Unit 1 — Theory of Operation

TM-ANT-056 — Open Handout TM Chapter: Chapter 2 ELOs: Understand the operating principle of the SHUNT-FED VERTICAL ANTENNA; identify key electrical characteristics Estimated time: 20 minutes

Step 1: Read the TM

Open TM-ANT-056. Read Chapter 2 — Theory of Operation completely.

Then come back here.

Chapter 2 Content

2-1. RADIATION PHYSICS

Grounded vertical radiator (tower or monopole) fed by a shunt wire tapped at a point where impedance equals 50 ω. A vertical radiator carries current that is vertically polarized; its electromagnetic wave radiates with E-field vertical. The ground (or radial counterpoise) serves as the electrical mirror-image of the above-ground element: a λ/4 vertical above a perfect ground is equivalent to a λ/2 dipole in free space, with radiation resistance Rr = 36.6 Ω.

2-2. RADIATION PATTERN

A quarter-wave vertical over an extensive ground plane radiates omnidirectionally in azimuth with a low-angle elevation lobe ideal for DX. The elevation angle of maximum radiation (θmax) depends on ground conductivity and number of radials: over a perfect ground θmax ≈ 0°; over average ground with 32+ radials, θmax ≈ 5–15°. The vertically polarized wave follows the earth’s surface better than a horizontal wave at low angles.

2-3. IMPEDANCE AND BANDWIDTH

Feed impedance: 50 Ω at tap point. The SWR bandwidth of a simple λ/4 vertical at HF is approximately 5–8% of center frequency for 2:1 SWR, typically covering one amateur band. Loading coils reduce bandwidth in proportion to their Q; top loading preserves more bandwidth than base loading because it maintains higher current along more of the radiator length.

2-4. GROUND SYSTEM DESIGN

Ground loss resistance Rg appears directly in series with the radiation resistance. For maximum efficiency: Rg < Rr. With buried radials, Rg decreases as Nradials increases: 32 radials each 0.25λ gives Rg ≈ 3 Ω. Elevated resonant radials achieve similar performance with only 4–8 radials. For tapped shunt wire spaced 0.1–0.2λ from tower — follow this requirement closely for efficiency.

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 SHUNT-FED VERTICAL ANTENNA as described in Chapter 2.

SC1-2. What determines the resonant frequency of the SHUNT-FED VERTICAL ANTENNA? Name the primary physical parameter(s).

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

SC1-4. What radiation pattern does the SHUNT-FED VERTICAL 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 SHUNT-FED VERTICAL ANTENNA - What determines its resonant frequency - The expected feedpoint impedance

→ Proceed to Unit 2