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:

Lesson Plan:

  1. Go over any questions from the homework from Chapter 6.

  2. 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!

  3. 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.

  4. SAFETY NOTE: Remind the students that one can be easily burned doing soldering.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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:


  11. 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.

  12. Show different types of baluns. Take a 300 ohm to 75 ohm TV balun apart to show that it really is small transformer.

  13. 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:

  1. Read Chapter 7

  2. Do the Chapter 7 Quiz

  3. Read Sections A1.6 and A1.7 in Appendix 1.

  4. 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

  1. Slide 1
    7.1 Equivalent Circuit of a Cable
    7.2 Types of Cables

  2. Slide 2
    7.3 Construction of a Coaxial Cable

  3. Slide 3
    7.4 Toroid Balun
    7.5 Coaxial Cable Stub Balun