Unit 1 — Theory of Operation

TM-ANT-009 — Open Handout TM Chapter: Chapter 2 ELOs: Understand the operating principle of the ACTIVE RECEIVING ANTENNA; identify key electrical characteristics Estimated time: 20 minutes

Step 1: Read the TM

Open TM-ANT-009. Read Chapter 2 — Theory of Operation completely.

Then come back here.

Chapter 2 Content

2-1. DIRECTIVITY AND NOISE REJECTION

Small loop or whip with integrated low-noise amplifier (lna), broadband 50 ω output. Receive-only antennas exploit directivity (front-to-back ratio) and aperture to separate signals from noise. A terminated long-wire or loop antenna achieves a cardioid or kidney-shaped pattern with deep null in one direction: F/B ratio typically 15–25 dB. This null can be steered toward interference sources (power-line noise, broadcast QRM) to dramatically improve signal-to-noise ratio on weak HF signals.

2-2. NOISE FIGURE AND SNR

At HF below 30 MHz, external noise (atmospheric, man-made) dominates over receiver noise figure. A receiving antenna with poor efficiency but good directivity can outperform a high-gain antenna pointed at a noise source. The key metric is signal-to-noise ratio (SNR) improvement over the reference antenna (typically a 40M dipole), not absolute signal level. An SNR improvement of 10–20 dB (1–2 S-units) on a target station makes marginal copy copy into solid copy.

2-3. TERMINATION RESISTANCE

Most directional receive antennas use a termination resistor (typically 500–900 Ω) to absorb backward-traveling waves and prevent re-radiation from the far end. This resistance determines the F/B ratio and the wave velocity factor along the antenna. LNA noise figure 0.8 dB, gain 20 dB typical, IP3 > +15 dBm, powered via bias-T. The termination must be a non-inductive resistor (carbon composition or metal film; not wirewound) mounted in a weatherproof housing.

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 ACTIVE RECEIVING ANTENNA as described in Chapter 2.

SC1-2. What determines the resonant frequency of the ACTIVE RECEIVING ANTENNA? Name the primary physical parameter(s).

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

SC1-4. What radiation pattern does the ACTIVE RECEIVING 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 ACTIVE RECEIVING ANTENNA - What determines its resonant frequency - The expected feedpoint impedance

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