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.)  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                           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

KHz means 1000 Hertz, MHz means 1,000,000 Hertz, and GHz means 1,000,000,000 Hertz

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.

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 usually 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

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 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.

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 1^st transistor is quieter than the receiver’s 1^st 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 1^st transistor out of the picture).

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

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 many people add these two numbers (and then maybe subtract the losses) to find the strength of the signal at the receiver.  But this sum has little value.  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.  Many 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.  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

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” (Radio Shack 278-1645), 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

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 gone and cannot be made up later.

Comparing some common antenna amplifiers

The following noise figures were measured by the author:

*  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 15-1109 is Radio Shack’s best.  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 2^nd 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 stat.  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 “combined 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.

An attenuator is a resistor network that can be used to reduce the gain of an amplifier.  6 dB attenuators are available at Radio Shack.  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.  The Radio Shack attenuator will not pass DC and thus must not be inserted between the mast-mounted amplifier and its power injector.

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.

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.

To reduce these voltages the antenna cable should have a grounding block (Radio Shack 15-509) 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 (Radio Shack 15-035, $7) 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 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.

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.  As with all ground wires, turns should be curves of large radius.

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.)

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