CRAC Class A (2025) — Questions 461–470

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.

Loss attenuates the forward signal so less power radiates (A), and on receive the loss is ahead of the receiver, lowering the SNR (B). Loss does not by itself force SWR > 1 (C) nor distort the signal (D).

Water raises the dielectric constant and loss, and corrodes the conductors, increasing loss (A) and eventually causing opens (B). The velocity factor can never exceed 1 (C), and SWR is always ≥ 1 (D) — both are physically impossible, so they are wrong.

UV degrades the outer jacket; once it cracks, water enters and ruins the cable (A). UV is not an RF source and does not cause mixing, intermodulation, or heating of the cable (B/C/D are false).

Air dielectric gives very low loss but, being open to air, must be sealed against moisture ingress (A). It is not band-limited to 30 MHz, not high-loss, and works below freezing (B/C/D are wrong).

Coax loss (especially dielectric loss) increases with frequency (A). Characteristic impedance is set by geometry and is essentially constant (C is wrong); reflected power and SWR depend on the load match, not on frequency alone (B, D are wrong).

Air-dielectric rigid coax has the lowest dielectric loss of these choices, so it has the lowest loss at VHF/UHF (A). Twisted pair and flexible coax have higher loss.

The Type N connector is designed for high frequencies (up to several GHz) with low loss and good impedance control, so it is preferred above 400 MHz (A). The M/RS-213/DB-23 choices are not the appropriate HF/VHF-UHF connectors for this purpose.