HamSandwich

Repeaters, simplex, and questionable antenna choices.

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T7C

PRACTICAL CIRCUITS

Antenna and transmission line measurements and troubleshooting: measuring SWR, effects of high SWR, causes of feed line failures; Basic coaxial cable characteristics; Use of dummy loads when testing

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T7C011 of 11

What is the primary purpose of a dummy load?

Why A dummy load is a non-inductive resistor, usually 50 ohms, that presents a matched load to the transmitter and turns the RF output into heat instead of radiation. That lets you tune, test, or measure power output without putting a signal on the air where it could cause interference. It also protects the transmitter by giving it a proper 1:1 SWR load during testing.
Watch out The choice about antenna efficiency is backwards: a dummy load is deliberately designed not to radiate, so its radiating efficiency is essentially zero. Modulation level and receiver signal-to-noise ratio have nothing to do with a resistive load.
Dummy load = 50 ohm resistor that turns RF into heat, not radio waves. Test without transmitting.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7C022 of 11

Which of the following is used to determine if an antenna is resonant at the desired operating frequency?

Why An antenna analyzer sweeps a small signal across a range of frequencies and reports SWR and impedance at each point, so you can see where the SWR dips to its minimum. That dip marks the resonant frequency, where the antenna's reactance is near zero and it presents mostly resistance. If the dip is above or below your band, the analyzer tells you to shorten or lengthen the antenna.
Watch out A frequency counter only measures the frequency of a signal fed into it, it tells you nothing about how the antenna is behaving at that frequency. A VTVM is an old-style voltmeter and a Q meter measures the quality factor of coils and capacitors on the bench.
Analyzer = SWR versus frequency curve. Find the dip, find resonance.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7C033 of 11

What does a typical RF dummy load consist of?

Why A dummy load replaces the antenna so you can test or tune a transmitter without radiating a signal. It must look like a perfect antenna to the transmitter, meaning a purely resistive 50 ohms with no reactance, so the resistive element is made non-inductive (carbon or thin-film rather than wirewound). All the RF power turns into heat, so the resistor is bolted to a heat sink or immersed in cooling oil to handle the transmitter's output.
Watch out The choice with 50 ohms of inductive reactance is the trap: reactance would store and reflect energy rather than absorb it, giving a poor match and an inaccurate test. Relay and power supply answers describe switching gear, not a load.
Dummy load = 50 ohms of pure resistance plus a way to dump the heat. No coils, no radiation.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7C044 of 11

What reading on an SWR meter indicates a perfect impedance match between the antenna and the feed line?

Why SWR is a ratio of the maximum to minimum voltage (or impedance mismatch) on the line, so a perfect match gives equal maximum and minimum, which is a ratio of 1 to 1. At 1:1 no power is reflected back from the antenna, and all forward power goes into the load. In practice a 50 ohm antenna feeding a 50 ohm coax reads 1:1; any mismatch drives the ratio higher, for example a 100 ohm load on 50 ohm line reads 2:1.
Watch out Zero is tempting because reflected power is zero at a perfect match, but SWR is a ratio and can never be less than 1; a meter reading zero simply means no power is being measured.
SWR is a ratio, and the best ratio is one to one. It never goes below 1:1.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7C055 of 11

Why do most solid-state transmitters reduce output power as SWR increases beyond a certain level?

Why When SWR is high, part of the transmitted power is reflected back into the final amplifier, producing high voltage and current peaks and extra heat in the output transistors. Solid-state finals have very little tolerance for that abuse compared to vacuum tubes, so modern rigs include an SWR sensing circuit that folds back drive power once SWR climbs past roughly 1.5:1 to 2:1. The reduced output is a self-protection feature, not a fault in the radio.
Watch out Cutting power does not lower the SWR at all: SWR is set by the mismatch between the feed line and the antenna, so the ratio stays the same no matter how much power you feed into the line.
Foldback protects the finals. High SWR cooks transistors, so the radio backs off to save itself.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7C066 of 11

What does an SWR reading of 4:1 indicate?

Why SWR is the ratio of maximum to minimum voltage along the feed line, and it is caused by reflected power from a load that does not match the line's characteristic impedance. A perfect match gives 1:1; any ratio larger than that means some power is being reflected back toward the transmitter. A 4:1 reading corresponds to a load roughly four times (or one fourth) the line impedance, such as 200 ohms or 12.5 ohms on 50 ohm coax, so it signals a significant mismatch. Most solid state transmitters start folding back power above about 2:1.
Watch out SWR is a ratio, not a decibel figure, so it never directly states gain or loss, and the 'good match' choice describes a reading near 1:1, not 4:1.
SWR: 1:1 is perfect, bigger number means bigger mismatch. The colon tells you it's a ratio, not dB.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7C077 of 11

What happens to power lost in a feed line?

Why Feed line loss comes from the resistance of the conductors, dielectric losses in the insulation, and small leakage. Energy that does not reach the antenna has to go somewhere, and by conservation of energy it ends up as heat in the cable. That is why long runs, higher frequencies, and thin coax all reduce the power actually radiated, and why a lossy line can run noticeably warm at high power.
Watch out High SWR causes extra loss, not the other way around, so the choice about increasing SWR reverses cause and effect. Harmonics are generated in the transmitter's nonlinear stages, not by a passive length of coax.
Lost RF does not vanish, it warms the coax: loss equals heat.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7C088 of 11

Which instrument can be used to determine SWR?

Why SWR is the ratio derived from how much power the transmitter sends toward the antenna versus how much bounces back. A directional wattmeter (or SWR meter, which works the same way) uses a directional coupler to sample forward and reflected power separately, and SWR follows from the ratio of those two readings. When reflected power is zero the SWR is 1:1; as reflected power rises, so does SWR.
Watch out A voltmeter or ohmmeter reads DC or low-frequency values at one point and cannot separate forward from reflected RF traveling waves, so neither can give you SWR. Iambic pentameter is poetry, not test gear, though 'iambic' does appear in ham radio as a keyer mode.
SWR needs forward AND reflected power, so you need the meter that knows direction: directional wattmeter.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7C099 of 11

Which of the following causes failure of coaxial cables?

Why Coax relies on a clean, dry dielectric between the center conductor and the shield. When water wicks in through a cracked jacket or an unsealed connector, it soaks the dielectric and braid, raising loss dramatically and corroding the shield, and the damage spreads along the cable. That is why outdoor connectors should be sealed with weatherproofing tape or sealant.
Watch out Solder flux is a concern for cold or corroded joints in connectors, but it is not the classic cable-killer; and coax does not care about how fast transmitter power varies or about long key-down operation as long as its power rating is not exceeded.
Water is coax's worst enemy: seal every outdoor connector.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7C1010 of 11

Why should the outer jacket of coaxial cable be resistant to ultraviolet light?

Why Sunlight breaks down the polymer in a coax jacket over time, making it chalky, brittle and cracked. Once the jacket splits, rain and humidity wick into the braid and dielectric, which raises loss dramatically and corrodes the shield. That is why outdoor runs use UV-resistant (usually black, carbon-loaded) jacket material, and why any nicked or cracked section should be replaced.
Watch out Jacket losses are not the issue: the jacket carries no RF, so UV does not directly add loss. The damage matters only because it lets water into the parts that do carry signal.
UV cracks the jacket, water gets in, cable dies. Outdoor coax is black for a reason.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7C1111 of 11

What is an advantage of foam-dielectric versus solid-dielectric coaxial cable?

Why Foam dielectric is mostly air bubbles, so there is less solid plastic to absorb RF energy as heat. That lower dielectric loss means foam cable typically shows noticeably less attenuation per 100 feet than solid polyethylene cable of the same size, especially at VHF and UHF. Foam also has a higher velocity factor, around 0.78 to 0.85 versus about 0.66 for solid PE, which matters when cutting phasing or matching sections.
Watch out Moisture resistance and voltage breakdown are actually the weak points of foam cable, not advantages: the porous foam wicks water more readily and the thinner effective dielectric withstands less voltage. Impedance is set by the conductor dimensions and dielectric constant together, so both types are made in 50 ohm versions.
Foam = more air, less loss. Its trade-offs are moisture and voltage handling, so any answer praising those is wrong.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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