For Radio Reception Diffraction Is

Reception in some areas, although the extent of the problem should be far less than for analogue television. 3.5 Impact on other terrestrial broadcasts Although reports of new structures causing problems to radio reception are rare, the possibility of difficulties cannot be ruled out entirely. Broadcast radio (FM, AM and DAB. The diffraction of radio waves on the spherical surface of the earth is one reason for the reception of radio signals beyond the limits of direct visibility when the transmitter and receiver are separated by the curvature of the earth. Diffraction is very important for radio communications! Diffraction is when waves bend around the corner of an obstacle. Medium gap: some diffraction, but mostly straight. Gap of wavelength size: most diffraction. The maximum effect is when the gap and wavelength are about the same size. Radio waves with wavelengths of kilometers diffract. Transmission and Reception. Radio waves surround us all the time, but the only way we can pick them up is with a radio receiver. The term radio also refers to the technology that allows information to be transmitted and received over radio waves. Signal diffraction isn't quite the same phenomenon as signal reflection. Signals diffract, or bend, around obstructions that can be many miles away, but the obstructions don't cause signal reflections by themselves. (That's not to say diffracted signals can't ever be reflected by other objects, however.).

10/24/08

Choosing a mounting site

Diffraction

Diffraction is the ability of a wave to bend around into the shadow formed by an obstruction.It doesn’t matter whether it is an absorbing or reflecting obstruction.Most OTA viewers depend on diffraction for their reception.The only exceptions are:

·Where the transmitting tower can be seen.

·Sometimes in cities with tall buildings reflection is more effective than diffraction.

VHF Diffraction

The direction the signal is moving is always perpendicular to the wave fronts.Thus if an antenna is mounted in the shadow of a building, the antenna should point at the top of the building, because that is where the wave is coming from.

Low frequencies diffract efficiently, but VHF diffracts poorly.UHF is another ten times worse.

UHF Diffraction

These diagrams use linear shading and thus are perhaps overly pessimistic.Reception might be possible where these diagrams show no signal.(Logarithmic shading would convey more optimism.)

TV Transmitting power allowed by the FCC:

To make up for the inability of UHF to reach into valleys, the FCC allows UHF stations to have higher power.

Analog:DTV:

Channels 2-6 :100 kilowatts50 kilowatts

Channels 7-13 :316 kilowatts160 kilowatts

Channels 14-69 :5 megawatts1 megawatt

The above numbers are approximate.The actual power rules are more complicated than this table, and stations can argue for and get a higher limit.But the goal in most cases is a 60-mile reception radius.

The above power numbers are ERP numbers (effective radiated power).ERP is defined as the transmitter’s RF power output times the gain of the transmitting antenna.UHF transmitting antennas usually have higher gain, so the disparity in transmitter electric bills is not as great as this table suggests.

Ground reflections

Often the signal waves are angled downward slightly, usually the result of diffraction over an obstacle in the distance.If there is mostly-flat ground in front of the antenna, the ground reflection can be efficient.

Instantaneous voltage diagram:Onedrive plex.

These two waves pass through each other without affecting each other.But the antenna responds to the instantaneous sum of the two overlapping waves.Where the two waves subtract, there will be places where reception is very weak.

Average power diagram:

The result is a striped region of alternating strong and weak layers parallel to the ground.Thus there are cases where lowering the antenna might put it in a stronger signal.So, while the following siting technique is unorthodox, the result is very credible:

ã King Features Syndicate.Reproduced here with permission.

What

Unfortunately a strong spot for one channel can be a weak spot for a different channel, so compromise might be necessary.

The ground doesn’t have to be as flat as you might guess for these layers to form.Weeds, shrubs, and trees are mostly transparent to VHF.A surface wet from rain will usually be 100% reflective.

Average power diagram for UHF:

This layering problem is greatest for UHF.The distance from a very strong spot to a very weak spot can be as little as three feet.But weeds might save you.Stand where the antenna will go and look toward the ground in the direction of the transmitter.If you see weeds or shrubs or trees, you are OK.If you see lawn or dirt or pavement then you likely have some layering.

An antenna has an aperture, over which all incoming signal is collected.In this diagram the aperture is positioned to collect signal from two layers.But adjacent layers always have opposite polarity, and subtract.Thus this antenna is picking up no signal at all (assuming a 100% efficient ground reflection):

For Radio Reception Diffraction Is The Study

When layering is present, if a larger antenna is necessary, choose an antenna whose aperture is wider, not taller.Otherwise you may find the new antenna works no better than the old one.

The ground reflection can be very helpful.Assume the power in the incident wave is P.If the reflection is 100% efficient, you might expect the power in the overlapped area to be 2P.But instead it will vary from 0P to 4P.(Power is the square of voltage.Where the voltage doubles, the available power goes up by 4.)

If you can put the antenna in the most intense spot, it will collect 4 times as much signal as with no ground reflection (assuming a 100% efficient ground reflection).

If a tree loses its leaves in the fall, reception behind it will improve dramatically.Many people get a TV for Christmas, and erect an antenna for it in January, and then wonder why it quit working in May.It’s the trees.

In the following simulation, a tree is modeled as a perfect sphere blocking 90% of the signal.

(The simulation was in 3 dimensions.The diagrams show the field strength in a plane through the tree center.Due to symmetry the diagrams look the same when viewed from above.The obstruction was coded as a disk, not a sphere, but the difference is minuscule in most places.)

If the antenna is behind a tree, it is in overlapping fields: a weak field that passes through the tree plus a weak field that is diffracted around the tree.Overlapping fields are complicated, with strong spots and weak spots.This will be true even if the tree is not a perfect sphere.If you get a UHF antenna to work behind a tree, you will likely see dropouts when the wind blows because the strong and weak spots will move around as the tree deforms.Even in a good-signal neighborhood it is inadvisable to put a UHF antenna behind a tree.

The farther away a tree is, the less of a problem it is.For far away trees, assume no signal penetrates the tree, and reception will be by diffraction around the tree.Trees block 100% of satellite signals.

Trees and VHF-high(The tree blocks 60% of the signal.)

In this case the wake tendrils are very broad.The tree is not likely to deform enough to cause a dropout.Reception might be slightly sensitive to wind.

Trees and VHF-low(The tree blocks 30% of the signal.)

An antenna in its wake will work fine for channels 2-6.

Skyline Multi-path

Diffraction

The following four diagrams are identical except for the view rotation.


Rays from the transmission tower come to earth after passing over a skyline ridge.This ridge could be a tree line 50 yards away or a mountain ridge 5 miles away.The rays diffract at the ridge, staying in a plane perpendicular to the ridgeline.The result is often overlapping rays.

Overlapping fields will result in weak signal spots and strong spots arranged in a regular pattern.

For UHF the strong and weak spots are often 5 to 20 feet apart.If you are in a neighborhood with overlapping fields, moving your antenna a few feet can make a huge difference in signal strength.The chimney might seem like the perfect site, but if the chimney is in a weak spot then the chimney is a mistake.

To make matters worse, the pattern of strong and weak spots will be different for different frequencies.You will want to find a spot that is strong for all the channels you want.But such a spot might not exist above your roof.In this case you must search for a spot that is the best compromise for your must-have channels.In the worst case you might need two antennas and a switch.

To make matters even worse, you will not likely discover that you are in such a neighborhood until after you have purchased and installed the antenna.To prove that you have strong and weak spots, you move the antenna (leftward and rightward, higher and lower) while keeping it perfectly pointed at the signal and watching the DTV signal strength indicator.(What?Your TV is not on the roof?Well maybe with some cordless phones you can get your wife to help you.Tape the phone to your head.)It is hard to keep a large antenna pointed correctly while devoting half of your attention to not falling off the roof, but a smaller antenna might not achieve a digital lock.

At this point a professional installer starts to look like the smart choice.But will he stick with it, or will he too quickly declare further improvements impossible and walk away?He will hesitate to raise his estimate, but he will not work at a loss.

These problems are UHF problems. VHF does the same thing, but with strong and weak spots 50 to 200 feet apart they are not very evident and there is usually not much you can do about them.

Non-uniform fields

Overlapping fields result in non-uniform fields: layered and continuously varying.An antenna in a non-uniform field doesn’t perform quite like one would guess.Normally an antenna captures all the radiation within its aperture.But in a non-uniform field some signal gets rejected.

Many people in this situation conclude they need a bigger antenna.But a bigger aperture gathers signal from a larger area, and this larger area is usually even more non-uniform, causing greater signal rejection.Many people have switched to a larger antenna and found no improvement.

There is more than one way to explain this counter-intuitive result.(These explanations sound totally different, but they are equivalent.)Probably the simplest explanation is that the antenna’s beam width gets narrower as the aperture gets bigger.The bigger antenna is now so directional that it cannot be pointed directly at both sources that produce the overlapped field.

A solution to this dilemma is an asymmetric aperture.Choose an antenna whose aperture is large in the direction of the layer, but small in the direction across the layer.Stacked antennas will have such an asymmetrical aperture.

Is a higher antenna always better?

No, for the first house below:

But after the signal has skimmed over several equal-height obstacles it will be necessary to go up about 5 wavelengths to find a full-strength signal, even if the transmitting tower can be seen from a lower spot.

For channel 2, five wavelengths would be 86 feet.A mast that long is impractical.But below 86 feet the signal strength is roughly proportional to the square of the height.Thus the rule of thumb: “Higher is always better” for VHF.

For the following text, “skyline” will mean the highest obstruction your antenna can see.The skyline could be the top of a house or distant hill.The top of a tree could be the skyline for UHF, but not VHF-low (for which trees are transparent).

The rules for UHF are a little more complicated than for VHF.UHF is more affected by obstructions and less affected by height.For UHF, 5 wavelengths is only about 10 feet.But a UHF antenna should be higher than this in these cases:

1.If at all possible, get the antenna above any obstructions.

2.If your skyline is less than 200 yards away then raising the antenna makes a significant difference.You would be a candidate for a tower.(When the skyline is farther than 200 yards, the benefit is usually too small to justify the effort a high mast requires.)

You will probably want to attach a VHF antenna to your chimney.That is also likely the best place for a UHF antenna.But if your chimney mount is still obstructed (by trees, etc.) then an unobstructed site closer to the ground will work better for UHF.The essential goal is to find a spot where your UHF antenna can see a distant skyline in the direction of the station.

Note that on the front of a hill, the antenna height often makes little or no difference (VHF and UHF):

Power lines

Power lines will reflect the signal.But that is just one reason to keep antennas away from power lines.If there is RF noise in the power lines, the lines will transmit the noise to a close antenna.

Many people have been killed when their antenna fell into power lines.Channel Master recommends that the antenna be kept away from power lines by a distance of twice the mast length plus twice the antenna length.

Attic antennas

If an indoor antenna is not as reliable as you want, an attic antenna is the next step up.If you are in a neighborhood with moderately strong signals, an attic antenna might work.But you are wasting your time installing an attic antenna in a poor-signal neighborhood.Most successful attic antennas are within 20 miles of the transmitter. (30 miles often works if you are on a hillcrest.)The problems with attic antennas are:

  1. The antenna might not be high enough above obstacles outside the house such as trees.
  2. It is hard to estimate the signal loss caused by the wood and other construction materials.
  3. Metal objects in the attic can block the signal.

Estimating the signal loss in ordinary construction materials requires knowledge of their water content.Exceptions are aluminum siding, stucco (which has an embedded metal screen), and foil-backed insulation, all of which totally block all signals.Concrete and most bricks have moderate water content, but their thickness is enough to block all signals.In a desert, plywood becomes so dry that it causes no signal loss at all, even for UHF.In any other place, there will be some moisture.Exterior wood is generally always wet inside, especially in north facing surfaces.(Paint does not prevent this.)The amount of water varies with the weather.Dry asphalt shingles are mostly transparent to TV signals, but the way they overlap encourages water to persist between them.The vapor barrier is often wet on one side or the other.The bottom line is that there is no way to predict the signal loss in these materials.It is usually a mistake to point an antenna through a surface that gets totally wet in rain.

Metals reflect signals.A metal object 8 inches long is big enough to reflect UHF.Smaller objects, such as nails, are of no concern.Wires and metal pipes effectively reflect VHF, as do plastic pipes containing water.If these reflecting objects are positioned to the side, to the rear, above, or below the antenna, they will have little effect on it, provided they are not too close.These objects should be further away than 2 feet for UHF, 4 feet for VHF-high, or 6 feet for VHF-low, and an even larger separation will help a little.(Some might wonder why these numbers are not proportional to the wavelength.It is because the lower frequency antennas are lower in gain.An antenna’s aperture depends on the gain as well as the wavelength.)

There should be no horizontal or diagonal wires or pipes in front of the antenna.A perfectly vertical metal vent pipe is invisible to TV signals, but its flashing at the roofline might not be.

Inner-city antennas

In good-signal areas, small low-gain antennas may work fine.Indoor antennas nearly always work up to 10 miles from the transmitter.They often work up to 20 miles for people who live on hillcrests, and sometimes 30 miles if the transmitting tower is visible.But if http://www.antennaweb.org says you are not a candidate for an indoor antenna then don’t waste your time and money on this.

If you are in a city and the transmitters are all around you, you might get tired of getting up to re-aim the antenna when you change channels.In that case, an omni-directional antenna, like a disk antenna, mounted in your attic might serve you well.But an omni-directional antenna will not work for DTV in a multi-path situation.Multi-path in cities occurs mostly when there is a big building blocking the direct path to a TV tower.If you see ghosting on some of your analog channels then you are probably not a candidate for an omni-directional antenna.See Omni-directional antennas.

Common misconceptions(optional reading)

What about reflections?

Many websites say ghosts are caused by reflections off nearby objects.But this is true maybe 1% of the time.Most such reflections are spherical waves that die off quickly due to spreading, too quickly to be delayed enough to produce a ghost.Only a plane surface can produce a reflection that will persist long enough, and most natural flat surfaces are at an angle that will reflect upwards away from any antenna.Building surfaces can produce ghosts.But like for a mirror, the reflection off a building surface is very directional.

What about front-to-back ratio?

Many websites say a high front/back ratio will block ghosts that arrive from the rear.But ghosts normally come from the front and are caused by the most direct path being blocked.

For radio reception diffraction is

Conceivably an obstruction could be just high enough to block the antenna but not high enough to block the signal reflecting off a building to the rear.But the blockage will necessitate a high gain antenna, and all such antennas have a high front/back ratio.So again the front/back ratio is something that never has to be considered.See Front/back ratio.

Related topics

Fading(Why a signal drops out for no apparent reason)

Field strength meters(for finding the strongest spots)

Fringe area reception

Multi-path interference(There are two kinds)

Painting an antenna(Is this OK?)

This page is part of “An HDTV Primer”, which starts atwww.hdtvprimer.com

NCPR broadcasts throughout the North Country on FM. FM is a truly wonderful thing … if handled correctly. To receive and enjoy our quality programs to the fullest, you'll need the three A's of radio:

  1. A good radio
  2. A good antenna
  3. A good location

If you have all three of these, you could potentially receive an FM station up to 100 miles away! However, most of us have, at best, only one of these items…

RADIOS: For good reception, a radio must have good selectivity (selectivity is a radio's ability to separate weak stations located nearby, on the FM dial, strong stations) and good sensitivity (the ability to receive weak, distant, stations at all!)

Car Radios: You already probably have a radio with these features. This radio is located in your automobile. Car radios have to be built to high standards to provide decent reception in a moving vehicle, in the presence of varying terrain, with a serious nearby source of interference (your engine!) … all while being bounced around on North Country roads. You've already probably noticed that FM radio reception is usually better in your car than in your house. This is partially because your car radio is probably better than your home radio.

Home/Office radios: Most home radios have poor selectivity and sensitivity (we like to call them 'junk' in the radio business). Typical $19.95 radios with analog tuning (as opposed to digital tuning, where the radio station's frequency is displayed in illuminated numbers) will work, but only near a radio station's transmitter. Most clock radios, under kitchen cabinet radios, 'boom boxes' , crank-up radios, etc. just don't work very well… especially when compared to a car radio!


Boston Acoustics Horizon Solo

Boston Acoustics Horizon Duo

Tivoli Model One

Sangean WR-2

SONY XDR-F1HD
Examples of diffraction

RadioBob Recommends:

  1. Ripping a car radio from a car, building a 12 volt DC power supply, constructing a really cool Honduras mahogany or Purpleheart cabinet, and finding some nice external speakers.
  2. OK, seriously, there are a few great radios out there… yes, they do cost more than a $19.95 plastic throw-away radio… but they perform MUCH better. They sound great, and are a quality product. Today, (early 2009) I'd recommend the Boston Acoustics 'Horizon Solo' clock/table radio for about $100. Or the stereo version (the Boston Acoustics Horizon Duo) for $150. Then there is the Tivoli 'Model One' if you don't need a clock, and like a 'retro' analog tuning dial (around $140)… or even the button-filled Sangean WR-2 (also around $140)
  3. If you happen to have a component 'stereo' system (typically separate amplifier/tuner, with separate speakers) your tuner already might have decent sensitivity and selectivity …. Try connecting a better antenna (see below) and see how your reception improves! If you'd like to buy a state-of-the-art FM tuner (at a very reasonable price) get the SONY XDR-F1HD (under $100). Reviews have stated that this is one of the best tuners ever built! Remember that you have to have an external amplifier and speakers for this one!

For Radio Reception In City Buildings Diffraction Is

ANTENNA/LOCATION LOCATION/ANTENNA ANTENNA/LOCATION LOCATION/ANTENNA

These two items are actually MORE important than the kind of radio you're using… and they are related in a big way. You can get the same reception in a bad location (using a great antenna) as you can in a good location (using a bad antenna)! But lets work on improving both!

Statement from Radio Bob: You MUST have an antenna (of some kind) to receive any signals on a radio!

Another statement from Radio Bob:FM Radio Waves travel more-or-less in straight lines. They are weakened by objects that get between the transmitter and receiver.

LOCATION:

  1. The closer you are located to an NCPR transmitter (check out this map) the better chance you have of receiving a clear signal from NCPR.
  2. The higher up your antenna is located, the better chance you have of receiving a clear signal from NCPR. In other words, if your radio has a built-in antenna, it will work better in your attic, than in your basement! Or if you have an outdoor antenna, it will perform better on the roof, than on your kid's old swing-set.
  3. If your house is on a hill, you'll get better reception than if it is in a valley.
  4. if there is a large object between your house and the NCPR transmitter (like a mountain, for instance) you will probably receive a poor signal!
  5. if your antenna is outside, it will perform better than if it is inside.


ANTENNA:

Someone once said that you MUST have an antenna to receive any radio reception at all, and she/he was correct! The least expensive radios ONLY have built-in antennas, with no provision for connecting an external antenna. With better radios, you have a choice.

Polarization Is A Property Of


  1. A radio with a built-in antenna
    Built-in antennas: Even the lowliest radio has some sort of antenna… typically built-in, with typically poor performance. Most clock/table radios use the power cord as an antenna. A 'walkman' or iPod with an FM radio uses the headphone cord as the antenna! Except in strong signal areas (nearby a transmitter) none of these perform very well. Signals they receive are usually variable; for example, when you walk around the room, the signal strength will change (usually for the worse!) Here's a photo of a radio with a built-in antenna:
    Radio Bob Recommends: If your radio has a built-in antenna, and you are nearly satisfied with its performance, try moving the AC power cord around… it may work slightly better draped over the dresser, instead of lying on the floor.

  2. Wire 'dipole' (usually supplied with better radios): This is a flexible wire antenna that comes packed with some radios, including the few I recommended earlier. Using this antenna will improve reception somewhat… but as with ALL antennas, it's not the perfect solution. It is attached to the back of the radio, then 'strung up' somewhere in the room as a 'T'… with the two ends extended as far as possible from each other. (this is the difficult part, because who wants an unsightly wire tacked on the walls of their living room…not even me!) It is also somewhat directional; it will (in theory) pick up radio stations better perpendicular to the horizontal portion of the 'T'. So if you listen to several stations broadcasting from different directions this might not be ideal. (you might try snaking the antenna out a window and somehow tacking up the 'T' portion outside for improved reception)
  3. Three antenna types: rabbit ears (above left); outdoor directional (upper right); outdoor omni-directional (lower right)
    Telescoping antenna(s)/'rabbit ears': Some 'boom boxes' and portable radios have one or two telescoping antenna rods. These perform somewhat better than the wire 'dipoles' because you can move the one (or two) rods around for optimal performance. You can buy a pair of 'rabbit ears' from Radio Shack for around $10.
    Radio Bob Tip: Don't bother with other indoor antennas that do NOT have a pair of unsightly long rods with them. Circular, ash-tray sized and other types of indoor antennas are meant for UHF television and won't work very will with an FM radio.
  4. Amplified indoor antennas: People ask me every day (well maybe every few months) about these things. … and I guess that my answer is something on the order of 'it all depends' . There are several of these available, manufactured by Terk, Audiovox and others… they are dipole or similar antennas, in an 'attractive' case with a small pre-amplifier intended to boost the signal before it gets to your radio. Technically there are several things wrong with this approach…. Typically your radio already has a very good pre-amplifier built in to its circuitry. A 'dipole' antenna connected to your radio should work just as well as an amplified antenna (if your radio isn't somehow 'reception challenged' that is!) The additional pre-amplification sometimes causes increased noise in the reception, especially if there are strong FM stations in the neighborhood and you're trying to receive a weak one!
    On the other hand, if you can't have an outdoor antenna, and an indoor dipole or pair of 'rabbit ears' is too unsightly, then a slick-looking indoor antenna might be the best for you…. However, I don't think it will 'work wonders' with your FM reception…. Let me know how they work for you. Email [email protected]
  5. Outdoor Antennas: If you can somehow manage an outdoor antenna, this is the way to go! Apartment dwellers, renters, cave-dwellers, submarine operators…. I'm sorry, as I know you probably can't install an outdoor antenna.
    The best thing for the rest of us to do is to use (or re-purpose) a TV antenna that's already on your roof. If you still use it to receive off-air TV, then get an inexpensive TV-FM splitter. Or if you've switched to cable-TV or satellite-TV, then just connect your old TV connection to your FM radio. You'll be surprised at how well this works!
    If you want to install a new outdoor FM antenna, there are two types to choose from, 'omni-directional' and 'directional'. An omni, will receive FM signals from all directions, like the Winegard HD-6010 ($20). This is a decent choice if you like to station-hop.
    If you mostly listen to only one station (NCPR perhaps?) or most of the stations you listen to are located in (more or less) the same direction, or you could use an antenna 'rotator', then you need a directional antenna. NCPR typically purchases $800 heavy-duty log-periodics like this puppy, but you can do almost as well yourself with a Winegard HD-6000FM ($25).

Installing an outdoor antenna can be fun, challenging, or both! Here's how:

  1. The easiest way to do this, is to have someone else do the installation. Hit the Yellow Pages or ask around.
  2. To do it yourself, 'somehow' mount your new FM antenna on a metal pipe. (you can get antenna mast in 5 and 10 foot lengths). Somehow, secure the pipe to the roof or side of your house (remember the higher the better!) . You can get tripod roof mounts and wall mounts from Radio Shack as well as antenna mast. (be careful, by the way!)
  3. If it's a directional antenna, aim it towards the station of your choice (actually its better to wait until the antenna is connected to the radio, then manually rotate the antenna for best reception and lock it down) Or purchase an antenna rotator… as well as sufficient cable to connect it to the rotator control unit, which will be inside your house somewhere.
  4. Then you need to somehow get the signal from your antenna down to your radio. In the 'old' days, a flat cable about ½' wide called 'twin-lead' was all that was available. Now everybody uses round coaxial cable. Your new antenna will probably have two screw terminals on it, so you'll need a coaxial 'balun' or matching transformer to connect to this type of antenna. The matching transformer will have an 'F' connector on the end away from the antenna. You can purchase coaxial cable with mating 'F' connectors already installed, or purchase a special crimp tool and install your own 'F' connectors on coaxial cable you purchase separately.
  5. Route the coaxial cable through the cellar, etc to the radio. Newer radios will also have an 'F' connector on them… and a switch, labeled something like 'internal' and 'external'… connect the new cable from your new antenna to your radio, switch to 'external' and hear more FM stations than you've ever heard before!

INTERFERENCE: Wait! I didn't mention interference! (However, as adults, we should at least discuss it)

The atmosphere is FULL of radiofrequency signals, some of which will conflict with each other. Here's a few examples of types of interference to NCPR we've heard about.

  1. iPod, Satellite Radio, FM Modulator (etc) interference: This has been a BIG problem for FM radio reception. These devices are designed to let you listen to their audio through your FM radio… they have a small FM transmitter which is designed to broadcast over a very limited range on an un-used FM frequency.
    In theory, this sounds perfectly acceptable, however several things can go wrong: Some of these units weren't designed properly and they broadcast over a MUCH wider area than they should. Some of these devices come pre-set to broadcast on 88.1…. unless they are re-set, they will cause interference to actual FM stations on 88.1 (like NCPR's transmitters in several communities!) Also, if improperly installed, some of these devices can cause tremendous areas of interference!
    You will probably notice this type of interference while driving. All of a sudden, Howard Stern will override your Morning Edition! (then disappear just as quickly) This may even happen at home, if you live in an area of heavy traffic.
    What to do? The FCC has been requiring much more stringent testing of these devices… hopefully (eventually) the older non-compliant devices will be replaced. Or if you happen to repeatedly see the same vehicle producing the same interference to your radio, you might ask the owner to switch the output frequency of his device. Or if it is you that is causing the interference … cut it out!
  2. Interference from other stations: Typically this can be cured through the use of a radio with better selectivity. If you hear another station while trying to listen to NCPR, try listening on another radio (a car radio, for example) . If the situation improves, you probably need a better radio. Another option is to use a directional antenna, and/or try to re-orient whatever antenna your radio uses. If you can (somehow) increase the amount of NCPR signal while decreasing the offending signal then your radio will be able to separate the two stations better.
  3. Multi-path interference: Sometimes the same FM signal can be received by your radio's antenna from several different places, at slightly different times! For example, if there is a nearby reflective surface (like a cliff face) the signal might arrive at your antenna directly from the NCPR transmitter, then a fraction of a second later after it has 'bounced' off the rock cliff face. The radio becomes 'confused' and noisy, harsh reception can occur. To cure this, try to either use a directional antenna, or try to re-orient your antenna to maximize your reception.
  4. Other sources of interference: There are plenty of other sources of interference possible, amateur radio operators (talk to them), computers, TV's, fluorescent lights, electric fences…. Try to determine which of these sources is causing the problem and isolate it…. And/or try to improve the FM signal getting to your radio …. Get a better antenna or a better location for it.