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T9B

ANTENNAS AND FEED LINES

Feed lines: types, attenuation vs frequency, selecting; SWR concepts; Antenna tuners (couplers); RF Connectors: selecting, weather protection

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T9B011 of 12

Which of the following connectors should be carefully taped for weather protection when used outdoors?

Why No common coaxial connector used by hams is truly waterproof in field service, so any outdoor connection should be sealed with tape or self-amalgamating tape and weatherproof sealant. Water that wicks into a connector soaks the coax dielectric and braid, raising loss and SWR and eventually ruining the feed line. The UHF (PL-259), BNC and Type N families all need this treatment, so sealing every outdoor connector is the right practice.
Watch out Type N is often called weather resistant because of its gasket, which tempts people to leave it bare, but it is only resistant, not sealed, and still needs taping when exposed.
If it lives outside, wrap it. No ham coax connector is waterproof by itself.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T9B022 of 12

What is the most common impedance of coaxial cables used in amateur radio?

Why Amateur transmitters, transceivers, SWR meters, amplifiers and most antennas are all designed around a 50 ohm system, so the standard coax for ham use is 50 ohm cable such as RG-58, RG-8X, RG-8 and LMR-400. Matching the feed line impedance to the equipment keeps SWR low and power transfer efficient. The other common coax impedance in the world, 75 ohms, is the television and video standard, not the ham standard.
Watch out 600 ohms is roughly the characteristic impedance of open-wire or ladder line, a balanced feed line, not coax. The very low values like 8 or 12 ohms are not standard transmission line impedances at all.
Ham radio runs on 50: 50 ohm rigs, 50 ohm coax. 75 ohms is for TV, 600 ohms is ladder line.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T9B033 of 12

Why is coaxial cable the most common feed line for amateur radio antenna systems?

Why Coax wins on convenience, not performance. Because the outer shield confines the RF field inside the cable, coax can be run alongside metal, through walls, taped to a tower leg, buried, or coiled up without upsetting its impedance or radiating, and standard connectors like PL-259 and Type N make hookup quick. Balanced open-wire line needs spacing from conductors, careful routing and usually a balun, so most hams accept coax's higher loss for the easy installation.
Watch out The low-loss claim is the trap: at the same impedance and size, open-wire or ladder line normally has less loss than coax (and is usually cheaper too), while large hardline or open-wire handles more power.
Coax is chosen for convenience, not for being lowest loss, highest power or cheapest.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T9B044 of 12

What is the major function of an antenna tuner (antenna coupler)?

Why An antenna tuner (more accurately an antenna coupler or matching network) is an adjustable network of inductors and capacitors that transforms whatever impedance the feed line presents into the 50 ohms the transmitter wants to see. It does not change the SWR out on the antenna itself; it just presents a friendly load at the radio end so the transmitter's protection circuits do not fold back the power. Modern solid-state finals need a close match to 50 ohms to deliver full output, which is why tuners are so common.
Watch out The choice about tuning in weak stations describes a receiver's tuning or preselector circuit, not an antenna coupler; the one about using an antenna for transmit and receive describes a T/R switch or duplexer, and automatic antenna selection is the job of an antenna switch.
A tuner tunes the transmitter's view of the load, not the antenna: it makes the rig see 50 ohms.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T9B055 of 12

What happens as the frequency of a signal in coaxial cable is increased?

Why Coax loss comes mainly from conductor resistance and dielectric heating, and both get worse as frequency rises. Skin effect pushes current into an ever thinner outer layer of the conductor, raising effective resistance, while the dielectric absorbs more energy at higher frequencies. That is why a cable rated at a fraction of a dB per 100 feet on HF can lose several dB per 100 feet at UHF, and why short runs or low-loss cable matter most on the higher bands.
Watch out The impedance choices are a trap: characteristic impedance is set by the conductor diameters and the dielectric, so a 50 ohm cable stays essentially 50 ohms across its whole usable frequency range.
Higher frequency, higher loss. Impedance is set by geometry, not by frequency.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T9B066 of 12

Which of the following connector types is most suitable as an RF connector for frequencies above 400 MHz?

Why The Type N connector is a constant-impedance 50 ohm design with a threaded, weather-resistant shell, so it maintains a uniform impedance through the junction and works well up to several GHz. That makes it the standard choice for VHF, UHF and microwave work, including everything above 400 MHz.
Watch out The PL-259 (UHF connector) is tempting because it is the most common ham connector, but despite the name it is not impedance-matched and its performance degrades above roughly 300 MHz; RG-213 is a coaxial cable, not a connector, and DB-25 is a multi-pin computer data connector.
N for Nice and high frequeNcy; PL-259 is for HF and 2 meters despite the 'UHF connector' name.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T9B077 of 12

Which of the following is true of PL-259 type coax connectors?

Why The PL-259 (with its SO-239 mating socket) is the classic "UHF" connector, a threaded screw-on design that handles high power and is the standard fitting on most HF and VHF radios and amplifiers. Despite the UHF name, its impedance is not well controlled, so losses and mismatch rise above roughly 300 MHz. For the HF and VHF bands where most hams operate, it is rugged, cheap and perfectly adequate.
Watch out The microwave choice plays on the misleading "UHF connector" nickname; for those frequencies you would use a constant-impedance connector such as type N or SMA. A PL-259 is neither watertight (outdoor joints must be wrapped or sealed) nor a bayonet type, which describes the twist-lock BNC.
PL-259 = "UHF" in name only: use it at HF/VHF, use type N when you go above a few hundred MHz.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T9B088 of 12

Which of the following is a source of loss in coaxial feed line?

Why Coax loss comes from several practical sources, and all three listed items add to it. Water in a connector or inside the braid changes the dielectric and adds heavy dielectric and resistive loss, often ruining the cable permanently. High SWR means power reflects and travels back and forth along the line, so the same energy passes through the lossy conductor and dielectric multiple times, multiplying the matched-line loss. Every connector and adapter adds a small amount of insertion loss and a possible impedance discontinuity, so stringing several in a run adds up.
Watch out Each single choice is a genuine loss source, so picking just one, such as high SWR, is only part of the picture. When a Technician question lists three plausible physical causes, the combined answer is usually the intended one.
Water, watts bouncing (SWR), and connectors: all three steal signal. Seal your PL-259s.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T9B099 of 12

What can cause erratic changes in SWR?

Why SWR depends on the impedance the feed line sees at the antenna, so anything that changes that impedance changes the reading. A loose or corroded connector, a broken braid strand, or an intermittent solder joint makes the connection resistance vary with vibration, wind, or heating, so the SWR meter jumps around unpredictably. Steady SWR that drifts slowly is normal (weather, ice, frequency change), but erratic, jumpy SWR almost always points to a bad mechanical connection.
Watch out Overmodulation is a transmitter audio problem that distorts your signal and splatters onto adjacent frequencies, but it does not change the impedance your feed line presents, so it does not make SWR jump.
Jumpy SWR = jiggly connection. Check the connectors first.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T9B1010 of 12

What is the electrical difference between RG-58 and RG-213 coaxial cable?

Why Both are 50-ohm coax, but RG-213 is physically much larger (about 0.405 inch diameter versus about 0.195 inch for RG-58). The bigger center conductor and shield mean less resistive loss per hundred feet at any given frequency, and the thicker dielectric also lets it handle more power. Impedance is the same, so the practical difference is attenuation and power rating.
Watch out The claim that RG-58 handles higher power reverses the relationship: thin cable has a small center conductor and thin insulation, so it loses more signal as heat and breaks down at lower voltages. Shield count is not the defining difference here, and there certainly is a significant difference between the two.
Fatter coax, fewer losses. RG-213 is the thick one, so it wins on both loss and power.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T9B1111 of 12

Which of the following types of feed line has the lowest loss?

Why Feed line loss comes mostly from conductor resistance and dielectric absorption. Air-insulated hardline uses a large center conductor, a solid copper or aluminum outer shield, and air (with only thin spacers) as the dielectric, so both loss mechanisms are minimized. That is why it is used for long runs, VHF/UHF, and repeater installations, at the cost of being stiff, heavy, and expensive.
Watch out Flexible coax at 50 or 75 ohms uses a solid or foam plastic dielectric that absorbs RF energy, so it loses more per 100 feet; impedance itself (50 vs 75 ohms) is not what determines loss.
Air beats plastic: the more air in the dielectric, the lower the loss. Hardline wins.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T9B1212 of 12

What is standing wave ratio (SWR)?

Why SWR compares the impedance of the load (usually the antenna) to the characteristic impedance of the feed line. When they match, all forward power goes into the load and SWR is 1:1; when they differ, some power reflects back and combines with the forward wave to create standing waves of voltage and current along the line. Numerically it is the ratio of the larger impedance to the smaller, so a 100 ohm antenna on 50 ohm coax gives 2:1.
Watch out The ratio of amplifier output to input power is gain, not SWR, and neither transmitter efficiency nor ground quality is what an SWR meter reads.
SWR = Standing Wave Ratio, but think 'System-to-Wire Ratio': it grades the match between line and load. 1:1 is perfect.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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