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
low frequency 3
KHz – 30 KHz 100 Km – 10 Km
KHz – 300 KHz 10 Km – 1 Km
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
high frequency 30
MHz – 300 MHz 10 m – 1 m
UHF ultra high frequency 300 MHz – 3 GHz 1 m – 100 mm
high frequency 3 GHz –
30 GHz 100 mm – 10 mm
high frequency 30 GHz – 300
GHz 10 mm – 1 mm
TV channel in the U.S. will always occupy 6 MHz of this spectrum.
2-6 occupy consecutive
spectrum from 54 MHz to 88 MHz. (with
one small gap)
7-13 occupy consecutive spectrum
from 174 MHz to 216 MHz.
14-69 occupy consecutive spectrum
from 470 MHz to 806 MHz.
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
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.
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.
(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
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:
dB = gain factor of 100
dB = gain factor of 10
3 dB = gain factor of 2
dB = no gain or loss
dB = a 20% loss of signal
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.)
a signal is receivable is determined by the signal to noise ratio (S/N). For TVs there are two main sources (classes)
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.
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.)
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
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.
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 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.)
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.
splitter 3.5-4 dB
splitter 5-6 dB
splitter 7-8 dB
75W-to-300W balun 0.2-2
dB (a balun
is an adapter)
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.
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.
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.
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.)
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.
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.
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
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.
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 water-proofer. 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
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:
- 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.
The 15-1253 is not suitable for
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:
- 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.
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
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.
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.
1: Still the King. The other 777x amplifiers probably behave the
best. It has the best FM trap, but few
people who need this amplifier need an FM trap.
3: The 10 dB variable attenuator is in
the power module. Be delicate when
adjusting the attenuator. It will break
4: The 15-1170 is modest but problem
free. It is a good 2nd amp in
a very long cable.
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
“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.
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.
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.
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.
never causes any equipment damage.
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.
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.
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.)
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.
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.
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.
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.
grounding method described above conforms fully to Channel Master
recommendations. It does not fully
conform to NFPA recommendations.
The National Electrical Codes (document NFPA 70) requires another wire be added to the grounding described above.
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.)
(Although these are safety rules, they also
reduce the pickup of appliance noise.)
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.
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?)
terminators (Are these necessary?)
Masts (What kind are available?)
Impedance (What is this?)
Polarization (Why are TV antennas horizontal?)
Mismatch (Does this matter?)
invented radio? This honor is generally
accorded to three people:
1864, James Clerk Maxwell declared that radio waves had to exist. Studying his “Maxwell’s Equations” convinced
him of this. His prediction was perfect.
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.
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
often called the “father of radio”.
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. There are two published accounts of earlier observations of
radio waves. They are not regarded as
discoverers because, while they realized they were seeing something new, they
were not able to provide descriptions that were helpful or believable:
1. Ben Franklin (the kite
experiment) in 1752. Radio waves are the
most probable explanation.
Loomis (dentist) in 1866. Loomis is said
to have set up a working radio telegraph using kites.
Both Franklin and Loomis thought they had found a conductive layer
in the atmosphere at an altitude reachable by a kite.
inventions of Edwin Armstrong:
The vacuum tube oscillator
The regenerative receiver (uses feedback to increase gain and
The super-heterodyne receiver (uses frequency translation downward
to reduce bandwidth)
The super-regenerative receiver (uses exponential
buildup of oscillations to increase gain)
Frequency modulation (more immune to interference)
This page is part of “An HDTV Primer”, which
starts at www.hdtvprimer.com