jpol2.htm
Lesson Plan:
- Check homework from Chapter 7.
- This is perhaps the most appropriate point in the course to deal
with the concept of decibels.
- Break the class up into small groups, ensuring that each group has
at one least person competent in terms of math skills
- Have them work through Chapter 8 Quiz questions 80 through 90.
Sections A1.6 and A1.7 in Appendix 1 in the RACSG will be
required
You may well have to do each of these on the board to
demonstrate how to get the right answer.
- Show a sample of different kinds of dipoles explaining how
to do the necessary calculations to determine the length of radiator.
The exam requires an understanding of how to use the formulae for
frequencies above and below 30 MHz. Refer the class to Appendix II of
the RACSG. Chapter 8 Quiz Questions 9, 10, 11, 22 fit
here.
- Show and explain the uses and reasons for 'accessories' for a
dipole: traps, baluns, insulators. A coil of RG213/U coaxial cable of about 6 - 12 turns,
30 cm in diameter forms an efficient, waterproof, balun for a Yagi-type
beam antenna.
- Show them a beam antenna. Dragging a Yagi for 20 metres into
the classroom is not practical but you can illustrate the topic with a
Yagi for 2 metres, or in a pinch, a simple TV receiving antenna.
Pictures of HF beams are useful; better still would be a video of a crew
installing a yagi atop a tower.
It would be a good idea here to
emphasize just how large a 20 M full size Yagi can be on the
ground!
Chapter 8 Quiz Questions 55, 56, 57, 65, 66, 67, 68, 75
require calculations.
- If the yagi is for the 2 M or 440 MHz bands, connect it to your HT
and using a fairly distant repeater and the HT's signal strength meter,
demonstrate directivity in the antenna. Emphasize that the "Antenna
Gain" comes from a corresponding antenna loss at a different azimuth
angle.
- Show the various types of vertical antennas commonly encountered –
on VHF and UHF: ¼ wave, 5/8th wave and 'rubber duck' antennas and
some variants such as the J-pole and Skypole.
Discuss the merits
of a mag mount as opposed to drilling a hole in the roof of one's
vehicle for a permanent mount. Chapter 8 Quiz Questions 26, 27, 28
require calculations.
- If members of your club have
built some unusual antennas by all means invite them along to the class
for a show and tell.
- Have the class construct a portable J-pole antenna for 2 metres,
using the run of coax with a connector at one end that they prepared in
Chapter 7 and some 300 Ohm TV twin-lead. This should take about 20 to
40 minutes of class time. See the Handout section of this lesson
for some sources of plans.
- Be sure to explain the reason for and use of dummy loads. If you can
secure the necessary high-wattage resistors and an empty paint tin,
building a dummy load is another class project to consider. You would
also require some chassis-mount SO-239 connectors. Discuss why
the paint tin must be filled with transformer oil.
- Demonstrate the use of an SWR meter to determine the SWR of an
antenna. Demonstrate how a high SWR value can be reduced by using an
antenna tuner. If you have access to the new breed of antenna analyzers
you can demonstrate how an antenna can be easily checked.
If
you have an antenna analyser, you can demonstrate how the SWR
varies as the load impedance varies by using the antenna tuner
to set up any SWR you want. Measure the SWR and then take a look at the
load resistance and reactance with the Analyser. This
reinforces the information on complex impedances from Lesson 4
Homework:
- Read Chapter 8
- Do Chapter 8 Quiz Questions
- Skim Chapter 9
Lesson 8 Quiz:
Printable copy of the Review
Quiz
Answers to Chapter 8 Quiz
1 |
A |
19 |
A |
37 |
A |
55 |
D |
73 |
B |
2 |
B |
20 |
A |
38 |
A |
56 |
B |
74 |
A |
3 |
B |
21 |
C |
39 |
A |
57 |
B |
75 |
D |
4 |
C |
22 |
A |
40 |
B |
58 |
D |
76 |
D |
5 |
B |
23 |
A |
41 |
D |
59 |
A |
77 |
C |
6 |
A |
24 |
B |
42 |
C |
60 |
D |
78 |
C |
7 |
D |
25 |
D |
43 |
C |
61 |
B |
79 |
C |
8 |
B |
26 |
C |
44 |
B |
62 |
A |
80 |
B |
9 |
C |
27 |
B |
45 |
B |
63 |
D |
81 |
B |
10 |
B |
28 |
A |
46 |
B |
64 |
B |
82 |
D |
11 |
C |
29 |
C |
47 |
D |
65 |
C |
83 |
C |
12 |
D |
30 |
C |
48 |
C |
66 |
A |
84 |
C |
13 |
D |
31 |
A |
49 |
B |
67 |
B |
85 |
B |
14 |
D |
32 |
D |
50 |
B |
68 |
B |
86 |
B |
15 |
A |
33 |
A |
51 |
C |
69 |
A |
87 |
A |
16 |
A |
34 |
B |
52 |
B |
70 |
C |
88 |
D |
17 |
D |
35 |
C |
53 |
B |
71 |
B |
89 |
D |
18 |
D |
36 |
B |
54 |
D |
72 |
B |
90 |
A |
Overhead Slide Versions of the Diagrams
- Slide 1
8.1
Current and Voltage Distribution in a Resonant Antenna
8.2
Distribution in a 3/2 Wavelength Antenna
- Slide
2 (Antenna Patterns)
8.3 Radiation Pattern
in a Simple Antenna
8.4 Radiation Pattern in a Full-Wave
Wire
- Slide
3 (Vertical Radiation Patterns of Horizontal
Antennas)
8.5 Effect of Image on Vertical Pattern
8.6 Vertical
Patterns for Various Heights Above Ground
- Slide
4 (Simple Antenna Types)
8.7 Dipole
8.8
Inverted Vee
8.9 Trap
8.10 Folded Dipole
- Slide
5 (Trap Antennas)
8.11 Basic Trap Antenna -
Details
8.12 Trap Construction
- Slide
6
8.13 Tuner Position for Long Wire Antenna
- Slide
7 (Directive Antennas)
8.14 Yagi-Uda
8.15
Cubical Quad
- Slide
8 (Vertical Antennas)
8.16 Vertical
Elevation Over Practical Ground
8.17 Providing a Ground
Reference
8.18 Base Loading
8.19 Top Loading
- Slide
9 (Miscellaneous Antennas)
8.20 Helical Beam
Antenna
8.21 Parabolic Beam Antenna