12/10/08

Antenna basics

 

The TV channels

Hertz (Hz) means cycles per second.  (Heinrich Hertz was the first to build a radio transmitter and receiver while understanding what he was doing.)  KHz means 1000 Hertz, MHz means 1,000,000 Hertz, and GHz means 1,000,000,000 Hertz

 

The radio frequency spectrum is divided into major bands:

                                                                     Frequency:                      Wavelength (in meters):

VLF     very low frequency                          3 KHz – 30 KHz              100 Km – 10 Km

LF       low frequency                                   30 KHz – 300 KHz         10 Km – 1 Km

MF      medium frequency                          300 KHz – 3 MHz           1 Km – 100 m

HF       high frequency  (a.k.a. short wave)       3 MHz – 30 MHz             100 m – 10 m    

VHF    very high frequency                        30 MHz – 300 MHz        10 m – 1 m

UHF    ultra high frequency                        300 MHz – 3 GHz           1 m – 100 mm

SHF    super high frequency                     3 GHz – 30 GHz             100 mm – 10 mm

EHF    extremely high frequency              30 GHz – 300 GHz         10 mm – 1 mm

 

A TV channel in the U.S. will always occupy 6 MHz of this spectrum. 

·         Channels 2-6             occupy consecutive spectrum from          54 MHz to 88 MHz.   (with one small gap)

·         Channels 7-13           occupy consecutive spectrum from          174 MHz to 216 MHz.

·         Channels 14-69        occupy consecutive spectrum from          470 MHz to 806 MHz.

Frequency Table

Any of these channels could contain either an analog channel or a digital channel.  Note that “channel 25-1” is a virtual channel name and does not indicate what channel the station physically occupies.

 

Channels 2-13 are the VHF channels.  They are split into two groups so that antennas will work better:  In general, an antenna designed for frequency N will also work well at 3N, but very poorly at 2N.

 

The wavelength of a radio wave is:       λ = 300/F     where F is the frequency in mega-Hertz and λ is the wavelength in meters.  Antenna elements are typically about a half-wavelength long.

 

Decibels

Decibels (dB) are commonly used to describe gain or loss in circuits.  The number of decibels is found from:

 

Gain in dB = 10*log(gain factor)       or

 

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

 

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

 

(Decibels can be used to describe changes in voltage.  But this website will use them only to describe changes in power.)

 

Noise

Whether a signal is receivable is determined by the signal to noise ratio (S/N).  For TVs there are two main sources (classes) 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 (radio frequency) 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.  On an analog channel, noise looks like snow.  If there were only a barely perceptible amount of snow, this would correspond to how noise-free a DTV signal must be for a DTV receiver to lock-on to it.

 

Signal Amplifiers, Preamplifiers

Many people think that connecting an external amplifier to the antenna will improve the performance of the antenna.  This is usually wrong.  Receivers always have more gain than is necessary.  (The receiver has an Automatic Gain Control circuit, AGC, which will reduce strong signals.  The AGC makes all stations the same strength at the demodulator.  When you add a preamplifier, the TV receiver lowers its own gain, usually by an equivalent amount.)

 

Normally the signal to noise ratio will be set by the receiver’s first transistor.  But if an external amplifier is added, the first transistor in that amplifier determines the S/N ratio.  (Since the external amp will greatly magnify its own noise as well as the signal, the receiver’s noise becomes insignificant.)  Since there is no reason to think the external amp’s first transistor is quieter than the receiver’s first transistor, there is generally no benefit to the S/N ratio from an external amplifier.

 

But an external amplifier will compensate for signal loss in the cable if the amplifier is mounted at the antenna.  Without this amplifier, a weak signal, just above the noise level at the antenna, could sink below the noise level due to loss in the cable, and be useless at the receiver.

 

RG-6 will lose 1 dB of the signal every 18 feet at channel 52.  For a DTV channel, 1 dB can be the difference between dropouts every 15 minutes (probably acceptable) and every 30 seconds (unwatchable).  This author recommends a mast-mounted amplifier whenever the cable length exceeds 20 feet.  (If you are in a good-signal area or you have no high-numbered UHF channels, you can to an extent ignore this advice.)

 

The preamplifier should have a gain equal to the loss in the cable (for your highest channel) plus another 10 dB (to keep the receiver’s first transistor out of the picture).

The amplifier can usually exceed this target by another 10 dB without causing trouble.

 

(If you follow the above rule, the cable length becomes irrelevant, and reducing the cable length yields no benefit.)

 

When figuring the cable loss, be sure to include the loss in any splitters and baluns.  If a 2-to-1 splitter were 100% efficient then you would figure a 3 dB loss since each TV gets half of the power.  But splitters are usually 80% to 90% efficient.

         2-to-1 splitter                3.5-4 dB

         3-to-1 splitter                5-6 dB

         4-to-1 splitter                7-8 dB

         75W-to-300W balun    0.2-2 dB         (a balun is an adapter)

 

The antenna and the amplifier both have gains measured in dB, and some people add these two numbers (and then maybe subtract the losses) to find the strength of the signal at the receiver.  But this sum is worthless.  The net gain in front of the amplifier should always be kept separate from the net gain that follows.

 

You might not need an amplifier if the antenna is too big.  But an amplifier can never make up for an antenna that is too small.

 

Receiver noise

Actually there is a reason to think the external amplifier is quieter than the receiver.  Long ago designers made an effort to make the TV’s first amplifier stage very quiet.  But now 90% of homes use cable or satellite boxes (strong sources) and most of the rest are rural homes using antennas that have mast-mounted amplifiers.  So the TV’s noise is rarely a factor.  Some TV makers no longer put any effort into making their sets quiet.

 

Suppose you live in an apartment 15 miles from the transmitter.  Your indoor antenna mostly works, but you are troubled by dropouts and some snow appears on analog channels.  Will adding an amplifier right at the TV improve things?  Yes, if it is quieter than the TV.  Unfortunately TV makers see no reason to publish the noise figures for their receivers.  So buying an amplifier for an indoor antenna is a total crapshoot.  This author recommends that you try a Channel Master Titan or Spartan amplifier, but make sure you can return it if it is no help.

 

Transmission cable

Twinlead (ribbon cable) used to be common for TV antennas.  It has its advantages.  But due to its unpredictability when positioned near metal or dielectric objects, it has fallen out of favor.  (Such objects, even if not touching the cable, cause a portion of the signal to bounce, return to the antenna, and get retransmitted.)

 

Coaxial cable is recommended.  It is fully shielded and not affected by nearby objects.  Transmission cable has a feature called its characteristic impedance, which for TV coax should always be 75 ohms.  (50-ohm coaxial cable is also common.  Avoid that cable.)  Although rated in ohms, this has nothing to do with resistance.  A resistor converts electric energy into heat.  The “75 ohms” of a coaxial cable does not cause heat.  Where it comes from is mathematically complicated and beyond our scope here.

 

But coax also has ordinary resistance (mostly in the center conductor) and thus loses some of the signal, converting it into heat.  The amount of this dissipation (loss) depends on the frequency as well as the cable length.

 

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

 

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

 

The above chart is only approximate.  There are many cable manufacturers for each type and there is no enforcement of standards.  If the mast-mounted amplifier gain exceeds the cable loss then it shouldn’t matter what cable you use.  But there are two problems with this:

1.    Some cable has incomplete shielding.  This is most common for RG-59, another reason to avoid it.

2.    When the cable run is longer than 200 feet, the low-numbered channels can become too strong relative to the high-numbered channels.  In this case, RG-11 or an ultra-low-loss RG-6 is recommended.  (These alternatives are expensive.)  Alternatively, frequency compensated amplifiers will work.

 

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.  3M Vinyl Electrical Tape is a good waterproofer.  Even better is an asphalt putty called “Coax Seal”, but it is so tenacious it should not be used for temporary connections.  Cover the connectors completely.

 

Baluns

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.

 

Comparing some common 4-to-1 baluns

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

The 15-1253 is not suitable for outdoor use.

 

Signal Amplifiers, Preamplifiers, part 2

There are two types of signal amplifiers:

Preamplifiers  (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.)

 

Never feed an amplifier output directly into another amplifier.  There should always be a long cable between the preamplifier and the distribution amplifier.  Placing the two amplifiers close together can cause overload and/or oscillation.

 

A mast-mounted amplifier’s most important characteristic is its noise level, usually specified by the noise figure.  But many manufacturers don’t take this number seriously.  If it is given at all, it is often wrong.  If all makers don’t do them right then comparison-shopping is not possible.  The author is inclined to rate amplifiers for their noise figures as follows:

      0.5 dB superb (anything better runs into thermal atmospheric noise)

      2.0 dB excellent

      4.0 dB fair

      6.0 dB poor

      10 dB  awful

 

The noise figure is a number you must subtract from the antenna’s gain.  The noise figure tells how much of the antenna’s gain you are throwing away by not buying a quieter amplifier.  This loss is irretrievable.  It is gone and cannot be made up later.

 

Comparing some common antenna amplifiers

The following noise figures were measured by the author:

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

*  measured at channel 30

**  +13V=FM trap in, -13V=FM trap out.

***  This is the longest RG-6 cable that satisfies the rule “The gain should equal the cable loss plus an extra 10 dB” at channel 30, assuming the power injector is at the TV.

Note 1:  Still the King.  The other 777x amplifiers probably behave the same.

Note 2:  Winegard’s best.  It has the best FM trap, but few people who need this amplifier need an FM trap.

Note 3:  The 10 dB variable attenuator is in the power module.  Be delicate when adjusting the attenuator.  It will break easily.

Note 4:  The 15-1170 is modest but problem free.  It is a good 2nd amp in a very long cable.

Note 5:  The 15-1108 is terrible.  It often oscillates unpredictably.  Very noisy.  I bought a second unit to prove to myself that the first wasn’t broken.  If you need 300W inputs, you can use a 15-1109 with a 15-1140 balun, but then the noise figure becomes 4.6 dB.

 

The “Cable length” from the above table is a telling statistic.  It makes clear that there is generally no good reason to buy a Radio Shack amplifier.

 

7777

The Channel Master 7777 preamplifier has separate inputs (and separate amplifier circuits) for VHF and UHF, which are then combined without loss.  There is a switch inside that will allow VHF and UHF input via the same connector.  The unit usually comes with the switch in the “separate input” position.  A second switch disables the FM trap.  You have to remove the 4 base screws to access the switches.

 

Receiver 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.  (Too little gain = dropouts, too much gain = overload.)  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.

 

Grounding outdoor antennas

For TVs, the main benefit of grounding is lightning protection.  Lightning is a powerful radio wave generator and any elevated wire is an antenna for it.  A lightning strike in your neighborhood can generate hundreds of volts, even thousands, on the coaxial line.  These voltages can damage your equipment.  (This is also called electromagnetic pulse, EMP.)

 

To reduce these voltages the antenna cable should have a grounding block (Radio Shack 15-923) at the point where it enters the house, and that grounding block should be wired to a ground rod driven into the ground as close as possible to the grounding block.  An effective ground rod is one driven deep enough to reach into moist soil.

 

The ground rod should also connect to the mast via a heavy wire.  #8 aluminum wire is readily available for this.  Ground wires should be as short and straight as possible.  Turns should be curves with a 6-inch radius.  Ground wires do not need insulation.

 

Some people will tell you “Don’t ground the coax.  That just makes the antenna a lightning rod”.  But the coax is already grounded through your receiver’s power cord, so you can’t prevent it from being a lightning rod.  All you can control is how much of your house the high current will go through before it reaches the ground.

 

Another advantage:  Appliance RF noise can travel up the outside of the coaxial cable to the antenna, and then back down on the inside to interfere with reception.  The grounding method described above will often eliminate that.

 

The grounding method described above conforms fully to Channel Master recommendations.  It does not fully conform to NFPA recommendations.

 

The NEC requirement

The National Electrical Codes (document NFPA 70) requires another wire be added to the grounding described above.

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

This 6-gauge wire, shown in red, connects the new ground rod to the breaker box (typically).  This wire will help absorb the lower frequency components of a direct strike.  If this seems like too much work for too little benefit, don’t be discouraged from at least installing the ground rod.  But if your antenna is situated where a direct strike is likely then installing this wire is strongly advised.  The wire should run close to the ground so that side flashes will likely arc to the ground.  It is OK to run this wire around the exterior of the building.  In this case keeping the wire 6” to 12” above ground is best.  The length of this wire is less important, but turns should still be curves of large radius.  (4-gauge aluminum can be used for this wire, but the rules forbid bare aluminum within 18 inches of the ground outdoors.)

 

Winegard and others recommend putting the antenna near the breaker box so that the house ground rod can ground the antenna.  But this author considers that to be overly risky, as does Channel Master.  Many people have been killed when their antenna fell into the power lines.  (Also power lines can interfere with TV reception.)

 

Some additional NEC rules

  1. Do not attach an antenna to the power line service entrance power mast.  Outside the building, the antenna coaxial and grounding wires shall not come closer than:
    1. 2 feet from exterior power lines of 250 Volts or less.
    2. 10 feet from exterior power lines of greater than 250 Volts.
    3. 1 foot from underground power lines.
    4. 6 feet from lightning rod wires.

 (Although these are safety rules, they also reduce the pickup of appliance noise.)

  1. If the antenna mast or wires come within 5 feet of a swimming pool, they must be bonded to the pool’s bonding grid.
  2. Grounding wires and grounding blocks are permitted to be interior to the building.  (An interior ground rod might be in soil too dry to conduct much.)
  3. Grounding connections must be constructed so that they will not come loose or corrode away.  (Any connection joining two different kinds of metal will corrode very rapidly if the connection can get wet.)
  4. An interior cold water pipe is acceptable as a ground rod if the connection point is within 5 feet of where the pipe enters the ground.  (You must verify that the underground water line is not plastic.)
  5. Indoor antennas (including attic antennas) are not generally susceptible to direct strikes.  In such cases a grounding block is not required by the rules, but is probably a good idea when the cable is longer than 30 feet.

 

There is nothing that you can do to guarantee that your electronics will survive a direct strike.  If you have any uncertainty about a safety issue, seek the advice of a registered electrician. 

 

Related topics

Balun wire positioning  (adjusting the balun wires)

DC block  (when are they necessary?)

F-connectors  (Should you assemble your own cables?)

Join-tenna  (a device for combining antennas)

Rotors  (motorized antenna pointing)

Splitters/combiners/diplexers  (What do they do?)

75-ohm terminators  (Are these necessary?)

Masts  (What kind are available?)

Impedance  (What is this?)

Polarization  (Why are TV antennas horizontal?)

Mismatch  (Does this matter?)

 

Historical trivia

Who invented radio?  This honor is generally accorded to three people:

 

In 1864, James Clerk Maxwell declared that radio waves had to exist.  Studying his “Maxwell’s Equations” convinced him of this.  His prediction was perfect.

 

In 1886, Heinrich Hertz correctly assembled a crude transmitter and receiver.  But Hertz was a professor just trying to prove Maxwell’s assertion.  It doesn’t seem to have occurred to him that radio waves were useful.

 

In 1894, Guglielmo Marconi read about Hertz’s experiment and instantly recognized that radio was a signaling device.  He devoted the rest of his life to developing practical equipment.  He is usually called the “father of radio”.

 

Many people are convinced that experiments by Nikola Tesla preceded Marconi’s by a few months.  But Tesla was disorganized and his work in this area had no impact.

 

The inventions of Edwin Armstrong:

1912: The vacuum tube oscillator

1914: The regenerative receiver (uses feedback to increase gain and reduce bandwidth)

1918: The super-heterodyne receiver (uses frequency translation downward to reduce bandwidth)

1922: The super-regenerative receiver (uses exponential buildup of oscillations to increase gain)

1933: Frequency modulation (more immune to interference)

 

 

 

 

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