Unit 1 — Theory of Operation
TM-GEAR-007 — Open Handout TM Chapter: Chapter 2 ELOs: Understand the operating principle of the COMMON-MODE CHOKES — RFI SUPPRESSION ON FEEDLINES; identify key specifications Estimated time: 20 minutes
Step 1: Read the TM
Open TM-GEAR-007. Read Chapter 2 — Theory of Operation completely.
Then come back here.
Chapter 2 Content
2-1 Differential vs. Common Mode
In a coaxial feedline, the desired signal travels as a differential mode: equal and opposite currents on center conductor and inner surface of shield. The core sees equal and opposite H-fields; they cancel. The core presents zero impedance to this mode.
Common-mode current flows on the outside of the shield. The core sees an unbalanced H-field and presents the full choking impedance Z_choke (series impedance) in the common-mode path. Result:
CMR (dB) = 20 × log10(1 + Z_choke / Z_path) Z_path = impedance to which common-mode current flows (typically 50–300Ω)
For Z_choke = 5000Ω, Z_path = 100Ω: CMR = 34 dB For Z_choke = 1000Ω, Z_path = 100Ω: CMR = 21 dB 2-2 Core Material vs. Frequency Core material determines the frequency range of effective choking:
| Material | Best range | Peak μ" | Application |
|---|---|---|---|
| #31 | 1–300 MHz | 3000 | HF antenna feedpoints, coax chokes |
| #43 | 1–100 MHz | 750 | HF balun windings, TLTs |
| #61 | 10–300 MHz | 125 | VHF chokes, 6M and 2M feedpoints |
| #73 | 1–40 MHz | 2500 | HF snap-on beads, power line chokes |
| #75 | 0.5–10 MHz | 5000 | Audio and low-HF chokes |
Why Theory Matters
You cannot build or use RF gear correctly without understanding how it works. Theory tells you: - What the component does and how it produces that effect - What the sources of loss, distortion, or error are — so you can recognize and minimize them - What the valid operating range is — frequency, power, impedance — so you stay within specifications - How to interpret results or system behavior that doesn't match expectations
If a component doesn't perform as expected, theory is where you look first.
Self-Check Questions
SC1-1. In one sentence, state the operating principle of the COMMON-MODE CHOKES — RFI SUPPRESSION ON FEEDLINES as described in Chapter 2.
SC1-2. What does Chapter 2 identify as the primary source(s) of loss or degradation in performance?
SC1-3. What key specification(s) (frequency range, power rating, insertion loss, impedance ratio) does the TM state?
SC1-4. What does Chapter 2 say the COMMON-MODE CHOKES — RFI SUPPRESSION ON FEEDLINES cannot do — what are its limitations?
SC1-5. List two formulas or relationships from Chapter 2 that govern the component's behavior.
Answer Key
SC1-1. See TM §2-1. Compare your sentence to the first substantive paragraph of Chapter 2.
SC1-2. See Chapter 2. Look for language about loss mechanisms, parasitic effects, frequency limits, or power constraints.
SC1-3. See Chapter 2. Look for numbers with units: %, dB, Hz, Ω, W, V.
SC1-4. See Chapter 2 and Chapter 1. Limitations are often stated as frequency range, power handling, or impedance range.
SC1-5. See Chapter 2. Equations or proportionality statements are the relationships that govern the component.
Checkpoint
Before proceeding, state without looking: - The operating principle of the COMMON-MODE CHOKES — RFI SUPPRESSION ON FEEDLINES - The primary loss or degradation source(s) - At least one key specification with its value
→ Proceed to Unit 2