10/9/08

Return to previous page

 

Gain  -  Gain is a measure of how much signal the antenna will collect.  The gain varies according to the direction.  If the maker specifies a gain for the antenna, it refers to the gain in the antenna’s best direction.  A higher best gain always results in a narrower beam.  No antenna has the exact same gain for every channel.  http://www.hdtvprimer.com/ANTENNAS/comparing.html shows how the gain varies by channel for the common antenna types. 

 

Antenna gains are usually specified in dBd or dBi.  If the gain is specified as “X dBd”, that means that the antenna is X dB better than a half-wave dipole.  “X dBi” means X dB better than an isotropic antenna.  A dBi figure is always 2.15 dB higher than a dBd figure.  (That is, the gain of a half-wave dipole is 2.15 dBi.)

 

Manufacturer’s gain specs are notoriously unreliable.  Buyers beware!

 

Gamma match  -  This is a geometry that allows the driven element to be connected directly to a 75-ohm coaxial cable, no balun needed.  Its problem is that it works well for only one channel.  Even makers of single channel Yagis tend to avoid this solution since single channel Yagis are often used for viewing other channels that are strong.

http://www.hdtvprimer.com/ANTENNAS/gamma.gif

 

Ganged antennas  (see Stacked antennas)

 

Ghosts  (see Multi-path interference)

 

Ground plane antennas  (see Vertical antennas)

 

Guy wires  -  Guy wires are support wires that hold the mast up.  They are usually necessary when the mast exceeds 12 feet in length.  In most places, the mast will eventually face an 80 mph wind.  (West Coast antennas don’t see more than 60 mph.)

            http://www.hdtvprimer.com/ANTENNAS/GuyWires.jpg

Since guy wires are usually steel they can disrupt the antenna, lowering its gain.  To avoid this, keep the attachment point at least 4 feet below the VHF antenna boom (2 feet for UHF).  If this is impossible, guys made from plastics are a possibility (Dacron, Kevlar, etc.), but stretchiness is a problem, along with cost and ultraviolet susceptibility.

 

HDTV antennas  -  An antenna made for analog TV will work fine for DTV.  There is nothing different about an antenna for DTV or HDTV.  Unscrupulous people have labeled their antennas “HDTV Antennas” as a marketing ploy.  The honest antenna makers have had to re-label their products likewise to avoid losing sales.

 

Impedance  -  A resistor limits current flow.  Resistance is defined by Ohm’s Law:  voltage = resistance * current.  Likewise, reactance limits current flow.  Reactance is somewhat like resistance, but it describes what coils and capacitors do.  Reactance is sort of a phase-shifted resistance, and the reactance of a device changes with frequency.

 

In a circuit that has both resistance and reactance, the term impedance is generally used.  Impedance equals resistance plus reactance. 

 

Resistance and reactance are so different from each other that they cannot actually be added together.  So impedance is a two-part number in which the resistance and reactance are kept separate.  It so happens that resistance and reactance obey the arithmetic of complex numbers (numbers with real and imaginary parts).  Thus impedance is commonly expressed as a complex number:  Resistance is the real part, reactance is the imaginary part.  Be aware that electrical people often use “j” instead of “i” to designate the imaginary part.

 

Thus “impedance” is a generalized version of the concept of “resistance”, and it can be used in place of the word resistance when there is no reactance.  (It is not wrong to do so, but it can confuse some.)

 

Adding more confusion, the word “impedance” is sometimes a one-part number in which the resistance and reactance have been “combined”, but this is less general and less common.  This number expresses the net magnitude of the impedance, but the phase information has been thrown away.  You must figure out from the context which version of “impedance” is being used.

 

Transmission lines have a property called their characteristic impedance.  See Transmission lines.  See “Terminators for 75-ohm lines”.  The characteristic impedance is usually 75 ohms (actually 75+0i) for coaxial lines and 300 ohms (actually 300+0i) for twin-lead.

 

Antennas have a terminal impedance.  Matching this impedance to the line is important.  See “Mismatch between antenna and feed-line”.

 

Formulas:

Inductive reactance:               X=w*L

Capacitive reactance:            X=-1/(w*C)

Impedance (2-part):                Ż=R+X*i

Impedance (1-part)                 mag(Ż)=sqrt(R*R+X*X)

 

Where:

R is resistance in Ohms

L is inductance in Henrys

C is capacitance in Farads

X is reactance in Ohms

Ż is impedance (2-part) in Ohms

f is the frequency in Hertz

w=2*p*f

 

Inductors (coils and transformers)  -  When a wire carries current, there is energy stored in the magnetic field around the wire.  Coiling the wire magnifies this effect.  Energy entering or leaving this field affects the circuit.  Inductance is a measure of the coil’s effect on the circuit.

 

The effect of inductance is proportional to frequency.  At TV frequencies, even the minuscule inductance of a straight piece of wire becomes important.

 

A transmission line (coaxial or twin-lead) is a geometry in which the inductance and capacitance cancel each other out, allowing an unimpeded energy transfer for long distances.

 

Indoor 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.  The most basic antenna is rabbit ears with a UHF loop.  If that doesn’t work then you should consider a disk antenna in the attic or something on the roof.

 

If you are a renter then you might have to learn to live with indoor antennas.  A popular indoor antenna is the Silver Sensor (ZHDTV1, UHF only), available from Amazon.com and others.  It is also a superb looking antenna that will actually dress-up your living room.

 

The highest performing indoor antenna is a two-bay (UHF only).  This is an outdoor antenna, but it is small enough to use indoors.  It is big, 13”x19”, and not especially pretty, but if you are desperate, some sacrifices are acceptable.  You will have to invent some kind of a stand for it.  Two-bay antennas are made by Channel Master (4220) and by AntennasDirect.com (DB-2).

 

You might benefit from putting the antenna on a longer cable so you can search the room for the strongest spot.  The best spots are usually pointing out a window in the direction of the stations.

 

A preamplifier will probably help.  The only ones you should consider are the amplifiers made by Channel Master (Titan or Spartan) and Winegard (AP series).  You probably can’t determine the noise figure for your receiver.  If it is below 3dB then a preamplifier will not improve anything.  So make sure you can return the preamplifier if it proves to be of no benefit.

 

Probably the best a renter can do is get a Channel Master 7777 amplifier, connect a two-bay to its UHF input, and connect rabbit ears to its VHF input.  If you are desperate to get a difficult VHF channel, get a 5-element single channel Yagi and suspend it a few inches below the room ceiling.

http://www.hdtvprimer.com/ANTENNAS/GainUHFnetIndoor.gif

In-line amplifier  -  One hundred feet is normally the longest RG6 allowed for satellite dishes.  If you can’t avoid a longer run then you need an in-line amplifier.  These small units take DC power from the coaxial cable, and do not interfere with the DC or 22kHz tone that the receiver sends to the dish.  Place the amp at about the midpoint of the cable.  (You need one in-line amp for every 100 feet of cable.)

 

Interference  -  Common sources of interference include:

·         Adjacent channel interference (a very strong station one channel up or down).  R1

·         Co-channel interference (two weak stations on the same channel).  R1

·         Multi-path interference (usually caused by the direct path being blocked).  R1,R2

·         A very close transmitter (a neighborhood FM station, police station, taxi company, etc.).  R1,R3

·         An industrial noise source (a factory, a clinic, a malfunctioning power transformer).  R1

·         Household appliances.  R4

·         Light dimmers.  R4

·         Fluorescent lights.  R4

 

Remedies:

R1 – See “Nulls in radiation pattern”.

R2 – See “Multi-path interference”.

R3 – See Overload.

R4 – Try one of these:

·         Try fixing or replacing the device.

·         Try replacing the device with a device containing some RF filtering.

·         Try putting an RF noise filter on the power cord of the bad appliance.

·         Try putting an RF noise filter on the power cord of your TV.

·         If the TV and the appliance are on the same house circuit breaker, move one to a different breaker.

 

Appliance noise  -  Household motors and fluorescent lights often produce noise of the “120 sparks per second” variety.  If you tune to an analog station (especially channels 2-6) you may see intense sparkles that are somewhat confined to a broad horizontal band.  If so, you must find the appliance and fix it or replace it.  Identifying the appliance is sometimes difficult.  You might have to shut off the house circuit breakers one at a time, watching to see when the sparkles go away.  If every breaker but the TV is off and every appliance on that breaker but the TV is off and the sparkles remain then the noise source is either in a neighbor’s home or is a bad transformer on a utility company pole.  (If you can walk around with a portable AM radio tuned to an unexplained buzzy hum, you might be able to further isolate the offending device.)  If the source is in your neighbor’s home, brush up on your diplomatic skills.  If it is the utility’s transformer, call them.  They are obligated to fix it.

 

Ionosphere  -  The ionosphere is a charged layer of the atmosphere that can reflect radio waves.  Actually it is a series of layers from 30-275 miles in elevation.  It enables communication beyond the horizon for a band of frequencies, and with multiple reflections can reach half way around the globe.  What those frequencies are depends on the time of day, the time of year, and time within the 11-year sunspot cycle.

 

These “skip” frequencies rarely go above 30 MHz and normally play no part in TV reception.  On rare occasions they will go above 50 MHz, allowing the signals of channels 2, 3, and 4 to travel far, allowing distant analog channels to be watched.  But this also allows atmospheric noise to travel far, so getting a digital lock on a distant DTV channel is unlikely.  Thus the most likely consequence of the ionosphere is to occasionally kill a DTV channel 2-4 that you normally get weakly.

 

Isotropic antenna  -  This is an antenna with equal radiation (reception) in every direction.

 

Nobody actually wants one of these.  It is mainly a concept for comparison.  For example, if an antenna is rated at 7 dBi, that means it is 7 dB better than an isotropic antenna.

 

Jack  -  A jack is a female connector.

 

Join-Tenna  -  These devices, made by Channel Master, are single channel diplexers (combiners).  They permit a single channel antenna to share a feed-line with a wideband antenna.  However they mess up reception for the adjacent channels.  Thus a Join-Tenna cannot be used for a channel adjacent to another channel you want.  (Since there are gaps between channels 4-5, 6-7, and 13-14, this restriction does not apply to them.)

 

The Channel Master website says Join-Tennas are available only for channels 6-69.  But units for 2-6 are sometimes findable.  Join-tenna is missing from the new Channel Master catalog, so these devices might not be available much longer.

 

There is a unique Join-Tenna for each VHF channel.  But UHF Join-Tennas are adjustable, and there are just three of them for covering 14-29, 30-49, and 50-69.  The seller will adjust it for you.  But they tend to be a little slow, so acquiring one of these can take some time.  It is usually not practical for you to adjust it yourself.       

http://www.hdtvprimer.com/ANTENNAS/JoinTenna.gif

 

Kilowatt  (see Watt) 

 

Lightning arrestor  -  A lightning arrestor is a device that protects the equipment.  It has a set of points or gas-discharge devices that will arc-over when a high voltage is present.  A lightning arrestor is important when the feed-line is twin-lead.  But coaxial systems don’t usually have lightning arrestors because the center conductor rarely acquires a high voltage, and the shield conductor is usually grounded directly.

 

Log-Periodic antenna (Log Periodic Dipole Array, LPDA)  -  The LPDA has several dipoles arranged in echelon and criss-cross fed from the front.  The name comes from the geometric growth, which is logarithmic.

http://www.hdtvprimer.com/ANTENNAS/lpda.gif

This is a very wideband antenna of medium gain.  Usually only about three of the dipoles are carrying much current.  The other elements are mostly inactive.  As frequency increases, the active elements “move” toward the front of the array.  Most VHF antennas are LPDAs.

 

TV LPDAs come in two types: straight and Vee.  The Vee type (LPVA) has a very slightly higher gain for channels 7-13.  The straight type has nulls 90° to each side that can be used to cancel out a ghost, but the Vee type has less overall radiation to the sides.

 

LPVA (see Log-Periodic antenna)

 

Log-Yag antenna  -  This hybrid antenna is an LPDA with some Yagi elements added.  It is a good wideband antenna and is a common VHF antenna.  Example: Winegard YA-1026.

http://www.hdtvprimer.com/ANTENNAS/LogYag.gif

Long-wire antenna  -  A long-wire antenna is long, but of no specific length or geometry.  Long-wire antennas are used for long distance reception on the AM and short-wave bands, but are seldom used above 20 megahertz.  This is not a good TV antenna because it is unpredictable and usually has low gain.

 

Loop antenna  -  Although a loop antenna will work at any frequency (if the loop is the right size), for TVs it is only common as a UHF antenna.  It is a weak antenna, comparable to a single dipole.  The performance of a 7.5-inch UHF loop antenna is described at http://www.hdtvprimer.com/ANTENNAS/comparing.html .

 

LNB (Low Noise Block-converter), LNBF  -  An LNA is a low-noise amplifier.  An LNB is an LNA plus a Block converter circuit that shifts the frequencies downward.  A DirecTV LNB shifts the Ku band frequencies 12.2-12.7 GHz down to 950-1450 MHz.  This conversion is necessary because satellite frequencies will not travel very far in ordinary coaxial cable.

 

An LNBF is an LNB plus a Feed horn assembly.

 

LNBs and LNBFs can be found at the focus point of a satellite dish antenna.

 

Masts  -  Radio Shack sells 1.25” diameter masts that are convenient for many antennas.  But if you put a heavy antenna like an 8-bay on a ten-foot mast of this type, it will sway with the wind and break in a high wind.  Heavy gauge 1.5” masts are generally available at stores that sell Channel Master or Winegard equipment.

 

Matching transformer  (see balun) 

 

Mismatch between antenna and feed-line  -  An antenna has an innate impedance measured in ohms.  If this value is the same as the transmission line’s characteristic impedance then the energy captured by the antenna will be efficiently transferred into the transmission line.  Otherwise a portion of the signal will be reflected (retransmitted).  The greater the degree of numerical mismatch, the more that will be rejected by the line.

 

At http://www.hdtvprimer.com/ANTENNAS/comparing.html you will see raw gain and net gain graphs.  By comparing these two graphs you can see the degree of mismatch for some common antennas.  For channels where the net gain and raw gain are about the same, the antenna is well matched.  For channels where the net gain is far less than the raw gain, the antenna is badly matched.

 

The antenna impedance is usually not just resistive, but also includes a reactive component (which means there seems to be a capacitor or coil in series with the antenna) and the reactance tends to be the bigger problem.  There are ways to cancel out this reactance, and also adjust the resistance.  But making the antenna better matched at one channel will make it worse matched at other channels, and there is rarely an overall improvement.

 

Presently there is no hardware commonly available to change the match, except for some indoor antennas.  To perfect the match for a given channel on an outdoor antenna requires the services of an engineer.

 

(The impedance of a half-wave dipole is 75 ohms, while a folded dipole is about 300 ohms.  Many people when they see these general geometries assume the antenna is 75 or 300 ohms.  That is usually not even close.  The other elements have a huge affect on the terminal impedance.  A CM-4228 has an impedance of 1000 ohms at channel 25, and 750 ohms at channel 58, yet a 300-ohm balun is a good overall match.  Guessing the impedance from “inspection” is something this author will not even attempt.)

 

Multi-path interference  -  This is a problem which, if severe, can prevent DTV reception even if the signal is strong.  The signal is reaching the antenna by more than one path due to diffraction around hills and trees and sometimes reflections off of structures.

 

There are two distinct categories of multi-path interference.  The first is “short delay” multi-path, delays of less than about 20 nanoseconds.  On analog channels there will be no visible ghosts.

 

Short delay multi-path  -  This is always caused by something directly in front of the antenna.  One common cause is a tree in front of the antenna.  There will be chaotically overlapped signals behind a tree.  This will mainly affect UHF reception.  The solution is to relocate the antenna (or cut down the tree).  If the antenna stays behind the tree, you will likely see dropouts on UHF channels when the wind blows.  And that’s for strong-signal areas.  In weak-signal areas you will likely get no UHF reception at all behind a tree.

 

The other common cause is an irregular horizon line (structures and trees in the distance).  These will cause overlapping fields, which will result in a regular pattern of strong and weak spots.  For UHF, moving the antenna right or left three feet or so can make a huge difference.  Moving the antenna is usually the solution.  Unfortunately a strong spot for one channel can be a weak spot for a different channel.  The same phenomenon happens for VHF, but since wavelengths are ten times as big, the strong and weak spots are ten time further apart, so moving the antenna to a strong spot is often too far to be practical.  For VHF the solution is usually a bigger antenna.  (The author faces a severe case of horizon-induced multi-path.  His UHF strong spots are always about 12 feet apart, but they can move with the weather.  His solution:  He erected two UHF antennas 6 feet apart, selected by a switch.  Whenever one antenna is in a weak spot, the other is guaranteed to be in a strong spot, so at least one antenna always works.)

 

Long delay multi-path  -  This is caused by a large obstruction like a hill or a large building.  If you tune in an analog channel close in frequency and from the same direction, you will see ghost images.  The offending signals are approaching the antenna mostly from the sides, but also in rare cases from the rear.  Actually all analog images have these ghosts, but without the direct path blocked they are too dim to see.

 

One solution is to move the antenna to a new spot where signals from the offending directions are less strong.  A move of 20 feet or more will likely be necessary.

 

The other solution is to select an antenna with little or no reception in the offending directions.  There are two workable strategies here.  If the analog channels show one really strong ghost (and maybe a number of weaker ones) then selecting an antenna with a null will work.  See “Nulls in radiation pattern”.  Otherwise an antenna with minimum radiation to the side and rear is the way to go.  The higher the antenna’s gain, the less reception it will have to the side.  (More advice on multi-path can be found at http://www.hdtvprimer.com/ANTENNAS/silver.html .  Although that article is written for the Silver Sensor antenna, the same principles apply to larger antennas.)

 

 

Multi-switches for DBS systems  -

http://www.hdtvprimer.com/ANTENNAS/multiswitch.gif

Every DBS satellite system has a multi-switch.  Often it is mounted within 3 feet of the dish.  Other times it is packaged with the three LNBFs in a single large head assembly.  Basically it connects each TV to one of four lines from the LNBFs.  Some multi-switches will also amplify the signal.  Multi-switches come in 1-receiver, 2-receiver, 4-receiver, 8-receiver, and other configurations.

 

When the user selects a channel, the satellite receiver must figure out which satellite and transponder the channel is on and whether the transponder has left or right circular polarization.  (A transponder is like a large channel.  Each transponder carries several stations.  Consecutive transponders have alternating polarization, which makes it easier for the receiver to separate the transponders.)  The receiver selects the satellite by placing (or not) a 22-kilohertz tone on the coaxial cable.  It selects the polarization by placing either 13 volts DC or 18 volts DC on the coaxial cable.  The multi-switch uses this DC both as a signal and as a source of power for itself and the LNBFs.

 

For DirecTV, the third satellite carries only a few transponders, and the combiner maps those into unused transponder frequencies of satellite B.  Thus the receiver functions as if there are only two satellites.

 

(This article needs updated for Ka band satellites.  Rumors are that Ka band transponders are down converted to frequencies above and below the 900-1500 MHz frequencies used for Ku band, that a 4-way multi-switch is always built into the dish, and that an 8-way outboard multi-switch exists that can always adapt this 4-way output to feed 8 receivers.)

 

Noise/Noise figure/Noise factor  -  For TVs there are two main sources of noise:

1.    Atmosphere noise.  There are many types of sources for this noise.  A light switch creates a radio wave every time it opens or closes.  Motors in some appliances produce nasty RF noise.

2.    Receiver noise.  Most of this noise comes from the first transistor the antenna is attached to.  Some receivers are quieter than others.

 

Receiver noise dominates on the VHF and UHF bands, and atmospheric noise is usually insignificant.

 

The noise figure and noise factor are the same thing, but the noise figure is expressed in dB.  Every amplifier has a noise figure.  The noise figure must be subtracted from the antenna gain.  Thus the noise figure tells how much of the antenna gain you are throwing away by not buying a quieter amplifier.  The amplifier in question is the OTA receiver or the mast-mounted amplifier, whichever the antenna connects directly to.

 

Noise management is discussed at http://www.hdtvprimer.com/ANTENNAS/basics.html .  If you do not manage the noise properly, you might be throwing away a large portion of the antenna signal.

 

Nulls in radiation pattern  -  Nulls in the radiation pattern can be useful.  If you rotate the antenna so that a null points toward an interfering signal, that signal is eliminated.  Some interference situations that might benefit from this trick include:

·         Adjacent channel interference (a very strong station one channel up or down)

·         Co-channel interference (two weak stations on the same channel)

·         Multi-path interference (usually caused by the direct path being blocked)

·         A very close transmitter (a neighborhood FM station, police station, taxi company, etc.)

·         An industrial noise source (a factory, a clinic, a malfunctioning power transformer)

 

For example, the Channel Master 4228 has nulls on both sides at 30 and 90 degrees:

http://www.hdtvprimer.com/ANTENNAS/cm4228az.gif

Yagi/Corner-reflector antennas have no nulls.  LPDA antennas have nulls at 90 degrees, but LPVA antennas have no nulls.  To make rabbit ears have nulls (at 90 degrees) lower them into a straight dipole.

 

See also The Two-Antenna Trick (indoor version) and The Two-Antenna Trick (outdoor version) .  These explain how to create a null in a controlled direction.

 

In a multi-path situation, a null will work if there is only one strong ghost.  (Find an analog channel close in frequency and from the same direction.  Examine it for ghosts.)  If there are multiple very strong ghosts then a better approach is a very high-gain (very directional) antenna with little reception to the side or rear.

 

Omni-directional antenna  -  This is a non directional antenna.  Its gain is low, but if you are in a city and surrounded by stations, this might be the perfect antenna.  Common omni-directionals include these disk antennas:

·         Channel Master 3000 SMARTenna, 22” circular VHF/UHF.  The 3038 amplifier is optional.

·         Winegard MS-1000 or MS-2000 MetroStar, circular VHF/UHF.  Amplifier optional.

·         Radio Shack 15-1634, 21” circular VHF/UHF.  Amplifier included.  (Since Radio Shack amplifiers tend to be noisy, the author would probably avoid this antenna.)

 

The gain of these antennas is no better than that of a typical indoor antenna, but if you mount this unit in the attic or on the roof, you will likely see a considerable improvement.  And you won’t have to re-aim it or endure dropouts when someone walks past the TV.  Most people will benefit by getting an amplified version, but if you are in a very strong-signal neighborhood, that won’t be necessary.

 

An important exception is if tall buildings block some stations.  In this case you will likely have multi-path interference. If you see strong ghosting on analog channels then you have multi-path and you are not a candidate for an omni-directional antenna.  Most DTV receivers have trouble with multi-path, even if the signal is strong.  You need a very directional antenna that can ignore signals coming from the wrong directions.

 

Overload  -  Signal amplifiers are supposed to be linear.  That is, the output is a magnified but otherwise unaltered version of the input.  But too much signal can make an amplifier non-linear, usually clipping off the tops and bottoms of the sine waves.  When this happens, all channels are affected, not just the one that is too strong.  In fact, the too strong signal is usually not a TV station.  A close FM station or police station is more likely.

 

If you add a good amplifier to your antenna system and your results get worse instead of better then you have overload, and you need to reconsider more carefully what you are doing.

 

Overload never causes any equipment damage.

 

An attenuator is a resistor network that can be used to reduce the gain of an amplifier.  3 dB and 6 dB attenuators are commonly available.  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.

 

If you are close to an FM station, there might be a narrow range between too much and too little amplifier gain.  You can make that range larger by using an amplifier with an FM trap or by using a more directional antenna.  VHF preamplifiers usually include FM traps that can optionally be disabled.  Freestanding FM traps are also available.  FM traps can either cover the entire FM band or can be single frequency traps that you tune to the offending station.  The former are less effective and tend to attenuate channel 6.  If the FM station is close enough you might need more than one FM trap.

 

Painting an antenna  -  Many people think TV antennas are unattractive.  Will painting an antenna to make it less visible affect the way it performs?  No.  Dielectrics affect antennas, but a layer of paint is too thin for this effect to be significant.  Many cell phone antennas are painted black.  HAM radio operators often use long-wire antennas that have an enamel coating.

 

Don’t use a paint that has embedded metals (metallic flakes, red oxide, graphite, carbon).  Enamel paint is best.  Rustoleum is a good choice.

 

Do not paint the terminals.  Do not paint the rivets that hold together the criss-cross feed line of a log-periodic antenna.

 

Parabolic antenna  (see Reflector antenna)

 

Parabolic dish antenna  (see Dish antennas) 

 

Parasitic elements  -  Elements not connected to any feed-line are called parasitic elements.  They are generally divided into two categories: reflectors and directors.  See Yagi antennas.  See Reflector antennas.  Despite having no connections, parasitic elements carry a considerable current.

 

Plug  -  A plug is a male connector.

 

Polarization  -  AM radio antennas are generally some sort of vertical wire.  This is because AM radio waves have vertical polarization.  Other directions are possible.  Most TV signals have horizontal polarization and thus require antennas with horizontal elements.  The broadcaster chooses the polarization.  That decision reflects what type of antenna is most convenient for that service and also how radio waves behave at that frequency.

 

If you hold an antenna perpendicular to the polarization of the signal, you will receive no signal.  The received signal strength varies with the cosine of the angle of the antenna relative to the signal polarization.  (Reflections off nearby objects commonly make this experiment imperfect.)

 

If a transmitter uses two antennas, one vertical and one horizontal, the result is most likely diagonal polarization, which the station could also produce with a diagonal antenna.  Vertical, horizontal, and diagonal polarizations are examples of linear polarization.  But if this station introduces a quarter-cycle delay into one of these two antennas, the result is circular polarization (or elliptical polarization if the two components are not equal).

 

Circular polarization can be visualized as a signal that spins as it moves through space.  But it is equally valid to think of circular polarization as just two independent overlapping signals that have linear polarizations that are perpendicular and have a quarter-phase shift.

 

Circular polarization can be right circular or left circular.  DBS satellites use circular polarization, with adjacent channels using opposite spin.  A right circular antenna will ignore a left circular signal, and visa versa.  DBS satellites alternate the polarization to make it easier for the receiver electronics to separate the channels.

 

A linear antenna can be used to receive a circularly polarized signal, but half of the transmitted power will be invisible to it.  A circularly polarized antenna can be used to receive a linearly polarized signal, but half of the received signal will be retransmitted with the opposite linear polarization.  Because of these problems, circular polarization has not become popular for terrestrial radio services other than FM broadcasting.  A small percentage of TV broadcasters use circular polarization, which improves reception in a moving vehicle equipped with a vertical antenna.

 

Power Injector  (see antenna amplifiers) 

 

Preamplifier  (see antenna amplifiers)

 

 

Return to previous page

 

 

 

 

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