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Subelement T7

PRACTICAL CIRCUITS

Section T7B

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

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
  • Correct Answer
    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?

  • Correct Answer
    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.

Another explanation: Your amateur radio station may interfere with a nearby radio receiver if your signal is so strong that the receiver cannot reject the signal even though your signal is not on the frequency to which the receiver is tuned. This is called overload.

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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
  • Correct Answer
    All 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 could you use to cure distorted audio caused by RF current on the shield of a microphone cable?

  • Band-pass filter
  • Low-pass filter
  • Preamplifier
  • Correct Answer
    Ferrite choke

A Ferrite Choke can be placed on a cable to reduce current flow conducted on the shield of audio (and power) cables.

The grey tubes or clamp on ferrite devices add inductance to the cable to block the common mode RF energy that may leak from the radio onto the cables attached to the radio.

Bandpass and low pass filters are too complex to design, test and implement compared to the simplicity of placing a ferrite choke on the cable.

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

How can fundamental overload of a non-amateur radio or TV receiver by an amateur signal be reduced or eliminated?

  • Correct Answer
    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?

  • Correct Answer
    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
  • Install a harmonic doubler on the output of your transmitter and tune it until the interference is eliminated
  • All these choices are correct

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 they don't have a good antenna, or that your signal is getting into their 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 of a VHF transceiver by a nearby commercial FM station?

  • Installing an RF preamplifier
  • Using double-shielded coaxial cable
  • Installing bypass capacitors on the microphone cable
  • Correct Answer
    Installing a 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.

Silly Hint: FM station plays music; "Band" reject filter

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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 that FCC rules prohibit the use of devices that cause interference
  • Make sure your station meets the standards of good amateur practice
  • Correct Answer
    All 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 should be the first step to resolve non-fiber optic cable TV interference caused by your amateur 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
  • Correct Answer
    Be sure all TV feed line 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

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

  • Your transmitter is slightly off frequency
  • Your batteries are running low
  • You are in a bad location
  • Correct Answer
    All 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.

Also, "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 bad.

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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
  • Correct Answer
    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

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