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T7B

PRACTICAL CIRCUITS

Symptoms, causes, and cures of common transmitter and receiver problems: overload and overdrive, distortion, interference and consumer electronics, RF feedback

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T7B011 of 11

What can you do if you are told your FM handheld or mobile transceiver is over-deviating?

Why In FM the audio level from your microphone controls deviation, how far the carrier swings away from its center frequency. Standard amateur FM voice is designed for about 5 kHz peak deviation; drive the audio too hard and the signal swings wider than the channel, sounding distorted and splattering into adjacent channels. Backing away from the microphone (or speaking more softly) lowers the audio drive and brings deviation back into limits.
Watch out Talking louder does exactly the wrong thing, since more audio means more deviation. Power level and heat have nothing to do with deviation; transmitter power sets how far you are heard, not how wide your signal is.
Over-deviating means too much audio: back off the mic. Louder equals wider in FM.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7B022 of 11

What would cause a broadcast AM or FM radio to receive an amateur radio transmission unintentionally?

Why This is a receiver problem, not a transmitter problem. Consumer broadcast radios are built cheaply, with little front-end filtering, so a strong nearby signal on a completely different frequency can get into the first stages, overload them, and be detected as audio. The cure is at the affected receiver: better filtering, a bandpass or high-pass filter on its antenna lead, or more distance, since your transmitter may be perfectly clean and legal.
Watch out Excessive mic gain, an overdriven audio stage or wrong FM deviation are transmitter adjustment faults that cause distorted or splattered audio on the amateur signal, not the reason a broadcast set hears you at all.
Cheap radio, poor filter: fundamental overload is the listener's receiver failing to reject your strong out-of-band signal.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7B033 of 11

Which of the following can cause radio frequency interference?

Why Interference can come from either end of the link. Fundamental overload happens when your strong on-channel signal simply swamps the front end of a nearby consumer device, even though your transmitter is clean. Harmonics are unwanted multiples of your operating frequency (2f, 3f, and so on) that can land in TV, VHF, or other bands. Spurious emissions is the broader category of any unintended output outside the necessary bandwidth, including mixer products and parasitic oscillations, and Part 97.307 requires transmitters to keep these suppressed.
Watch out Picking just harmonics is tempting because they are the classic cause, but that ignores overload, where the interference is really a weakness in the victim device's ability to reject a strong nearby signal.
Interference comes from a dirty transmitter (harmonics, spurs) or an overloaded receiver, so all of the above.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7B044 of 11

Which of the following might be the cause of low RF power output from a solid-state transceiver?

Why Solid-state final amplifiers have no easy way to tolerate reflected power, so modern transceivers include a protection circuit that senses SWR and automatically reduces (folds back) drive when the load is badly mismatched. A high SWR on the feedline means a large portion of the power is being reflected back toward the radio, and the protection circuit throttles output to keep the finals from overheating or failing. So an antenna system fault, bad coax, or wrong-band antenna usually shows up first as reduced power out.
Watch out Low SWR is the goal, not a fault, since it means the antenna is accepting the power. Noise figure describes receiver sensitivity, and poor linearity would produce distortion and splatter rather than a drop in power.
High SWR equals power foldback: the rig protects its transistors by turning itself down.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7B055 of 11

Which of the following might reduce interference by an amateur station to a non-amateur over-the-air radio receiver?

Why Most interference to consumer receivers comes from fundamental overload: the amateur's clean, legal signal is simply so strong at the neighbor's antenna that the receiver's front end cannot reject it. The cure has to be applied at the receiver, where a filter (high-pass, band-reject or common-mode choke) at the antenna input keeps the out-of-band amateur energy from reaching the receiver's input stages. Once the strong signal is kept out of the front end, the receiver can handle the signals it was designed for.
Watch out Adding a filter at the transmitter only helps when the transmitter is actually radiating harmonics or spurious emissions; it does nothing for fundamental overload, because you cannot filter out the very frequency you are trying to transmit. Changing modes does not reduce the fundamental signal strength either.
Overload happens in the victim's receiver, so fix it there: filter at the receiver's antenna input.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7B066 of 11

Which of the following actions should you take if a neighbor tells you that your station's transmissions are interfering with their radio or TV reception?

Why The first step in any RFI complaint is to rule out your own station as the source: check that your transmitter is clean (no spurious emissions or harmonics), that your antenna system and grounding are sound, and then test by tuning your own TV or radio to the same channel while you transmit. If your own receiver is unaffected, the problem is very likely poor immunity in the neighbor's equipment, which is cured with filters, ferrite chokes, or better shielding on their end. FCC rules require you to avoid causing harmful interference with spurious emissions, so verifying your own gear is both good practice and your responsibility under Part 97.
Watch out Installing a "harmonic doubler" would multiply your output frequency and create more spurious energy, not less; it is a made-up cure. Calling the FCC is not a first step, and the FCC expects amateurs and their neighbors to resolve routine interference locally.
Clean your own house first: if your own TV on that channel is fine, the trouble is in the neighbor's set.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7B077 of 11

Which of the following can reduce interference to a 2-meter band transceiver from a nearby commercial FM station?

Why A nearby commercial FM broadcast station (88 to 108 MHz) puts a very strong signal into the receiver front end, which is close enough in frequency to overload it. A band-reject (notch) filter placed in the antenna line attenuates that FM broadcast band while passing the 144 to 148 MHz signals you want. Removing the offending energy before it reaches the first receiver stage stops the front-end overload.
Watch out An RF preamplifier is the opposite cure: it amplifies the strong broadcast signal along with everything else and usually makes overload worse. Double shielding and bypass capacitors address energy leaking into cables, not a strong signal arriving through the antenna.
Strong out-of-band signal? Notch it out. Filter first, never a preamp, which only amplifies the problem.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7B088 of 11

What should you do if something in a neighbor's home is causing harmful interference to your amateur station?

Why When a neighbor's consumer device is radiating noise into your station, the practical fix is a cooperative one: help locate the offending device (switching things off one at a time is the usual method), let your neighbor know politely that Part 15 devices must not cause harmful interference to licensed services and must stop if they do, and first verify your own station is clean and properly installed so the problem cannot be blamed on you. All three steps are part of the recommended approach, so the all-inclusive choice is correct.
Watch out Each single step is true but incomplete. In particular, leading with the rules lecture without first checking your own station or helping find the device tends to turn a technical problem into a neighbor dispute.
Interference etiquette: check yourself, help them hunt, inform politely. All three, every time.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7B099 of 11

What should be the first step to resolve non-fiber optic cable TV interference caused by your amateur radio transmission?

Why Cable TV is a sealed, shielded system: signals are supposed to stay inside the coax. If your transmitter gets into it, the cable's shielding has been compromised somewhere, and loose, corroded or poorly installed F connectors are the most common leak point. Tightening or replacing the connectors restores the shield and usually cures the problem at its source, so that is the first thing to check before adding any filters.
Watch out A high-pass filter on the TV input is a classic cure for HF energy entering an over-the-air TV receiver, but it does nothing about RF leaking into a cable system through bad connectors; a low-pass filter belongs on the transmitter output, and a preamplifier would amplify the interference along with the signal.
Cable trouble? Check the cable first. Tighten the connectors before you reach for a filter.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7B1010 of 11

What might be a problem if you receive a report that your audio signal through an FM repeater is distorted or unintelligible?

Why All three can produce garbled audio through a repeater. If your transmitter drifts slightly off the repeater input frequency, the repeater's discriminator recovers distorted audio. Speaking too loudly or with your lips against the microphone overdeviates the FM signal, splattering beyond the receiver's filter and clipping the recovered audio. And a poor location, meaning a weak or multipath-ridden signal into the repeater, makes the received audio noisy and broken up.
Watch out Each single choice is a real cause, so picking just one is incomplete; the pool includes an "all" option precisely because distorted repeater audio has several common sources.
Bad repeater audio: off frequency, over deviating, or out of range. Three causes, so pick them all.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T7B1111 of 11

Which of the following can eliminate distorted voice transmissions?

Why Distorted audio on transmit is often caused by RF feedback: some of your own transmitted signal is picked up by the microphone cable and coupled back into the audio stages, where it gets rectified and re-modulated. A clip-on ferrite bead or several turns of cable through a ferrite core raises the impedance to common-mode RF current on the outside of the cable without affecting the audio inside it, so the stray RF never gets back to the mic input. Other cures for the same problem are better station grounding, moving the antenna farther from the operating position, and reducing power.
Watch out Adding feedline does not lower SWR, it just adds loss that masks the reading at the radio, and the squelch control only affects the receiver, not your transmitted audio.
Distorted audio on transmit = your own RF sneaking back in. Ferrite on the mic cable chokes it off.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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