RFI Noise And Filters
Understanding And Solving RFI Noise
CB world Informer Article on RF Noise, Interference, and Filters. January 1997 Issue.
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With the increasing popularity of complex and computer-aided home telephone units manufactured by numerous companies worldwide comes a corresponding increase in both the quantity and severity of telephone interference caused by local radio broadcast transmitters. The new telephones, both domestic and imported, are the most susceptible and delicate ever built, and few manufacturers of the devices have given much consideration or effort to designs which include resistance to RFI and other forms of electromagnetic interference.

Making matters worse is the fact that ground terminal connections for telephones are rarely available at the location of an extension telephone, and telephone wiring is all unshielded and exposed.

Telephone interference is caused by radio signals produced in the immediate vicinity that are intercepted by the mass of telephone wiring in the home and outside on telephone company elevated wiring. Signals enter the phone on different conductors, working their way through phone circuits and causing current flow, resulting in voice distortion or noise.

Most telephone interference can be remedied by the simple installation of a telephone filter that plugs simply into the rear of the phone. These devices are designed to insert a choking effect, or loss at radio frequencies, into the phone wiring. They have no effect on the telephone operation. About the only realistic way to resolve phone interference short of making internal circuitry changes to the phone is by choking the RF signals before they enter.

There are two ports of entry that interfering signals can enter a phone unit. The first, and most common, is through the house wiring and into the telephone set directly as mentioned above. The second is through the handset cord (cord attaching the handset to the phone body). In nearly all cases a telephone line filter will be part of the solution to eliminate the interference. But in cases where the line filter is not completely effective another small filter device in the handset lead may be needed to bring back quiet enjoyment of the unit. An easy way to judge for yourself if the handset cord is suspect is to make a short handset cord about 6-12 inches long with the cord and tools available at most radio parts stores. If the interference is not present when the short cord is used to connect the handset with the phone body then the handset cord will probably have to have its own filter installed. If the phone has a speaker then simply disconnect the handset cord and run the test with the internal speaker (and a line filter installed).

If a line filter and handset filter are both installed and interference persists, then it's time to recognize that the telephone itself is inherently hypersensitive to external electromagnetic fields. Possible cures are replacement of the phone with a different type or brand, or internal circuitry modifications done by a local technician. Generally in our experience the worst offenders of telephone interference susceptibility @ are AT&T and Panasonic manufactured units. The best performers are built by Radio Shack/Tandy.

If you go shopping for phone filters obtain a unit with at least 30db measured attenuation in the RF range (3 to 30 Mhz.). If the filter manufacturer doesn't publish his figures, shop elsewhere. And get one that is designed to prevent BOTH common mode and differential mode interference.

For the most part telephone interference is the easiest type of interference to deal with, but sometimes it can be insidious. Don't be afraid to experiment with different combinations of filters, phone locations, or lead lengths to seek a final conclusion.


Lowpass filters are primarily a passive device used in the transmission and reception of radio signals in the BF frequency range (3-30 Mhz). Their intended purpose is to prevent the radiation of signals above 30 Mhz. that often emanate from transmitters due to the mixing of various signals in the transmitter's internal circuitry. The filters should be thought of as a frequency-selective bypass device. 'Mat is,- the unit will pass -through without attenuation (loss) those transmitted and received signals below 30 Mhz and short circuit (between coaxial line center conductor and outer shield conductor) those signal products whose frequency is above 30 Mhz.

The point at which the loss through the filter is measured as -3db (half of the power lost) is called the cutoff frequency. Above this point as frequency increases attenuation also increases, usually at a rate of rapid ascent. Lowpass filters in receiving operations work the same way. They prevent the reception of frequencies above 30 Mhz. which, generated locally by broadcasters can frequently disturb HF reception.

Many filters produced over the past 30 years or so have been either poor by design or installed by the user in such a way that the filter's ability to work was compromised, or both. The result was the expense of a lowpass filter that did not contribute to enhanced station ability or reduction of interference.

Here's what to look for when selecting a good lowpass filter. First, find a filter whose cutoff frequency is close to 30 Mhz. Many filters don't reach the amount of frequency spectrum between 30 and 450 Mhz. that is allowed to pass through. There's plenty of possibilities for interference and noise to occurring this range. If you're only interested in 30 Mhz. and below it's best to decide up front to get rid of everything else. Further, a low cutoff point pushes the VHF frequency arrange above 50 Mhz. farther into the stopband of frequencies where the attenuation is greatest. Second, be sure that the filter has sturdy housings and is not put together with "pop" rivets or hardware that will corrode and rust.  Third ask the manufacturer for a typical sweep curve of the filter so you can gauge the performance against other companies' published figures. If the figures are unavailable, shop elsewhere. Ask what insulation material is used and what the expected voltage breakdown of the filter is. If it's not insulated with a modern material such as Teflon sheet or thick mica and insulated to 2,000 volts or higher, shop elsewhere. Ask what kind of warranty is offered, if it's not at least one year and unconditional, shop elsewhere. Ask what kind of impedance passivity the filter has. If its VSWR at 50 ohms is greater than 1.2 to 1 anywhere in the passband (DC-30 Mhz.), shop elsewhere.

Once a filter is selected and purchased it's up to you to install it properly. Most filters are installed by simply connecting coaxial lines and hanging the filter in open space or mounting the unit to the rear frame of radio gear. But try to keep in mind that the filter is used to remove VHF energy above 30 Mhz. Once the removal is accomplished the VHF signal, is applied to the case, and if the case from that point to ground is long (more than several feet) the signal will easily re-radiate or simply not be absorbed and the value of the filter will be lost. Always mount the filter at ground level and as close as possible to a ground rod connection point. Keeping the leads short ensures that high frequency energy will be directly shunted (absorbed) by the earth, and hence removed from the transmission line. Mount the filter outside if you have to and cover with a rainproof enclosure but always keep those leads short - then relax and enjoy!


Highpass filters used with modem television receivers are passive devices intended to block the reception of frequencies below 54 Mhz. and allow to pass signals above that frequency. The television range used today extends from 54 Mhz. to 806 Mhz., not inclusive. Cable television frequencies extend from 54 Mhz. to 300 Mhz. in most systems but as high as 500 Mhz. in some of the larger cities with 70 or more channels.

When interference occurs to TV reception it's important to try to recognize first what the nature of the specific case is and from where it comes. If voice and video distortion both occur and it is believed that a strong local transmitting source -such as a CB or Amateur Radio station is involved then it's generally pretty easy to determine what to do next. Here's how.

If the interference occurs to only one TV channel or perhaps two channels spread way apart then the most likely cause is harmonic signal generation from the transmitter source. This type of interference can only be solved, at the transmitter by filtration and it's not the most common type of malady. A more frequent type of interference is when the local transmitter interrupts the reception of many or all channels, inducing wavy lines or audio noise into the system. This specific case is called fundamental overload and is caused by large signal voltages present in the immediate area.

There are two primary ports of entry that locally generated radio signals can reach and disrupt TV circuitry. The first is through the TV's antenna or cable line and the second is through the AC power line. Here's how to tell which case you have. Disconnect the antenna or cable line from the back of the set, and drop it to the floor. Have the station owner transmit again and observe the screen. If interference disappears then you know that the offending signal was entering the, TV through the antenna line and a highpass filter installation is the next step. If interference persists then the AC line is part of the problem and AC line filter may also have to be installed. Either way, a combination of simple disconnection tests can provide a wealth of data from which a solution can be drawn.

If a highpass filter is part of the program here's how to choose an appropriate unit. Be sure that the filter is designed to attenuate BOTH common mode and differential mode interference. Common mode is where the shield and center conductor of the TV's antenna coaxial line are both electrified by a locally generated signal. Differential mode is where the center conductor alone is electrified and the shield maintains its neutral (ground) integrity. Common mode is the most common of modem cases, and a good highpass filter should have a common mode loss of 20db or more. If manufacturers do not publish their loss figures, shop elsewhere.

Always mount the filter as close as possible to the input (antenna) connector of the TV set or VCR. Generally it's best to place the filter between the incoming antenna or cable lead and the first item to which the cable lead is connected. But it may be more effective in some cases to connect the incoming, line to the VCR and install the highpass filter to the input connector on the TV receiver. It's important to keep leads short and connections tight.

But keep in mind that all cases of interference differ somewhat, and that experimenting with different combinations of protective devices is normal in the pursuit of good results. Make notes as you go and don't be discouraged if early results are not satisfying. Most interference cases can be solved without a great deal of investment or effort.


The success of all receiving operation, regardless of frequency or application, can be defined as the pursuit of a single goal  maximum signal to noise ratio. The larger the signal and the lower the offending background noise the better the reception.

Unfortunately, the strength of the received signal is, for the most part, a fixed quantity. With the antenna and receiver in use not. much can be done to improve. the delicate balance between signal strength and atmospheric noise.

But there is another type of noise that is all-too-common in the modern receiver setup, and it may be described as environmental noise. This is a type of noise that is usually wide spectrum, amplitude modulated (AM), and locally generated. It is especially a problem of the modern computerized world. Environmental noise is caused by local arcing connections of AC power lines, computer "hash" type noise emitted by typewriters, fax machines, television sets, VCRs, heating & cooling systems, and just about anything else electric in the home or office. Most of the noises do not travel a great distance but can cause harmful effects to radio reception and can be difficult to find and correct.

We have seen many sad cases of environmental noise. Station owners purchase expensive, delicate receiving equipment costing thousands of dollars and then suffer poor performance because of local noise generated on their own property or from a nearby source. So here's a simple, almost cost free method of hunting down these insidious noise gremlins. It's cheap, easy, and you may even enjoy the "hunt".

Put a PL259 or other connector on about 50 feet of RG58 coaxial cable. On the other end of the cable fray back the ends, cut back the shield, and attach about, 18" of wire to the center conductor (clip-leads work well). What you have created is a simple sensing antenna. Connect the cable end with the connector to a receiver that covers the 25 to 50 Mhz. frequency range, select a clear channel in that spectrum area, and place the receiver in the AM mode. Then move around the house or property with the sensing antenna and listen for noises in the receiver. Moving the sense antenna near electrical appliances will be very educational. It's almost hard to believe how much noise is generated by the computer in a fax machine or other computerized devices. Also fluorescent lights can be nightmarish! Best bet  if you're a serious listener issue a total ban on fluorescent lightning for as far away as you can dictate or negotiate. Fluorescent lights are based on an arcing principal and are very bad offenders.

Fixing noise problems in your own home or office is usually not difficult. Installing EMI filters on AC power leads of computerized devices ordinarily stops the AC line cord and house wiring from acting like a transmitting antenna for the noise. Commercial telephone RFI filters work well to prevent the same effect from fax machines. Just-about any grounding, shielding, or filtering methods are helpful if you're-after a secure, noise-free environment. Remember  those generated noises only have to transmit the distance between the noisy device and your receiving antenna, and that may be only a few dozen feet!

Power line noises must be repaired by local electric company workers, but most power companies accept noise complaints and deal with them internally. Finding the noise source yourself with an AM radio in your car or handheld unit is a big help to getting quick service. Keep in mind that an arcing high voltage line connection is both a point of power (and revenue) loss for the company as well as a fire and/or service loss hazard. Be sensible-take noise reduction just as seriously as you take receiver choice, antenna choice, or any other facet of good station design.


Proper safety grounding of telecommunications equipment is one of the most important but least understood elements of good installation practice. Earth neutral connections provide numerous benefits to equipment owners including personal electric shock safety, protection from voltage surges caused by lightning and power line delivery variations, and reduction or elimination of electromagnetic interference from nearby sources. Here  are some basic tips to follow when designing an installation in which grounding is an integral part:

  1. Start with the location. Electronic equipment, especially transmitting gear, should always be located at ground level or below ground where the distance from equipment chassis to the earth terminal connection point is as short as possible. In all cases try to keep the ground leads less than 10 feet in length running in a straight line. If an elevated site is mandatory then all connecting leads such as transmission lines, rotator lines, AC feeder lines, etc., should reach ground level first (where lightning protection can be installed) and then routed to their proper destinations (antenna on roof, AC system, etc.).
  2. Choose an electrode wisely. Don't use cold water pipes or AC service neutrals to achieve ground. Both of these often travel very long distances before actually reaching earth ground and they are often full of joint connections through these sources in transmitting applications frequently increases local interference because they become part of the radiation pattern at ground level. Grounding should always be done with the shortest distance to the actual dirt entry point where a rod may be driven. Ground rods come in many sizes but a.lengthof,6 feet or.-more is highly recommended. Use rods that have a bright dipped copper clad finish to the steel core or solid brass for best long term results. Keep the earth around the rod wet often to increase effectiveness and dissipation capability.
  3. Always add weather protection to ground rod connections. Products such as "Liquid Rubber," RTV Compound, commercial aquarium sealers, or roof patching tar make fine coverings for electrical joints and they'll prevent corrosion and rust. Use an anti-oxidant compound to coat the conductors before connecting them as a further protection from weatherization. Many are available, but among the better ones are Burndy Penetrox, Ideal Noalox, or I.C.E. #601 or 602. All are easily applied and available from electrical supply houses or hardware stores.
  4. What kind of wire to use in making ground leads? Copper definitely, but remember that the length of ground leads is far more important than wire size or type. Use conductors of #12 or larger, covered or bare. But keep 'em short!
  5. Always ground coaxial cable shields, but be sure to do it by routing the coaxial cable to the ground rod joint. Don't ground cable shields by attaching a wire to the shield in some convenient fashion and running a long length of wire from that point to a ground rod. The effect is mostly lost that way. Route the cable to the rod and insert a grounding block or some homebrew means, then route the cable to the equipment. As always, keep the leads short!
  6. Check the condition of ground connections every six months or so. Keep in mind that the rod connections are exposed to a big variety of outdoor vermin!


It's probably done most often for the simple convenience of time and effort, but there's little to be gained and frequently a lot to lose by using cold water pipes, gas pipes, and electrical outlet box connections as RF or lightning protection grounds.

Good grounding is a critical and integral part of good telecommunications station design. Whether the application is receive only, transceiving, data delivery, or otherwise modern solid state equipment is internally delicate, and good grounding is a key factor in maintaining clean spectrum operating and overvoltage protection. Unfortunately it is seen as a quick ten minute afterthought to many installations.

In their haste to finish ground connections are commonly made with a piece of "off the shelf" wire connecting radio equipment chassis to whatever is nearby that may eventually reach ground. The most important factor in good neutral connections is length of lead from chassis to earth entry point - not the specific materials of wire sizes used. Here are a few guidelines to follow when installing ground connection systems:

  1. Cold water pipes make poor grounds in most cases because the length of copper pipe to earth is often very long. Also lead over ten feet probably should be avoided for most applications. Additionally, pipes of this type connect through numerous solder-sweated joints, bends, and possibly even conversion to plastic pipe (a good insulator) before reaching ground. The fact that the pipe may have water inside is irrelevant. When such systems are used in transmitting service the piping becomes part of the radiating structure and ground level radiation will often be severe, causing interference to other services or neighbors.
  2. Never, ever, ever use natural gas pipes for ground connections. In a lightning event a seam crack or rupture of a gas line can be explosive. Hot water lines used in conjunction with gas water heaters should be avoided for the same reason. Be sensible-stay well away from dangerous ignition sources!
  3. When designing a telecommunication installation keep equipment at or below ground level if possible. Locate the equipment close to an outside wall where short grounding connections can be made. Or drive a ground rod through the floor downward into a crawl space if present where short distance ground can be found. Borrow or rent a hammer drill to drill a hole through concrete slabs or floors where a ground rod may be inserted. Ground underneath such places is nearly always moist and very conductive. If drilling through a slab be sure to avoid pipes that may be in the concrete! Consult the builder or house plans.
  4. If the facility must be elevated off the ground run ground wire straight down to keep the distance as short as possible, and be sure to route all antenna leads, rotator wires, etc., to ground first (where lightning protection devices are installed), and then up to the equipment.
  5. Electrical service box connections generally make poor grounds for the same reason as cold water pipes. The leads are lengthy, the wire size small, and the integrity of the earth connection is often compromised by age, poor initial installation, corrosion, dissimilar metal conversion, loose screws, etc.

The moral is simple - put some effort in good grounding. Keep leads short, wire size large, connections tight and weatherproof, and grounding electrodes wet. It will probably save you from more headaches than aspirin!


This is a subject that we just had to write about. In the lightning protection business we come into contact with many people who have had both dangerous and disastrous experiences with Mother Nature. And one that has perplexed antenna users for decades is the very common damage and destruction to radio equipment when connected to a so-called "DC Grounded" antenna system.

For many years' antenna manufacturers have touted the positive advantages of owning and operating a station with antennas whose feed systems are a direct DC short across the input terminals, and hence both sides of the coaxial feeder cable are placed at "ground" potential at the antenna site. In reality, there are no such advantages to this kind of feed system but it is singly the most dangerous ever used from a lightning perspective.

The reason is pretty easy to both explain and understand. Lightning bolts that streak from clouds to ground frequently hit exposed metallic structures like towers and high antennas. This is simply because the metallic nature of the object electrically shortens the striking distance between ground and sky. When a large voltage potential is reached between the two during a storm the metal antenna acts like a prod, sticking up in the air and drawing the first arc.

Lightning wants to reach ground, and that's pretty much all it wants. And it will get what it wants in the easiest and least resistive way possible. Just about anything in the way can be easily vaporized out of the way by a good sized lightning blast. If ten different paths to ground are presented to a striking bolt (such as numerous transmission line conductors, the tower frame, etc.) then the currents will divide quite  nicely between all of them, with the larger amount of current flowing in the path of least resistance and so on.

"DC Grounded" type antennas provide a very neat dual path for those lightning currents. Some of the blast will flow down the shield of the cable to ground level earth terminal connections while the rest will simply flow down the center conductor and ravage the radio connected at the other end. Keep in mind that at the point of impact a bolt of lightning can easily deposit 50,000 volts or more respective to ground. And for an instant the voltage at the radio equipment end will be the same. By the time the balance of the surge comes to an end the equipment will have long since been toasted, probably beyond repair.

The myth is that "DC Grounded" antennas offer good lightning protection. The legend is that antenna manufacturers have been claiming it for decades. The fantasy is that some of them still actually believe it. But it's not all hopeless. Here's how you can tell if your present antenna is one of these and what you can do about it. Disconnect the transmission line at the equipment end and measure across the center and outer conductors with a VOM on the R x 1 scale. If only a few ohms are measured then the antenna at the other end is a DC Grounded type. If you're satisfied with the performance of the antenna otherwise and wish to continue using it then you have two choices. First, disconnect the antenna whenever a storm approaches and hope you'll always be there to do it on time. Or second, install a blocking-type lightning arrestor that will shunt center conductor voltage to ground while blocking voltage from passing through the arrestor. Be sure to install the arrestor at ground level and ground the body of the device well.

If you're in the market for an antenna and wish to enjoy a bit of protection select the ones offered that use capacitor or link feed systems. Capacitor feed systems such as gamma matches are excellent feed systems and lightning protectors as well. They isolate the center conductor and force lightning into the shield.

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CB world Informer Article on RF Noise, Interference, and Filters. January 1997 Issue.


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