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Subelement T7
Station equipment: common transmitter and receiver problems; antenna measurements; troubleshooting; basic repair and testing
Section T7B
Common transmitter and receiver problems: symptoms of overload and overdrive; distortion; causes of interference; interference and consumer electronics; part 15 devices; over-modulation; RF feedback; off frequency signals
What can you do if you are told your FM handheld or mobile transceiver is over-deviating?
  • Talk louder into the microphone
  • Let the transceiver cool off
  • Change to a higher power level
  • Talk farther away from the microphone

The louder the voice signal in an FM transmitter the greater the frequency deviation of the modulator. If we are over deviating the frequency then we need to reduce the amplitude of our voice. Therefore the correct answer is to talk farther away from the microphone or not talk so loudly.

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Tags: frequency modulation best practices arrl chapter 5 arrl module 11

What would cause a broadcast AM or FM radio to receive an amateur radio transmission unintentionally?
  • The receiver is unable to reject strong signals outside the AM or FM band
  • The microphone gain of the transmitter is turned up too high
  • The audio amplifier of the transmitter is overloaded
  • The deviation of an FM transmitter is set too low

Microphone gain might cause distortion, splatter or over deviation, but it wouldn't cause the signal to jump into a broadcast AM or FM band - they're too far away from the amateur bands.

Similarly, if the audio amplifier of the transmitter were overloaded, it might distort, but wouldn't shift the frequency or cause harmonics to be radiated.

If the deviation of an FM transmitter were set too low, the signal would sound very quiet, the opposite of loud or even loud enough.

The only remaining choice is the right one, that the receiver is unable to reject the strong signals outside of the broadcast band.

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Tags: arrl chapter 9 arrl module 23

Which of the following can cause radio frequency interference?
  • Fundamental overload
  • Harmonics
  • Spurious emissions
  • All of these choices are correct

There are many things that can cause interference with radio signals. All three of those listed here can cause interference, therefore the correct answer is "All of these choices are correct".

Fundamental overload - This is a case when the transmitted signal is so strong that it overloads the receiver which prevents proper reception of the desired signal.

Harmonics - When a sine wave is distorted (not pure) it creates harmonics that are integer multiples of the fundamental frequency; these harmonics end up getting into receivers at these harmonic frequencies interfering with operation on those frequencies. See this reference: 5.0 Transmitter Harmonic Emissions.

Spurious emissions - There are a number of undesirable emissions that can interfere with normal signal reception. Most of these can be termed Spurious emissions. From ITU, 1.145 "Spurious emissions include harmonic emissions, parasitic emissions, intermodulation products and frequency conversion products but exclude out-of-band emissions."

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Tags: noise and interference arrl chapter 9 arrl module 23

Which of the following is a way to reduce or eliminate interference from an amateur transmitter to a nearby telephone?
  • Put a filter on the amateur transmitter
  • Reduce the microphone gain
  • Reduce the SWR on the transmitter transmission line
  • Put an RF filter on the telephone

Since the problem is that the telephone is acting as a receiver and should only be acting as a telephone, the only solutions would be to reduce the power of the transmitter so it's not detected by the telephone (acting as a receiver), or to stop the telephone from acting as a receiver.

Although sometimes a high SWR (Standing Wave Ratio) can cause unwanted radiation near the ground that results in a strong signal appearing at a nearby telephone, the problem is still that the telephone is acting as a receiver.

The microphone gain being too high could cause splatter on adjacent frequencies or over-deviation, but can't turn a telephone into a receiver.

The only remaining choice is to keep the RF energy from even getting to the telephone by installing an RF filter on the telephone.

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Tags: arrl chapter 9 arrl module 23

How can overload of a non-amateur radio or TV receiver by an amateur signal be reduced or eliminated?
  • Block the amateur signal with a filter at the antenna input of the affected receiver
  • Block the interfering signal with a filter on the amateur transmitter
  • Switch the transmitter from FM to SSB
  • Switch the transmitter to a narrow-band mode

The problem here is that the non-amateur radio is receiving signals in the amateur radio band, so a filter on the amateur transmitter won't help - the signal in the amateur band will still be radiated.

Changing the transmitter from FM to SSB will only change the mode of the interference, and it's likely that it will still be received by the TV receiver.

The bandwidth of the transmitted signal isn't the problem - the problem is that the TV is receiving signals in the amateur radio bands. So the only way to reduce or eliminate the signal is to block the amateur radio signal before it gets into the TV, using a filter at the antenna input of the affected receiver.

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Tags: arrl chapter 9 arrl module 23

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?
  • Make sure that your station is functioning properly and that it does not cause interference to your own radio or television when it is tuned to the same channel
  • Immediately turn off your transmitter and contact the nearest FCC office for assistance
  • Tell them that your license gives you the right to transmit and nothing can be done to reduce the interference
  • Install a harmonic doubler on the output of your transmitter and tune it until the interference is eliminated

If you can't keep your signal out of your own TV, it's unlikely that you can keep it out of your neighbor's. It's much easier to experiment with your own TV.

Once you're sure that your TV is okay and not receiving your signals, you can look for differences in your neighbor's TV set up. It could be that he doesn't have a good antenna, or that your signal is getting into his cable TV feed line.

Tuning to the same channel lets you know that it isn't harmonic radiation from your station, or at least that the harmonic radiation could be reduced using a filter.

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Tags: arrl chapter 9 arrl module 23

Which of the following can reduce overload to a VHF transceiver from a nearby FM broadcast station?
  • RF preamplifier
  • Double-shielded coaxial cable
  • Using headphones instead of the speaker
  • Band-reject filter

The best way to eliminate the interference is to not let it in your radio. Just like if you have a noisy roommate, best way to get rid of the noise is not letting them in the room in the first place!

A band reject filter knocks out the frequency range from the FM broadcast station before it ever gets to your radio and has a chance to cause interference.

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Tags: arrl chapter 9 arrl module 23

What should you do if something in a neighbor's home is causing harmful interference to your amateur station?
  • Work with your neighbor to identify the offending device
  • Politely inform your neighbor about the rules that prohibit the use of devices that cause interference
  • Check your station and make sure it meets the standards of good amateur practice
  • All of these choices are correct

Cooperation with others is the best way to start solving an interference problem. Many devices, especially digital computers and peripherals, generate RF interference. Turning off devices one at a time is a good way to locate an offending device. For more information see: RF interference

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Tags: troubleshooting noise and interference best practices arrl chapter 9 arrl module 23

What is a Part 15 device?
  • An unlicensed device that may emit low-powered radio signals on frequencies used by a licensed service
  • An amplifier that has been type-certified for amateur radio
  • A device for long-distance communications using special codes sanctioned by the International Amateur Radio Union
  • A type of test set used to determine whether a transmitter complies with FCC regulation 91.15

Part 15 devices are things like Wireless Routers or the little switching power supplies that you plug in the wall, or even computers. They can put out some low powered radio signals, but they're supposed to be limited to very low powered signals that could be filtered out or avoided by placement of a radio receiver.

Since they're not intended to radiate RF outside their intended band, they can't be citizen's band radios, and amateur radios don't share any frequencies with Citizen's Bands.

Part 15 devices radiate tiny amounts of radio frequency energy, and as such are not likely to be useful for long distance communications. Indeed, you should be able to get rid of interference from a Part 15 device by locating the receiver at a greater distance from the device.

91.15? Nice try, but no, it's not a test set.

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Tags: arrl chapter 9 arrl module 23

What might be a problem if you receive a report that your audio signal through the repeater is distorted or unintelligible?
  • Your transmitter is slightly off frequency
  • Your batteries are running low
  • You are in a bad location
  • All of these choices are correct

How do each of these cause your signal to be distorted or unintelligible?

"Your transmitter may be slightly off frequency" - If your transmitter is slightly off frequency the receiver will not be able to demodulate the signal and it may sound garbled, distorted and low in volume.

"Your batteries may be running low" - When your batteries are low the audio amplifier cannot generate the proper amplitude signals; the signal is limited by the lower than normal voltage which clips the signal causing it to be distorted.

"You could be in a bad location" - As in real estate, location, location and location are vital. Multipath signals can cause an interference pattern that can make the received audio sound badly.

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Tags: noise and interference batteries troubleshooting radio operation arrl chapter 6 arrl module 14

What is a symptom of RF feedback in a transmitter or transceiver?
  • Excessive SWR at the antenna connection
  • The transmitter will not stay on the desired frequency
  • Reports of garbled, distorted, or unintelligible voice transmissions
  • Frequent blowing of power supply fuses

Your own transmitter output can be picked up by the sensitive circuits inside your microphone. This is called RF feedback. The symptom is that the transmission may be distorted, garbled or unitelligible.

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Tags: noise and interference radio waves troubleshooting arrl chapter 9 arrl module 23

What should be the first step to resolve cable TV interference from your ham radio transmission?
  • Add a low-pass filter to the TV antenna input
  • Add a high-pass filter to the TV antenna input
  • Add a preamplifier to the TV antenna input
  • Be sure all TV coaxial connectors are installed properly

Once it is known that the TV interference is originating from your ham radio transmission, and that your own equipment is functioning within acceptable parameters, your first step is to determine how best to solve or reduce the effects of the interference at the TV.

While it's possible to reduce the effects of the interference by installing a preamplifier at the TV antenna input, you will only be masking the problem rather than solving it. Also, as well-intentioned as they might be, installing filters might not prove very useful because their effective frequency ranges might be so close to those of the TV that the TV signals themselves could just as easily get filtered out with the interference.

Usually the problem will be that something connected to the TV antenna input is acting as an antenna, picking up your transmissions. This is especially true for loose connectors, mismatched connectors, frayed cables, and other damaged or corroded connections. Therefore, checking to be sure that all TV coaxial connectors are installed properly is a good way to satisfy that first step.

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Tags: arrl chapter 9 arrl module 23

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