Unit 1 — Theory of Operation

TM-ANT-020 — Open Handout TM Chapter: Chapter 2 ELOs: Understand the operating principle of the END-FED QUARTER-WAVE (EFQW) ANTENNA; identify key electrical characteristics Estimated time: 20 minutes

Step 1: Read the TM

Open TM-ANT-020. Read Chapter 2 — Theory of Operation completely.

Then come back here.

Chapter 2 Content

2-1. END-FED MATCHING

Quarter-wave wire element fed at high-impedance end via 9:1 unun with short counterpoise. An end-fed wire presents a high impedance at its end: ~2500–5000 Ω for a half-wave wire; ~1000–2000 Ω for a quarter-wave wire. This high impedance is transformed down to 50 Ω by the matching transformer or unun. Design values: ~1000–2000 Ω → 50 Ω via 9:1 unun. The transformation ratio n² = Zwire/Zcoax; for a 49:1 unun, n = 7, so the wire impedance appears as 49×50 = 2450 Ω from the wire side.

2-2. RADIATION PATTERN

An end-fed wire radiates in a pattern that varies with length and frequency. A half-wave wire at 10 ft height produces a bidirectional broadside pattern similar to a dipole. At harmonic frequencies the pattern has multiple lobes and nulls. Gain per design: 0 dBi (vertical, omnidirectional). The pattern is affected by the run of the feed line if common-mode current is not controlled.

2-3. COUNTERPOISE REQUIREMENTS

A counterpoise (short wire or coax braid) is required as a reference conductor for the feed voltage to work against. Without a counterpoise the coax braid serves as the counterpoise, causing RF current on the braid. Recommended counterpoise: 0.05λ minimum length wire from the unun ground terminal, or a 1:1 choke isolating the antenna current from the coax outer.

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 END-FED QUARTER-WAVE (EFQW) ANTENNA as described in Chapter 2.

SC1-2. What determines the resonant frequency of the END-FED QUARTER-WAVE (EFQW) ANTENNA? Name the primary physical parameter(s).

SC1-3. What feedpoint impedance does Chapter 2 predict for the END-FED QUARTER-WAVE (EFQW) ANTENNA in free space? How does that change over real ground?

SC1-4. What radiation pattern does the END-FED QUARTER-WAVE (EFQW) 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 END-FED QUARTER-WAVE (EFQW) ANTENNA - What determines its resonant frequency - The expected feedpoint impedance

→ Proceed to Unit 2