CRAC Class C (2025) — Questions 631–640

A horizontal dipole's maximum radiation is broadside (perpendicular) to the wire. North–south orientation puts the broadside lobes toward east–west — exactly where the correspondent is. The standard broadside-maximum case is the half-wave dipole, i.e., each arm = λ/4 (A). Each arm = λ/2 (B) would make a full-wave dipole with end-fire/null-broadside pattern, poor toward east–west. The reference answer marks A.

The launch angle of a horizontal antenna is set by the interference pattern between the direct wave and its image reflected from the ground; the key variable is height above ground expressed in wavelengths (A). Element pointing direction (B), boom direction (C), and element length (D) affect azimuth/pattern/gain, not primarily the elevation angle.

Long-distance (DX) contacts rely on one or more ionospheric hops, which need a low launch angle to reach far; short- distance (near-vertical-incidence, NVIS) contacts need a high launch angle to return to ground nearby (A). B is reversed; C/D are not the governing principles.

Since launch angle depends on height in wavelengths, a greater absolute height (more wavelengths) yields a lower angle suited to DX, while a lower height gives a higher angle for nearby work (A). B is reversed; C/D are not the primary planning rule (though higher frequencies need more wavelengths to reach the same angle, the stated principle links height to distance).

The optimal launch angle for a point-to-point DX path is found geometrically from the ionosphere's effective height, the path distance, and the number of hops — a simple triangle/geometry computation (A). Power (D), sunspot number (B), or ground conductivity/obstacles (C) are secondary or irrelevant to the basic angle geometry.

Ground conductivity affects the image (mirror) effect of the earth, which acts as a reflecting ground plane for the antenna. Higher ground conductivity gives a stronger, more ideal image current, improving low-angle radiation and hence long- distance (DX) HF performance — especially on the launch of the skywave (天波). Option B is wrong because lower conductivity weakens the image and worsens performance; C is wrong because ground conductivity at the launch point does affect skywave; D misstates the optimum.

When a horizontal dipole is brought very close to ground (1/10 λ), the antenna is no longer in free space; its image (mirror) current strongly couples to it, and the radiation resistance drops substantially. Thus the feed-point impedance (the resistive part) decreases. Options B, C and D are incorrect.

The radiation elevation angle (辐射仰角) is set by the phase relationship between the antenna and its ground image, which depends on height measured in wavelengths (height-to- wavelength ratio), not on absolute height. Option B (absolute height ignoring wavelength) and C (height above sea level) are therefore wrong; D confuses physical tilt with the elevation- angle mechanism.

An inductively loaded (加感) antenna can be tuned with a coil to resonance and to the same feed-point impedance as a full- size antenna, so B is too absolute. But the coil adds loss resistance and makes the current distribution less uniform, so radiation efficiency is always lower than a full-size element — C and D (equal or better than full-size) are false. A states both facts correctly.

This is a trapped triband dipole: each added inner segment forms a 1/4-wavelength (plus the trap) section at the next lower band, so the cumulative lengths are A = 1/4 λ at 28 MHz, A+B = 1/4 λ at 21 MHz, A+B+C = 1/4 λ at 14 MHz. Option B (half-wave) and C/D (each segment its own band) are wrong.