Module 14 - Radio Frequency Interference
- Reading Assignment
- Radio Frequency Interference
- Types of RFI/Direct Detection/Overload/Harmonics
- Noise Sources
- RFI Mitigation
- TV Interference
- Interference to Us
- Interference By Us
- FCC Rules
- Quiz 14
- Recommended Reading
Read Chapter 5, Pages 5-18 to 5-24 & 8-6 to 8-8 in your text before you continue.
Radio Frequency Interference
Unwanted, unintentional signals from some electronic device that interferes with radio wave reception. This is a very broad statement. Interference can come from many sources and we will explore them in this module.
What are filters? A low-pass filter is an electronic filter that passes low-frequency signals but attenuates (reduces the amplitude of) signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter.
A high-pass filter (HPF) is filter that passes high-frequency signals but attenuates (reduces the amplitude of) signals with frequencies lower than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter.
A band-pass filter is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.
Types of Radio Frequency Interference
Direct detection - offending signals get into the electronics circuits to cause interference. Remember detection is another word for demodulation. If you were to look into the circuitry of demodulators or detectors, you would find a simple diode is what does detecting. Some of you will remember the old crystal radio. It was just a piece of quarts and a spring poked into it. Attach a set of headphones and you have a receiver. Many things will act as a diode and detect or demodulate signals. The stronger the signal the more chance of detecting the signals.
A cordless phone, a cord phone and even filings in teeth can act as a crystal receiver
Overload - strong signal that overwhelms the weaker, wanted signal so much the receiver cannot reject them. A very strong transmitter close by your receiver will exhibit symptoms of fundamental overload. The signals do not even have to be on frequency or even be close.
Harmonics - even multiples of the offending signal that coincide with the wanted signal. Remember every time you mix two frequencies together, they will result in four frequencies, the sum, the difference & the two originals. A harmonic, if you don't recall, is a multiple of the fundamental frequency. In other words if you are sending a signal on 7.000 MHz, the harmonics are 14.0 MHz, 21 MHz, 28 MHz, 45 MHz etc. and you will also be sending the harmonics. Now, it is true these harmonics are usually suppressed by a very great degree. (measured in db, 3 db = a magnitude of X2, 10 db is a magnitude of 10X) However, they can still be a problem especially if the transmitter is running high power.
Now when these harmonics mix with other harmonics or signals the possibilities exist that a signal may be produced that is directly on some ligament receive frequency.
Sometimes, one of the harmonics can even be the local oscillator of your own receiver. For example, if you have a superheterodyne receiver that is receiving 10 MHz with a 2 MHz Intermediate Frequency and you are using a 12 MHz Local Oscillator to mix with the 10 MHz signal to obtain a difference of 2 Mhz, you have an image frequency of 14 Mhz because 14 MHz - 12 MHz also obtains a difference of 2 MHz your IF. Is this a problem yes, but only if there is a signal on 14 Mhz. Receivers are designed to avoid these image frequencies, but sometimes it is really a challenge.
We will go on to discuss various interferences and ways to deal with them in this module.
Noise can be heard on your receive as popping or scratching or hiss, to name a few. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. These effects can range from a simple degradation of the desired signal to a total loss of signal. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit, the Sun or even the Northern Lights.
The textbook covers noise sources very well on Page 5-21, please review them. You'll be amazed at what can be a transmitter!
Mitigation or moderating the problems of RFI can take several forms. You might not be able to eliminate interference to you or causing interference completely, however you can lessen it.
Some of the things you can do yourself are; Proper grounding, filtering and the use of current chokes or current baluns to remove signals from the outer shield of your antenna feedline.
Proper grounding is the easiest, most cost effective way of mitigating incoming or outgoing interference. Flat strap grounding is the most effective for RF grounding.
Make sure your connectors are not loose.
It is important not to have RF floating around the shack because of improper grounding or lack of filtering because it can cause a feedback by getting into the modulation circuits and cause garbled, distorted or unintelligible transmissions.
TVI is a major problem for Hams. Not because we cause a lot of it, but because it is not unusual for people that don't understand anything about radio to blame the easiest target, Hams.
It is not as bad a problem as in the past because a high percentage of consumers are using cable tv, or satellite for their tv reception. Both of these methods use a shielded coax lead which helps to keep unwanted signals from reaching the tv receiver. Also the impedance is only 75 ohms and that also helps.
In the days of outside antennas, often 300 ohm unshielded twin lead that had deteriorated over the years added to the problem. The neighbor says, well I didn't have the problem before you started transmitting. That might be true, but the neighbor still has the obligation to keep their antenna system in good repair.
Most of the time you will find if you tell your neighbor what they can do to resolve the problem on their end, they will be happy to do so.
Often the problem is the result of broken shielding somewhere in the cable, loose, broken or corroded connections. Many times this can be solved by proper cable maintenance by cable supplier.
Use a Low Pass TVI Filter (cuts off signals above 54 MHz, which is where TV signals start but passes signals below that frequency.) between your transmitter and antenna or tuner and reduce your transmitter harmonics by 50 dB! Harmonics could be the source of interference to your neighbors TV, radio, VCR, telephone, etc.
Another method is to treat the problem at the tv receiver end. A high pass filter designed stop TV, FM, VCR interference from amateur radio, pagers, CB and other RF sources. They are made for all antenna and cable systems using 75 ohm coaxial cable. This filter design uses five shielded sections and 17 elements to provide 70 dB attenuation below 50 MHz. It has a very sharp cut-off at 52 MHz, so any signals below that frequency are highly attenuated. It simply attaches in-line to a TV set's antenna input.
Take interference complaints seriously. Make sure that you're really not the cause (demonstrate that you don't interfere within your own home). Offer to help eliminate the RFI, even if you are not at fault. Click here to consult the ARRL RFI Resources for help and assistance.
Interference to Us
Lets deal with interference to us.
Squash obnoxious power line & computer hash/noise by a bunch! You can filter and reduce AC power line RFI, hash, noise, transients, surges generated by computers, motors, RF transmitters, static/ lightning by 30 db and up to 60-80 dB with good earth ground by using a good power line filter.
A tunable DSP audio filter can simultaneously eliminate heterodynes, reduce noise and reduce interference in nearly any mode. Adaptive noise reduction reduces fatigue and the high pass-low pass & tunable band pass filters give you effective tolls to dig out the weak ones.
Filters attenuate (reduce) interfering signals - but do not totally eliminate them. High-pass - generally on the receive side. Low-pass - generally on the transmit side. Band-pass - used within most radio
RFI snap-on ferrite chokes are selected for RFI suppression, you can: Install on DC power line to keep ignition noise from your mobile rig and amplifier; Remove RF on cables and wires to your radio station, including audio cables; Isolate RFI on feedline from your station. Many have a two-piece design, snap and lock securely around your cable. Install end to end along rigid cable or loop multiple turns with smaller soft cable. These are also referred to as common mode chokes. Ferrite Choke (Beads) can be used for the same purpose by running the wires though them.
Interference By Us
QRM - Hamspeak for Interference from nearby signals. Caused by other hams or other users of the frequencies. Operators should avoid interfering with other users of the frequencies.
Nobody owns a frequency, we all share what we have available. That being said, it is courteous and correct to allow hams who have a long time schedule (Nets - more info next module) for traffic on a certain frequency at a certain time to be able to use the frequency. If a group regularly schedules use of a frequency it is very little inconvenience to a single ham to move a few KHz away. This will happen more on simplex operation.
Repeater channels usually have scheduled nets and they are generally the owner of the repeater, so since they let others use the repeater when the net is not active it is more than courteous to wait until the frequency is clear.
However, when no scheduled traffic is present, it is just a big party line and we all need to play nice.
The FCC defines Harmful Interference as that which seriously degrades, obstructs, or repeatedly interrupts a radio communication service operating in accordance with the Radio Regulations.
This Interference is disruptive but not intentional. Usually caused by close by simplex operation on very close frequencies. Seldom a problem on VHF frequencies, often occurs on HF frequencies. Deal with it as best you can and help others avoid harmful interference..
The proper course of action if your stations transmission unintentionally interferes with another station is to properly identify your transmission and move to a different frequency.
If you receive a report that your stations transmissions are causing splatter or interference on nearby frequencies, you need to heck your transmitter for off-frequency operation or spurious emissions.
Spurious emissions are kind of noisy unwanted emissions that are beyond the frequency you intend to use. They can be caused by problems with your transmitter, but more often are caused by improper operation. For example, over modulation of your transmitter, especially on digital modes. Check your mike gain or ALC (Automatic Level Control) if your transmitter has one. There are some inexpensive programs and equipment that will monitor your digital signals and show poor modulation results.
Generally, modulation that drives your transmitter to 1/2 its rated power is best for digital operation, both for the transmitter components, and for a clean output signal.
Willful Interference is defined by the FCC as 'Intentionally causing Interference.' This becomes a legal and law enforcement issue. This is rare and there are procedures to deal with this. (ARRL Official Observers can help). This is like road rage. As in other aspects of life, some people become belligerent or are just plain nasty. This kind of behavior is seldom caused by licensed amateurs, rather by unlicensed people. A licensed amateur causing harmful interference will soon find themselves fined or having their license revoked by the FCC.
Remember, use common sense and courtesy. Keep your equipment in proper operating order. No one owns a frequency; be a good neighbor and share. Yield to special operations and special circumstances.
Code of Federal Regulations, Title 47, Part 15 (47 CFR 15) is an oft-quoted part of Federal Communications Commission (FCC) rules and regulations regarding unlicensed transmissions.
It is a part of Title 47 of the Code of Federal Regulations (CFR), and regulates everything from spurious emissions to unlicensed low-power broadcasting. Nearly every electronics device sold inside the United States radiates unintentional emissions, and must be reviewed to comply with Part 15 before it can be advertised or sold in the US market.
Part 15 contains a general provision that devices may not cause interference and must accept interference from other sources. It also prohibits the operation of devices once the operator is notified by the FCC that the device is causing interference.
Frequently encountered types of "Part 15" transmitters include:
- 802.11x wireless LAN (e.g. WiFi) 2.4 GHz, 5 GHz
- 802.15x PANs (e.g. Bluetooth, ZigBee) 2.4 GHz
- Cordless phones: 900 MHz, 1.9 Ghz, 2.4 GHz, 5 GHz
- Low-power broadcasting, often by hobbyists, or on college or high school campuses.
- Small FM radio transmitters designed to hook to the audio output of an iPod or other portable audio device and broadcast the audio so that it can be heard through a car audio system that is not equipped with an audio input.
- Very low power transmitters, often referred to as "talking roadsign", "talking houses" or "talking billboards", which will air a repeating loop of highway construction, traffic, promotional or advertising information. A sign placed near the transmitter is used to entice passersby (nearly always in automobiles) to tune in. The talking house gets its name from the fact that many such transmitters are installed at houses that are up for sale, thus enabling a passerby to find out details about the interior of the house without actually touring the building. Many talking houses have been noted by DXers apparently using unauthorized power levels and antenna systems, and thus audible far beyond the limitations authorized under Part 15. The FCC has also found some of these devices to exceed their limits.
- Some wireless microphones and headsets that broadcast to a receiver which amplifies the audio. Wireless microphones allow the user to move about freely, unlike a conventional microphone, and are thus popular with musicians. Some professional wireless microphones and 'low power auxiliary' stations(including those labeled as "UHF") must be licensed under Part 74, Subpart H of the FCC's rules. However, as of January 2010, many professional wireless microphones, and other Part 74 certified 'low power auxiliary' stations with a 50 mW output or less, can be operated in the "core TV band" (TV channels 2 through 51, except 37) frequencies without a license under a waiver of Part 15 rules. This waiver is expected to become permanent. Units using the high UHF channels (700Mhz band) revoked from the TV bandplan in June 2009 became illegal to operate in June 2010.
- Toys such as the popular late-1970s Mr. Microphone and its imitators, which would broadcast the user's voice to a nearby radio receiver. Variations on this type of transmitter were advertised for sale in radio magazines as far back as the 1920s.
- Walkie talkies intended for children's use, baby monitors, and some older cordless phones all operate on frequencies in the 49 MHz band (or rarely at the upper end of the AM broadcast band) and have been known to interfere with one another.
- Remote Controls for various toys, garage door openers, etc.
RFI from and to unlicensed devices is the responsibility of the users of such devices Bottom line - If your station is operating properly, you are protected against interference complaints. BUT - Be a good neighbor because they may (probably) not be familiar with Part 15 rules and regulations.
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RFI is an important side of Ham Radio. The ARRL site has information and links. Google can be your friend.
View the complete Technician Question Pool in Chapter 11 of your Ham Radio License Manual.
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