CRAC Class C (2025) — Questions 571–580

An SWR of 4:1 simply indicates poor overall matching as seen at the transmitter port (A). The reading by itself does not determine a unique impedance, nor does it state efficiency (C) or gain (D). (Note: with a known 50 Ω reference a 4:1 SWR could correspond to 200 Ω or 12.5 Ω, but the question's answer key marks only A as the correct statement.)

SWR is the ratio of the maximum to minimum envelope of the standing wave along the line — equally valid for voltage (A) or current (B), since the standing-wave ratio is the same for both. C and D are false: standing waves increase peak voltage/current and raise losses; they do not reduce them.

A high SWR makes the power amplifier see a bad load, which can overheat and damage the final power semiconductors, so reducing power protects them (A). SWR exists on the feed line, not "inside the radiated wave" (D), and fuses/line limits are not the reason (B, C).

Lower SWR means less reflected power and therefore less additional loss in the feed line, improving transfer efficiency (A). Option B concerns common-mode currents (a separate issue addressed by a balun/chokes), and C/D are unrelated to SWR.

Intermittent or poor connections make the impedance vary, so the SWR reading jumps around (A). Phase modulation, over- modulation, or simply high cable temperature do not by themselves make the SWR reading unstable.

An SWR meter samples the RF envelope amplitude; SSB voice has a constantly varying envelope, so the reading fluctuates even with a healthy system. Measuring on a steady carrier (CW) gives a stable value (A). B/C/D misattribute the fluctuation to the antenna, over-modulation, or a fault.

Standing waves are only observable when the line length is a significant fraction of a wavelength. Mains is 50/60 Hz (wavelength of thousands of km), so household wiring is tiny compared to λ → no visible SWR (A). The underlying physics is the same; only the scale differs, so B/C/D are wrong.

Amateur transmitters, receivers, and antennas are designed around 50 Ω coax (A). 75 Ω is CATV, 300 Ω is twin-lead (not coax), and 93 Ω is uncommon for our use.

Coax is unbalanced and shielded, so it is easy to route and has low coupling to nearby objects (A). It is not the lowest-loss (air- dielectric line wins), nor necessarily the highest-power or cheapest (B/C/D are wrong).

The key RF selection parameters are characteristic impedance (must match 50 Ω for a good match) and attenuation per unit length at your operating frequency (A, B). Center-conductor cross-section/current rating and dielectric voltage/temperature (C, D) matter for DC power wiring but are not the primary criteria for choosing an RF feed line.