Unit 1 — Theory of Operation

TM-ANT-073 — Open Handout TM Chapter: Chapter 2 ELOs: Understand the operating principle of the YAGI-UDA DIRECTIONAL ANTENNA; identify key electrical characteristics Estimated time: 20 minutes

Step 1: Read the TM

Open TM-ANT-073. Read Chapter 2 — Theory of Operation completely.

Then come back here.

Chapter 2 Content

2-1. PARASITIC COUPLING PHYSICS

Driven element with parasitic reflector and one or more directors providing forward gain; boom-mounted. Gain is achieved through parasitic coupling: the driver excites the reflector and director elements by near-field induction. The reflector (5% longer than the driven element) carries current that lags by approximately 160° and re-radiates energy forward. Directors (5% shorter) carry current leading by approximately 140° and focus energy forward. Each additional director adds approximately 1 dB of gain at optimal spacing (0.2–0.25λ).

2-2. ELEMENT DESIGN RULES

For a 3-element Yagi on VHF: driver = 0.473λ, reflector = 0.505λ, director = 0.440λ. Spacing: reflector to driver = 0.2λ, driver to director = 0.25λ. These values produce forward gain of 7–8 dBd with front-to-back ratio of 20–25 dB. Gain estimate: 8–12 dBi forward. At UHF, dimensional tolerance is critical — elements must be within ±1 mm of design length for proper pattern formation.

2-3. FEED IMPEDANCE AND MATCHING

Feed impedance: 50 Ω (via gamma match or T-match at driven element). The driven element impedance drops below 50 Ω when directors are added (typically 20–40 Ω). A gamma match, T-match, or delta match raises this to 50 Ω. The gamma match uses a parallel conductor tapped on the driven element to form an L-network; the shorting bar position and gamma rod length are adjusted for 50 Ω + j0 Ω at the design frequency.

Why Theory Matters for Antenna Construction

You cannot build a working antenna without understanding the underlying physics. Theory tells you: - What determines resonant frequency — and therefore how cutting or loading errors affect performance - What radiation pattern the antenna produces and why physical layout matters - What feedpoint impedance to expect — so you know whether a matching network is needed - What the sources of loss are: conductor resistance, ground losses, impedance mismatch

If the antenna doesn't resonate where expected, or SWR is high, theory is where you diagnose the cause.

Self-Check Questions

SC1-1. In one sentence, state the operating principle of the YAGI-UDA DIRECTIONAL ANTENNA as described in Chapter 2.

SC1-2. What determines the resonant frequency of the YAGI-UDA DIRECTIONAL ANTENNA? Name the primary physical parameter(s).

SC1-3. What feedpoint impedance does Chapter 2 predict for the YAGI-UDA DIRECTIONAL ANTENNA in free space? How does that change over real ground?

SC1-4. What radiation pattern does the YAGI-UDA DIRECTIONAL ANTENNA produce? What are the nulls and maxima directions?

SC1-5. List two formulas or relationships from Chapter 2 that govern the antenna's electrical 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. For most antennas the primary parameter is physical length relative to wavelength. Loading (coils, capacitors) shifts this.

SC1-3. See Chapter 2. Free-space feedpoint impedance is a theoretical value; ground proximity, height, and nearby conductors modify it significantly.

SC1-4. See Chapter 2. Directional patterns are usually shown in terms of azimuth and elevation radiation patterns.

SC1-5. See Chapter 2 and Appendix A. The key equation usually relates length to frequency, or impedance to element geometry.

Checkpoint

Before proceeding, state without looking: - The operating principle of the YAGI-UDA DIRECTIONAL ANTENNA - What determines its resonant frequency - The expected feedpoint impedance

→ Proceed to Unit 2