Chapter 7:
Transmission Lines
Objectives:
When the students have finished this section they should be
familiar with the characteristics of the different types of transmission
lines and the connectors used in Amateur Radio, and how to use them.
Apparatus:
- Samples of balanced transmission lines
(open wire
feed, TV twin lead, ladder line)
- Samples of unbalanced transmission lines
(RG58/U, RG8/U, RG213/U, heliax)
- Coax connectors
(PL259 with and without a reducer, SO239, type N, barrel connectors,
adapters that change from type of connector to another)
- Soldering irons, solder
- Crimping tool
- Baluns (300 ohm to 75 ohm TV balun at least)
- SWR meter suitable for VHF
Lesson Plan:
- Go over any questions from the homework from Chapter 6.
- A large part of this chapter should be show and tell: handing
around samples of the various types of transmission lines in common use
in Amateur Radio. The properties of each are well documented in the
text. Point out that dealing with heliax is beyond the scope of the
course but if they become serious about operating in the UHF bands then
they will probably become more acquainted with it!
- Next, pass around samples of the various connectors and
show how they are used joining coax to a chassis, a rubber ducky to
an HT, and so on. Let them play with these. Pictures of various connector types.
SAFETY NOTE: Remind the students that one can
be easily burned doing soldering.
- The next chore is to teach the class how to install a PL259 on
RG58/U and RG8/U. This involves teaching the basics of soldering. Crimp
or twist-on connectors are not really recommended.
- Explain that the final product should be checked
with a VOM to ensure that the cable is not shorted in any
way and that all solder joints are conductive.
- If you intend to have the class construct a simple vertical or
J-pole antenna when you get to Chapter 8 then ensure that the coax
length is appropriate for this project.
- Explain that it is crucial for cables mounted outside in the
weather that no water be allowed to enter the cable. Type 'N'
connectors may be used for a waterproof joint. Show how butyl
rubber electrical tape can be used to seal around a standard UHF
plug. The photo shows the butyl rubber tape partially
wound around the plug. Notice that the tape is moulded to fit
the cable and plug shell tightly and handles more like
plasticine than electrical tape.
- Make the point that quality coaxial cable is important to the
proper operation of a station, especially at VHF and UHF frequencies,
and for low received noise levels. If you have any contaminated coaxial
cable (old RG8/U cable usually shows this effect) show how the copper
shield of this cable is black and oily where the shield of newer types
(such as RG213/U) does not contaminate and will remain bright for
years.
- Explain that while many connector types are available, not all are
appropriate for Amateur Radio use, and some are for special purposes.
The photograph,
shows some connector types:
- Type SMA will handle up to 10 W at 10 GHz. The sample is installed
on a miniature hardline cable
- Type BNC is commonly available. It is for indoor use at low power
only and may be noisy at VHF frequencies and up. Maximum usable
frequency is about 220 MHz.
- Type F is a TV receiving type connector, noisy and unreliable. It is
not recommended for Amateur Radio uses.
- RCA Phono Jack is surprisingly reliable for frequencies up to 450
MHz, small, and easily obtained. It is the connector of choice for low
power (signal) circuits inside projects. Not recommended for power
circuits. For frequencies above 450 MHz, use SMA.
- Putting a BNC connector on a cable is a bigger
challenge. Perhaps the easiest way is by crimping if you can get your
hands on the right tools and if the resulting connector is to be used
exclusively indoors.
- Show different types of baluns. Take a 300
ohm to 75 ohm TV balun apart to show that it really is small
transformer.
- Demonstrate the SWR meter. An easy demo here is an SWR meter in
line from a 2 metre HT to a simple antenna. Explain that you will come
back to the SWR meter when you do Chapter 11.
If you can get some of your fellow Amateurs to turn out for this
class then the task is much easier to accomplish.
Homework:
- Read Chapter 7
- Do the Chapter 7 Quiz
- Read Sections A1.6 and A1.7 in Appendix 1.
- Skim Chapter 8
Chapter 7 Quiz:
Printable copy of the Review
Quiz
Answers to Chapter 7 Quiz
1 |
C |
14 |
A |
27 |
B |
40 |
B |
53 |
C |
2 |
D |
15 |
C |
28 |
B |
41 |
D |
54 |
C |
3 |
A |
16 |
D |
29 |
A |
42 |
A |
55 |
A |
4 |
A |
17 |
D |
30 |
D |
43 |
A |
56 |
A |
5 |
C |
18 |
C |
31 |
B |
44 |
A |
57 |
B |
6 |
C |
19 |
C |
32 |
B |
45 |
C |
58 |
C |
7 |
A |
20 |
B |
33 |
A |
46 |
C |
59 |
D |
8 |
A |
21 |
D |
34 |
B |
47 |
B |
60 |
A |
9 |
B |
22 |
A |
35 |
B |
48 |
D |
61 |
A |
10 |
D |
23 |
D |
36 |
A |
49 |
B |
62 |
D |
11 |
C |
24 |
A |
37 |
B |
50 |
D |
63 |
A |
12 |
A |
25 |
A |
38 |
D |
51 |
B |
* |
* |
13 |
A |
26 |
B |
39 |
D |
52 |
D |
* |
* |
Overhead Slide Versions of the Diagrams
- Slide
1
7.1 Equivalent Circuit of a Cable
7.2 Types of Cables
- Slide
2
7.3 Construction of a Coaxial Cable
- Slide
3
7.4 Toroid Balun
7.5 Coaxial Cable Stub Balun