Before 1998, the FCC would never allow stations in the same city to occupy adjacent channels. (There were exceptions for channels 4-5, 6-7, and 13-14 because gaps exist between those channel pairs.) But due to improved receiver technology, the FCC now allows any channel assignment.
When a signal is 10 to 15 times more powerful than that of an adjacent channel station, most receivers become unable to receive the weaker station. To receive a far away station, you might need to use a directional antenna to reduce the strength of a nearby adjacent channel station. See “Nulls in radiation pattern”. But if both stations are in the exact same direction you might be out of luck. There are some frequency selective filters that can “trap” out a channel, but they are seldom able to reduce an adjacent channel by more than half. (e.g. Winegard UT-2700 Dual Trap)
When adjacent channel stations broadcast from the same tower or adjacent towers they must have an agreement that neither will exceed 10 times the other’s power.
Antenna amplifiers - Many people think adding an amplifier to their antenna will improve the performance of the antenna. The truth isn’t that straightforward. http://www.hdtvprimer.com/ANTENNAS/basics.html describes when and why an amplifier is important. There are two types of signal amplifiers:
Preamplifiers or Mast-mounted amplifiers - These should be mounted as close to the antenna as possible. Usually the amplifier comes in two parts:
Distribution amplifiers - These are simple signal boosters. They are often necessary when an antenna drives multiple TVs or when the antenna cable is longer than 150 feet. Distribution amplifiers don’t need to have a low noise figure, but they need to be able to handle large signals without overloading. Commonly, distribution amplifiers have multiple outputs. (Unused outputs usually do not need to be terminated.)
Antenna aperture (capture area) - An antenna has an aperture area, from which it captures all incoming radiation. The formula for the aperture area of any TV antenna is A=Gl2/4p where l is the wavelength and G is the gain factor over an isotropic antenna (not dB).
Antennaweb.org - This website is the best for predicting what kind of antenna you need. Type your full address. When it returns, click on “View Street Level Map” and verify that it is on target. If not, click on exactly where your house is on the map.
The program knows the terrain around your house and compensates accordingly. But it doesn’t know about local obstructions such as trees, buildings, and artificial embankments. If you have some of these local obstructions, you will probably need a stronger antenna than this site recommends.
The Consumer Electronics Association (a manufacturers group) runs this site. Its mission is to promote digital TV.
Attenuator - This device will decrease the strength of the signals passing through it. 3 dB and 6 dB attenuators are commonly available. A 6 dB attenuator will reduce a signal to one-quarter of its power (one-half its original voltage). It employs a resistor network designed to not cause any reflections in the transmission lines.
If an antenna system needs two amplifiers, where the output of one amp feeds into the other amp, too much gain (overload) can result and an attenuator is usually the simplest solution. If you don’t have two amplifiers, it is unlikely that you will ever need an attenuator.
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.
Az-el dish mounts - (Azimuth-elevation mounts) This dish mounting method is uncommon. For a C-band dish, it would require three motors: one to rotate the dish assembly, one to elevate the dish, and one to rotate the feed to match the polarization of the satellite. Equatorial mounts are much simpler and much more common. Equatorial mounts normally lack one motor (the latitude motor) and thus track only geo-stationary satellites, while az-el mounts can track the whole sky.
Oval dishes for DBS systems employ a motor-less equatorial mount, sort of.
Balun - A balun is an adapter that adapts a balanced line to unbalanced line. If a balanced transmission line (such as twinlead) is connected directly to an unbalanced line (such as coaxial cable) the two lines become a long-wire antenna, which is undesirable for VHF and UHF. All baluns are passive bi-directional devices. They are usually above 90% efficient. There are two types:
4-to-1 balun - This will connect 300-ohm twinlead to 75-ohm coaxial cable. This balun is usually a ferrite transformer.
1-to-1 balun - This will connect a 75-ohm balanced load to 75-ohm coaxial cable. This balun is often just some ferrite beads slipped over the coax.
For twin-lead to be 300 ohms the spacing between the wires must be about 6 times the wire diameter (more if there is insulation). If this is not true for the 300-ohm balun’s wires, the effect is equivalent to a small “point capacitance”. If the spacing is less than 6 diameters then the point capacitance will be positive, otherwise it will be negative.
The consequence of the point capacitance is hard to predict. Most likely it will reduce the antenna’s net gain on some channels, but it could actually improve the antenna on other channels. The effect is probably a few tenths of a decibel for UHF, insignificant for VHF.
Here is a rational approach to this dilemma:
1. Judiciously make a list of your must-have UHF channels.
2. Use the receiver to determine the weakest station on the list.
3. Note the signal strength of this station three times:
a. With the balun wires spread wide apart
b. With the balun wires pinched together
c. With the balun wires in a middle position
4. Leave the wires in the best position.
The beam width is normally measured to the “half-power points”. That is, the beam width is the number of degrees between the points where the gain is 3 dB less than for the antenna’s strongest direction.
The antenna’s maximum gain can be found from the beam width using the formula: G=41000/(A*B) where
· G is the raw gain factor (relative to isotropic, not in dB)
· A is the beam width, in degrees, in the elevation plane
· B is the beam width, in degrees, in the azimuth plane
This is an approximate formula, but it tends to be highly accurate for common, one-directional TV antennas.
Some conclusions can be drawn from this equation:
1. For any given channel, whichever antenna has the highest raw gain will have the narrowest beam. (This applies to antennas with “searchlight” like patterns like the 8-bay or a Yagi.)
2. It is often pointless to try to distinguish weak-signal problems from multi-path problems. An antenna that improves on one will also improve on the other.
Blonder-Tongue, Wade, and others make antenna equipment for the CATV/MATV market. This equipment costs two or three times as much as consumer grade equipment, but it is more ruggedly constructed.
Capacitance hat - This is a device sometimes found at the end of an element. It can be a cross, a disk, a ball, a loop, or just about anything conductive. It makes the element behave as if it is longer, maybe 10%-30% longer than it really is. It can save space with almost no drop in performance. Capacitive hats are uncommon in TV antennas. (The Channel Master 3671B has six capacitive hats.)
Type: Center conductor: Cable diameter:
RG-59 20-23 gauge 0.242 inches
RG-6 18 gauge 0.265 inches
RG-11 14 gauge 0.405 inches
This author usually recommends RG-6 for all TV antennas. It can be stapled in place using a staple gun with common 9/16” T25 staples. How long the cable lasts depends solely on how long you can keep water out of it. See Transmission lines. See “Terminators for 75-ohm lines”.
Many people believe coaxial cable wears out. It doesn’t. It can be water-damaged, UV-damaged, or physically damaged. (Many plastics age some, making them more prone to physical damage.) But if its connectors are well installed it could last 100+ years.
Co-channel interference - When the station you want is not receivable because of another station on the same channel, you have co-channel interference. The interfering station can be very far away and very weak, yet it can contribute enough “noise” to make the closer station hard to receive. The remedy is a new antenna that is both stronger in the forward direction and weaker in the interfering direction. See “Nulls in radiation pattern”.
Combo antennas (for VHF and UHF) - This is the most common outdoor TV antenna. The front third of the antenna is a Yagi/Corner-reflector UHF antenna, and the remaining two-thirds is a Log-Periodic Dipole Array (straight or Vee type) antenna for VHF. Presently there are almost no VHF digital stations. But after 2009 VHF DTV stations will be more common.
Connector types - There are many types of RF signal connectors (BNC, N, UHF, Reverse SMA, TNC, phono, etc). But the only connector generally used for TV antenna systems is the F connector. See F connector.
DBS (Direct Broadcast Satellites) - These satellites use Ku-band frequencies (12 GHz) and are receivable with an 18-inch dish. The newest DirecTV satellites use Ka band (30 GHz). See “Multi-switches for DBS systems”.
In the US, the DBS Subscription Services (DSS) are DirecTV and Dish Network. The Canadian DBS broadcasters are Star Choice and Bell ExpressVu. C-band is not usually called a DBS system.
DC Block - Some splitters will pass DC through all 3 legs. If such a splitter came between the power injector and the mast-mounted amplifier then the DC would go some place unintended. Some antennas and some TVs are effective short circuits for DC. That short circuit would kill the DC at the amplifier, and thus the amplifier would not work.
A DC Block is just a coupling with a capacitor inside that will not pass DC. This capacitor also will not pass the 60Hz AC that Winegard uses. Putting the DC Block in the line with the “short circuit” will enable the amplifier to work properly.
Some splitters have a DC Block built into one side. These are always clearly labeled.
Gain in dB = 10*log(gain factor) or
In some situations this is more complicated than using gain or loss factors. But in many situations, decibels are simpler. For example, suppose 10 feet of cable loses 1 dB of signal. To figure the loss in a longer cable, just add 1 dB for every 10 feet. In general, decibels let you add or subtract instead of multiply or divide. There are some special numbers you might want to memorize:
20 dB = gain factor of 100
10 dB = gain factor of 10
3 dB = gain factor of 2 (actually 1.995)
0 dB = no gain or loss
-1 dB = a 20% loss of signal
-3 dB = a 50% loss of signal
-10 dB = a 90% loss of signal
Diffraction (The bending of radio waves) - Diffraction is the ability of a wave to bend into the shadow created by an obstruction. Diffraction is pictured at http://www.hdtvprimer.com/ANTENNAS/siting.html .
Dipole antenna - The most basic of all antennas, the dipole is popular for TV reception because of its predictability. A freestanding half-wave dipole has a torroidal (doughnut-shaped) reception pattern, and has a terminal impedance of 75 ohms. But if this 75-ohm antenna is connected directly to 75-ohm coaxial cable without a 1-to-1 balun, some signal picked up by the dipole will flow onto the outside of the shield conductor where it will be retransmitted and lost. (Also some signal picked up by the shield will flow onto the dipole, disrupting the gain, the radiation pattern, and the impedance. This could be an improvement, but more likely it is the opposite.)
The dipole is 75-ohms at only one frequency. For other frequencies its terminal impedance will include some reactance, which will prevent a good match with the feed-line. See impedance. See “Mismatch between antenna and feed-line”. Making the dipole diameter larger minimizes the reactance, giving the dipole a larger bandwidth. The dipoles with the largest bandwidth are double-cone dipoles, but bowtie dipoles are somewhat effective at the same thing.
TV dish antennas are used for satellite reception both for C-band (requiring an 8-foot dish) and Ku-band. DirecTV and Dish Network use Ku-band, and their satellites are powerful enough that an 18-inch dish is adequate except in Alaska. DirecTV also uses Ka band.
A couple tree leaves are enough to disrupt Ku-band and Ka-band reception. But diffraction has little effect on the signal path. If there were a 25-inch diameter hole through the crown of the tree, and if the dish could be pointed at the satellite through that hole, then the tree would have no effect on reception (until the wind blew).
Dropouts (weak signals) - On DTV channels you will never see snow, ghosts, or interference, but you will see dropouts. When the signal is corrupted or becomes too weak, you will see “block errors” (parts of the screen that are shifted or obviously wrong), sound dropouts lasting a few seconds, or image freezes lasting a few seconds. All of these errors are crude, unsubtle errors. If these are not present, your image is perfect.
The causes of dropouts are:
· Weak signal (You might need a better antenna system.)
· Interference (See Interference.)
· Fading (See Fading.)
Elements - An antenna is made up of several elements that work together. They transfer energy back and forth between each other before the feed-line finally absorbs it. If one element is removed, the drop in performance is usually much worse than the decline in the element total would suggest.
The manufacturers can get very creative when it comes to counting the elements. Dipoles in an LPDA are usually counted as two elements. Sometimes each rod in a reflecting plane is counted as one. Real antenna people never do these things.
F/B (Front to Back Ratio) - This ratio tells how good the antenna is at rejecting signals from the rear. It is seldom truly important because interference seldom comes from the rear, but it can happen. This ratio is the gain factor in the forward direction divided by the gain factor from the rearward direction. But since gains and F/B ratios are usually given in dB, you normally get the F/B figure by subtracting the rearward gain from the forward gain (both in dB).
The F/B ratio is mostly a marketing gimmick, and is not a very useful number. Respectable antenna textbooks don’t define it. The true ratio often varies dramatically from channel to channel. This author has seen two common definitions for the F/B ratio. One uses the gain in the 180-degrees direction as the rearward gain. The other uses the X-degrees direction, where X is the rearward direction (from 90 to 270 degrees) with the most gain. It is usually impossible to tell which is being used if a radiation pattern is not available. (Of course, if you have the radiation pattern then you don’t need the F/B ratio.)
There is a quick push-on version of the F-connector, but these are never reliable long term, even indoors. Even good connectors require protection from the weather. How long the cable lasts depends solely on how long you can keep water out of it. 3M Vinyl Electrical Tape is a good waterproofer. Cover the connectors completely. Even better is an asphalt putty called “Coax Seal”, but it is so tenacious it should not be used for temporary connections.
It is inadvisable for you to assemble F fittings onto coaxial cable. The result often proves unreliable over the long run. It is better to purchase ready-made cables of the proper length. These machine made cables are highly reliable.
But if you need an odd-length cable you might have to learn how to do this. Your first few cables will probably be unreliable, but eventually you will get good at it. Don’t buy the smaller crimp tools. They will never make reliable crimps. Even the largest crimp tools will hurt your hands. Leather gloves help. An extreme amount of crimping force is necessary. Otherwise the junction resistance will rise slowly over a period of months. Most importantly, get the right fittings. There are three different sizes, one each for RG59, RG6, and RG6QS.
This author has not had good luck with screw-on fittings. Some people recommend RCA “Centerpin” F-fittings and other easy-assemble fittings. I have never used these “quickie” F-connectors, so I have no recommendation on them. Even bad connectors always work for a few months. Then they start to fail intermittently in a manner that seems unrelated to the connectors, causing much grief and wasted time. Just the possibility of this scares me away from trying anything I have not used before. Store-bought cables are safer. Usually the only advantage of making your own is that you can drill a quarter-inch hole, not a half-inch, through your roof.
When the sun warms up the land, a warm air layer near the ground can add a third path. The warm air causes a bending called refraction, which is identical to the “mirage effect”.
Which of these paths will be the strongest is hard to predict. The ground-reflecting path is usually weak if the reflection point is forested. The bent path can be enhanced by focusing, making it stronger than the direct path.
These paths will add together at your antenna, and considering phase, subtraction is a possibility. Whether subtraction occurs depends on the length of the bent path. Since the warm air layer is always either growing (sun up) or shrinking (sun down) this path length is always changing. For UHF the path length need change by only ten inches to turn addition into subtraction. If both path signals are about the same strength, your DTV channel will dropout momentarily. If you see two dropouts that were N minutes apart then you will probably continue to see dropouts every N minutes. This is fading. There is no cure for it, but a stronger antenna will make it less likely.
The above is but one scenario for fading. There are many variations on this depending on the terrain.
FCC data base, accessing - The FCC keeps an online database for all its licensees, and it is relatively easy for you to use to find a station’s location, transmitting power, and other data. Note that the accuracy of this data is not guaranteed. And when it is accurate, it tells what the station is allowed to do, not what it is actually doing.
1. Go to http://www.fcc.gov/mb/video/tvq.html .
2. Type the letters of the station call sign into the call sign box and press enter.
3. You will be shown a list of records. Probably there will be from 1 to 4 records in the list. You should select one of these records by clicking on it. The status column shows the reason for the record:
You should select the most recent record for the channel number you want.
4. sometimes, instead of going to the record you selected it goes to the first record and you must scroll down to the desired record.
Interpreting the record - The FCC defines “Effective Radiated Power” (ERP) as the actual transmitted power multiplied by the transmitting antenna’s gain (relative to isotropic, not in dB) in its best direction. Antenna gains of 10 are common, so the electric bill is not what the ERP suggests.
The record will say whether the antenna is directional or omni-directional. If it is directional you can click on “Relative Field polar plot” to see the radiation pattern. From this plot, find the field value for your direction. Since this is a voltage number it must be squared before multiplying it by the ERP to get the power radiated in your direction.
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.
Channels 2-6 : 100 kilowatts 50 kilowatts
Channels 7-13 : 316 kilowatts 160 kilowatts
Channels 14-69 : 5 megawatts 1 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. Note that UHF transmitting antennas usually have higher gain, so the actual disparity in transmitter power is not as great as this table suggests.
Field strength meters - Sometimes readers ask where they can get a portable signal strength meter, thinking this will allow them to make objective studies of the signal. A signal strength meter is not as useful as you might think. The standard meter works well for analog stations. But for DTV, a strong meter reading is no guarantee that you have found a good reception spot if certain types of multi-path are present.
An additional complication is that there are two generations of DTV demodulators. The newer generation identifies the contributions of the weaker paths in a multi-path situation and subtracts them out. A portable signal meter based on one generation will not find the correct antenna aiming/positioning when the TV’s demodulator is of the other generation.
The ultimate arbiter of where the signal is good for your receiver is the receiver itself. This author recommends that the signal strength readout provided by the receiver be used to search for the best antenna spots. The quality of the result justifies finding ways around the two problems you will discover when you try to do this. The first problem is that your HDTV is probably not on the roof, thus you can’t see the signal readout. The second problem is the receiver’s meter probably reads zero when it can’t demodulate the signal. This second problem is nasty for people in fringe areas. There are portable signal strength meters made for DTV, but they are rare and terribly expensive, and if they use a demodulation scheme different from what your receiver uses, their results might not apply to your receiver.
FM receiving antennas - The FM radio band is 88-108 MHz, a 20 MHz band adjacent to channel 6. Some VHF TV antennas work well over this band, but others don’t. There is no easy way to tell if a given TV antenna will work well for FM. To receive far away FM stations, you should probably get an FM Yagi.
Folded dipole - The terminal impedance of a folded dipole is four times that of a simple dipole of about the same dimensions. Other ratios are possible by using two rods of differing diameters, or by adding a third rod. The impedance of a freestanding half-wave folded dipole is 300 ohms, but the presence of nearby elements will usually lower this considerably.
Fringe area reception - If you live in the fringe, you can’t afford to make any mistakes. You must find a mounting site that is high and not behind any trees. You might benefit by searching your property for the spots with the strongest signals. You need a Channel Master 7777 amplifier. You need the biggest antennas made. You might need single channel Yagis for VHF. For UHF, start with a Channel Master 4228 or a Winegard 9032. If with this you don’t even get a digital lock then that station is hopeless. If you get a digital lock but you see too many dropouts then consider ganging two antennas (see http://www.hdtvprimer.com/ANTENNAS/16bay.html ) Another improvement you can make is to get a quieter amplifier. Research Communication in Kent England makes a UHF amplifier with a 0.4 dB noise figure. It is expensive and takes some time and effort to acquire but it will be like enlarging the antenna by 40%.
In the fringe, weather affects reception. You will never get dropout-free reception every day beyond 60 miles for UHF, 80 miles for VHF, no matter how good your antenna is. Beyond these distances, dropout-free reception on 9 out of 10 days is considered a good result.
Using the above techniques, you can surpass what www.antennaweb.org predicts for you. To force antennaweb.org to give you a more optimistic prediction, find a house in your area that is situated a little better than yours (higher and less obstructed) and type in that address.
This page is part of “An HDTV Primer”, which starts at www.hdtvprimer.com