Unit 4 — Operating Procedures and Calculations
TM-GEAR-007 — Open Handout TM Chapters: Chapter 5, Appendix A ELOs: Install and operate the COMMON-MODE CHOKES — RFI SUPPRESSION ON FEEDLINES correctly; interpret performance data; compute derived quantities Estimated time: 30 minutes (includes 3–4 practice problems)
Step 1: Read the TM
Open TM-GEAR-007. Read Chapter 5 — Operating Procedures and Appendix A completely.
Then come back here.
Chapter 5 Content
A CMC is a passive device; there is no operational procedure other than installation. Install CMCs at:
- Antenna feedpoint (highest priority)
- Entry panel where coax enters the shack
- Any point where the feedline changes direction sharply
- Audio and control cables if RF on those cables is suspected
Signs of inadequate common-mode chocking: RF in the shack (microphone pickup, TVI), pattern distortion on receive, SWR that changes when hands are placed near the feedline.
Appendix A — Reference Formulas
| Passes | Z_choke at 14 MHz (Ω) | Note |
|---|---|---|
| 6 | ~1500 | Adequate; marginal for dipoles |
| 9 | ~3000 | Good for most HF antennas |
| 12 | ~5000 | Excellent; use for multiband or high-power |
| 15 | ~7000 | Maximum practical (coax fills toroid bore) |
Key Formulas Summary
Key mathematical relationships from Appendix A:
(See Appendix A in the TM)
Operating Notes
Work through all steps in Chapter 5 in sequence.
Installation and operating discipline: - Always verify polarity and orientation before making connections — RF transformers and baluns are phase-sensitive - Route feedlines away from parallel conductors — parallel runs create mutual coupling that degrades isolation - Ground all exposed metalwork at a single chassis point — multiple grounds create loops - Record settings, frequencies, and power levels for every test — you need baseline data for comparisons
Practice Problems
Work these before reading the answer key below.
P4-1. Using the operating procedure from Chapter 5 and the formulas from Appendix A: State the installation steps you would take to put the COMMON-MODE CHOKES — RFI SUPPRESSION ON FEEDLINES in service on a 40m (7.150 MHz) station. List steps in order.
P4-2. From Chapter 5: what installation or setup detail produces the best RF performance with the COMMON-MODE CHOKES — RFI SUPPRESSION ON FEEDLINES? What is the tradeoff if you omit or shortcut that step?
P4-3. Chapter 5 specifies an operating procedure for a specific use case. State the first three steps of that procedure from memory.
P4-4. Appendix A gives a formula for computing a result from measured values. Pick one formula and compute a worked example using made-up but realistic values. Show all work.
Answer Key — Practice Problems
P4-1. Compare your list to Chapter 5. Steps should include: select mounting location → connect to feedline/antenna → verify polarity/orientation → apply power or signal → verify operation → record baseline.
P4-2. See Chapter 5. The most important installation detail is usually physical orientation, lead length, or ground bonding — the tradeoff if omitted is degraded isolation, increased SWR, or common-mode current leakage.
P4-3. See Chapter 5, steps 1–3. Copy exactly then close the TM and state from memory.
P4-4. See Appendix A for the formula. Your arithmetic is correct if your result has the right units and is physically plausible.
Checkpoint
Before proceeding: - [ ] You can state the operating procedure from memory (at least the first 5 steps) - [ ] You can compute the derived quantity from Chapter 5 / Appendix A without looking - [ ] You understand what a degraded or unexpected result tells you about the installation
→ Proceed to Unit 5