Radiation pattern -
Shown above is the 3-dimensional radiation
diagram for a 9-element Yagi antenna that the author uses to receive channel 12. To interpret this diagram, imagine that
the antenna is at the origin. The
length of a line from the origin to any point on the surface is proportional to
the gain in that direction.
2-dimensional pattern diagrams are more common. The following two diagrams describe a
Channel Master 4228.
Reflector antenna - Radio waves will reflect off of a large
conducting plane as if it was a mirror.
A coarse screen will work just as well.
The reflector eliminates most reception from the rear while at least
doubling the forward gain.
Reflector antennas are very common.
The screen reflector shown above has the gain
of about 4 dipoles. The parabolic
reflector focuses the signal onto a single dipole, but its bandwidth is a little
disappointing. The corner reflector
has a little less gain but much greater bandwidth. The corner reflector has roughly the
gain of three dipoles. It is a good
medium gain antenna, widely used for UHF.
If you need more than 25 dBi then the paraboloid dish is the only
practical choice, but for VHF or UHF they are huge.
RFI (Radio Frequency
Interference) (see interference)
Rotors (antenna
rotators, motorized antenna aimers)
-
When a highly directional antenna is employed,
a rotor becomes necessary when the desired stations are in multiple directions.
A rotor is a minor nuisance for analog
stations, but it becomes a major problem for digital stations. This is because it takes 10 times longer
to discover the correct aim for a DTV station.
(Instead of simply looking at the level of snow, you must count the
dropouts that occur over a half-minute period, then re-aim and try again.) The author discovered that for weak
stations it took him 5 to 10 minutes to achieve the optimum aim, and during that
time he missed about half the program dialog.
Because of this he quit using that antenna.
For an ideal rotor, the antenna would always
point where the indoor controller indicated.
But the Radio Shack rotor the author was using suffered “creep”, a small
directional error accumulation. The
creep would build up fastest during the short jerky movements typical during
station searches, and soon the antenna would be pointing far from where the dial
said, making the dial calibrations useless.
The author later switched to the Channel Master
rotor that has an infrared remote control.
This rotor doesn’t seem to suffer as much creep, but then, this antenna
isn’t used very often. But it can
creep. The controller has a
microprocessor that counts the number of times the rotor is used. After a certain number, the controller
will recalibrate to eliminate the accumulated creep. It does this by rotating the antenna to
the limit in one direction, then to the limit in the opposite direction, and
then to the requested channel direction.
If you must have a rotor, get one like this (or one that doesn’t creep in
the first place).
If you like TiVo then you will not want a
rotor. TiVo-like recorders can’t
make antenna adjustments.
This author recommends that if your stations
are in just two directions, you will be happier with two antennas. A common splitter can be used to combine
the two antennas. Doing this will
cost you a lot of signal strength (Half of the signal from each antenna gets
rebroadcast out the other antenna.
Figure a 3.5dB loss at the splitter.).
A Join-Tenna is a device that can avoid this loss.
Another way to avoid the 3.5 dB loss is to use
an antenna switch to combine the two antennas.
The author likes the Radio Shack 15-1968 switch, which has an infrared
remote control. But again, the TiVo
can’t work the switch.
Servo motor - This is a small motor in a C-band dish
feed assembly. This motor selects
whether vertical or horizontal polarization is to be received.
Single channel Yagi - The ultimate antenna for VHF is a single
channel Yagi. No other type has as
much gain. But these are large
antennas, and they work well for only one channel, so these are desperation
antennas. Reasonably priced single
channel Yagis are available from Wade.
See “Stores and websites supplying antenna equipment ”. See
http://www.wade-antenna.com/cutchannel.htm on the Wade website. See also Join-Tenna.
S/N (Signal to Noise Ratio) - Whether a signal is receivable is
determined by the signal to noise ratio.
Noise management is discussed at
image/e_learning/Splitters_combiners_diplexers_files/basics.html . If you do not manage the noise properly,
you might be throwing away a sizable portion of the antenna signal.
Splitters/Combiners/Diplexers -
A splitter is a sort of
“Y-adapter”. All splitters are
bi-directional, and thus will serve also as combiners. These devices are usually 85%-95%
efficient, whether used as splitters or combiners. There are four basic types. They look almost identical, but are very
different devices.
50MHz-900MHz splitter/combiner - Commonly available in 2-way, 3-way,
4-way, and 8-way types, this is the splitter you want if you have multiple TVs
or want to gang two identical antennas.
The splitter is designed so that it will not cause reflections, but if
you leave an output unconnected then there will be reflections. (See Terminators.) If a 2-way splitter were 100% efficient,
you would figure a 3dB loss since each TV would get half the power. Some splitters incorporate a DC Block in
one output so that the splitter can be between an amplifier and its power
injector. (See DC Block, Antenna
amplifiers)
900MHz-2200MHz splitter/combiner - These can be used with satellite
systems, but be sure you know how control issues will be resolved.
VHF/UHF splitter/combiner - This device will split UHF off from the
VHF, which would be necessary for TVs with separate VHF and UHF inputs. The device will also combine a VHF
antenna with a UHF antenna. The
advantage of this splitter is that there is no 3dB loss. (Technically, this device should be
called a diplexer.)
Satellite/OTA diplexer - This device will allow a satellite dish
and an over-the-air antenna to share the same cable. The loss in the device will not be 3dB,
but there will be some small loss.
Check for whether DC is blocked to the OTA port.
Channel Master has OTA preamplifiers that will use the DC voltage from
the satellite system.
Note:
Splitters in cable systems with “on demand” features must pass
5MHz-900MHz.
Stacked antennas - The use of multiple antennas is called
stacking or ganging.
The antennas must be identical.
Dipoles are commonly stacked horizontally
(collinearly), vertically (broadside), and in echelon (end-fire).
When dipoles are stacked horizontally, the
horizontal beam-width becomes very narrow.
This is because they do not add in-phase for directions not straight
ahead. Similarly, when stacked
vertically, the vertical beam-width becomes narrower.
The same principles apply when stacking whole
antennas, not just dipoles. See
“The two antenna trick” at
image/e_learning/Splitters_combiners_diplexers_files/silver.html .
Standing waves, SWR - If a transmission line ends in nothing
(either an open or a short), 100% of the signal gets reflected back toward the
source. The forward moving signal
and the backward moving signal pass through each other without affecting each
other. But at each point in the
line the two signal voltages add together, and considering phase, some places
they will subtract. The result is a
stationary voltage pattern, called a standing wave, arrayed along the
transmission line.
If an incomplete load (something other than 75 ohms) causes less than a
100% reflection, then the standing wave pattern doesn’t show complete
cancellation. The ratio of the
highest and lowest voltages that can be found along the line is called the
Standing Wave Ratio (SWR).
SWR=Vmax/Vmin
An SWR of 1 is good: There
is no reflected signal.
An SWR of infinity is bad: There is total reflection.
An SWR of 1.5 is not too bad.
There are formulas that will predict the SWR if
the load impedance is known.
Stores and websites
supplying antenna equipment -
None of the vendors listed here has contributed financially or materially
to this website or this author.
The following manufacturers’ websites have
store locators:
·
Channel Master (
www.channelmaster.com ) (Fry’s is usually poorly stocked)
·
Winegard (
www.winegard.com )
·
Antennacraft (
http://www.antennacraft.net )
·
Wade (formerly Jerrold) (
www.wade-antenna.com ) **
·
Radio Shack (
www.radioshack.com )
The following manufacturers sell online:
·
Antennasdirect.com (
www.antennasdirect.com ) A.k.a.
Terrestrial Digital
·
Radio Shack (
www.radioshack.com )
Manufacturers to avoid:
·
Terk – Severe hype. The equipment works,
but Terk marketing prevents intelligent decision-making.
·
Xium – Totally fraudulent
antennas
Online stores:
·
Warren Electronics (
www.warrenelectronics.com )
(Channel Master, Winegard, Antennacraft)
·
Dishplace.com (
www.dishplace.com )
(Channel Master, Winegard)
·
SummitSource.com (
www.summitsource.com )
(Winegard,
Antennacraft)
·
DTVgear.com (
www.dtvgear.com )
(Channel Master, Winegard)
·
Wholesale Electronics Inc. (
www.weisd.com )
(Channel Master, Winegard)
·
SolidSignal.com (
www.solidsignal.com )
(Channel Master, Winegard, Antennacraft, Blonder Tongue*, Terrestrial Digital)
Honorable mention:
·
Stark Electronics (online catalog, telephone sales only) (
www.starkelectronic.com )
(Channel Master, Winegard, Antennacraft, Wade, Blonder Tongue*)
* Blonder Tongue makes CATV antennas, which are
slightly more rugged and more expensive than consumer grade antennas.
** Wade is presently the only maker of
inexpensive VHF single channel Yagis.
Wade sells only through distributors, but the distributors don’t stock
much. Stark stocks some Wade. Otherwise you must go to the Wade
website, find your local distributor, call him and order the antenna, and the
antenna will be shipped out of Ontario.
Assume 2 weeks. While Wade
sells full lines of TV antennas for CATV and consumers, other vendors are easier
to deal with.
Terminators for
75-ohm lines -
A 75-ohm transmission line must end in a load equal to 75 ohms or much of
the signal will be reflected backwards, possibly getting retransmitted out the
antenna. Signals can literally
bounce back and forth inside transmission lines.
All TVs and amplifiers have 75-ohm inputs, so normally there is no
problem.
If you use a splitter but forget to connect one
output to anything, some of the signal will be reflected back towards the
antenna. Depending on the
frequency, the signal power lost this way can exceed the power the unused leg
would have consumed. Viewers of the
connected TV might discover some stations seem to be missing. The reflected signals can increase the
likelihood that the amplifiers will overload.
These problems can be insignificant, or they can be more confusing than
significant. Getting 75-ohm
terminators and connecting them to the unused outputs of the splitter will stop
the reflections.
Tilt angle -
If the
horizon (including the tree tops) is a few degrees above horizontal, you will
benefit from tilting the antenna up to point at the horizon, although for
low-gain antennas this benefit is insignificant.
If your visible horizon is many miles away,
this signal might arrive at different elevation angles on different days. Some authors will recommend a tilt motor
so that you can always maximize your signal strength. But this author believes that high-angle
days are always strong signal days and the minor improvement in tilting the
antenna up is not necessary. This
author recommends a tilt motor only when a rotor is employed and the horizon
angle is different in different directions.
Transmission lines -
The common
types are coaxial cable, twin-lead, twisted-pair, wave-guide, and strip-line (PC
etch). Coaxial cable is recommended
for all TV systems. Although lamp
cord can be used for transmission line, it has some problems.
Twin-lead (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.)
All high frequency transmission lines have a
property called characteristic impedance.
Twin-lead is usually 300 ohms, while coaxial cable for TVs 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. A transmission line must end in a load
equal to the line’s characteristic impedance in order to prevent signal
reflections in the cable.
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.
Trees as obstructions - A tree has very little effect on
VHF-low, but a significant effect on VHF-high.
But the big problem is UHF.
A tree with leaves blocks about 90% of a UHF signal. The space behind the tree is an overlap
of the signal going through the tree and the signal diffracting around the tree. Such overlapping fields have an
alternating pattern of strong and weak spots separated by only a few feet. An antenna in a strong spot might work
nicely until the wind blows, deforming the tree and moving the spots. Thus, for DTV stations, you are likely
to see dropouts when the wind blows.
Even in a good-signal neighborhood it is inadvisable to put a UHF antenna
behind a tree.
If the tree loses its
leaves in the fall, reception behind it will improve dramatically. Many people get a TV for Christmas, and
erect an antenna for it in January, and then wonder why it quit working in May. It’s the trees.
The farther away a tree is, the less of a
problem it is. For far away trees,
assume no signal penetrates the tree, and reception will be by diffraction
around the tree. See Diffraction.
Trees block 100% of satellite signals.
Turnbuckle - This is a device for tightening guy
wires. Since one side has
left-handed threads, a turnbuckle will tighten the guy wire without twisting it.
For the type pictured below, you must
tape over the turnbuckle after adjusting it or vibration will cause it to loosen
with time.
Twin-lead
transmission line (ribbon cable) -
Twin-lead (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.)
For UHF, twin-lead should be kept at least an inch away from anything
metal, and should not be allowed to touch anything dielectric (plastic).
For VHF, the twinlead should not be allowed to touch anything metal, even
if insulated, and should not lie against anything dielectric.
UHF - The Ultra High Frequency spectrum is
everything from 300 megahertz to 3.0 gigahertz.
One of the bands within this spectrum is the TV UHF band, which goes from
470 megahertz (channel 14) to 806 megahertz (channel 69). Each of these stations occupies 6
megahertz of that spectrum.
Velocity factor - Signals travel at the speed of light. However the speed of light inside a
transmission line is slightly slower than the speed of light in a vacuum. It depends on the dielectric constant of
the material holding the electric and magnetic fields.
Most transmission lines employ polyethylene
(PE) or a PE-air mixture. The speed
of light in PE is 66% of the speed of light in a vacuum. The speed of light in air is about 100%
that of a vacuum. A coaxial cable
with solid PE between the conductors will have a velocity factor of 0.66.
For twin-lead, the electric and magnetic fields
are partly in the PE and partly in the air around the twin-lead. As a result, the velocity factor is
somewhere between 0.66 and 1.00, usually around 0.90.
PE is somewhat lossy at high frequencies. To reduce the loss, microscopic air
bubbles (foam) are often blown into the PE of coaxial cables. This also raises the velocity factor to
0.75 or 0.85, but it makes the cable less rugged.
The velocity factor is usually of no use to
Average Joe installing a TV antenna.
In the rare cases where the transmission delay is important, the velocity
factor becomes important.
Vertical antennas - There are countries where TV signals
have vertical polarization, but not in North America. U.S. TV stations are allowed circular or
elliptical polarization if they want.
But fewer than 10% of U.S. stations do this. Most stations figure the vertical
component of circular polarization is wasted power. See Polarization.
If a local TV station uses circular or
elliptical polarization then you can probably receive it reliably using either a
vertical or a horizontal antenna. A
vertical antenna can be a good choice if you want TV reception in a moving
vehicle.
The common vertical antennas are the
vertical dipole and the ground plane antenna. Three common versions of the ground
plane antenna are:
1.
A quarter-wave vertical whip with 3 or 4 horizontal ground plane
“radials”. This is a small version
of the popular CB antenna.
2.
A quarter-wave vertical whip that uses the metal roof or trunk lid as the
ground plane.
3.
A ¾-wave vertical whip with a coil in the middle. This is a scaled-up version of a popular
cell phone antenna.
VHF - The Very High Frequency spectrum is
everything from 30 megahertz to 300 megahertz.
It is divided into many bands for different purposes (police, fire,
aircraft, etc.) Two of the bands
within VHF are:
1.
The TV VHF-low band. This band goes from 54 megahertz
(channel 2) to 88 megahertz (channel 6).
2.
The TV VHF-high band. This band goes from 174 megahertz
(channel 7) to 216 megahertz (channel 13)
Each channel occupies 6 megahertz of this
spectrum. (There is a 4 MHz gap
between channels 4 and 5.)
Watt, Kilowatt, Megawatt - A watt is a unit of measure that tells
the rate at which energy is produced or consumed. A kilowatt is a thousand watts. A megawatt is a million watts. One watt is one joule per second. 745.7 watts is one horsepower.
Wavelength
- For every frequency there is a
wavelength. They are related by the
formula
f=300/l
where:
f is the frequency in
megahertz.
l is the wavelength in meters.
Antenna elements are usually about a half
wavelength long.
Yagi antenna - A Yagi antenna has several elements
arranged in echelon. They are
connected together by a long element, called the boom. The boom carries no current. If the boom is an insulator, the antenna
works the same. The rear-most element is called the reflector. The next element is called the driven
element. All the remaining
elements are called directors.
The directors are about 5% shorter than the
driven element. The reflector is
about 5% longer than the driven element.
The driven element is usually a folded dipole or a loop. It is the only element connected to the
cable. Yet the other elements carry
almost as much current. The more
directors you add, the higher the gain becomes.
Gains above 20 dBi are possible.
But the Yagi is a narrowband antenna, often intended for a single
frequency. Yagi antennas are
described in more detail at
image/e_learning/Splitters_combiners_diplexers_files/types.html .
Yagi/Corner-reflector antenna -
This antenna is a hybrid between a Yagi and a corner reflector antenna. The directors determine the gain on the
high channels, while the size of the corner reflector determines the gain on the
low channels. It attempts to be a
wideband antenna, but in fact it does a mediocre job on the low channels. Although the Yagi/Corner-Reflector is
not the best antenna, it is the most common UHF TV antenna, mainly because it
can be mounted on the front of a VHF antenna without degrading the VHF antenna.
2-Bay antenna - This is a two dipole reflector antenna. An indoor version called the “Double
Bow”, made in China, is sold by Channel Master (4149) and Radio Shack. It has some flaws (a ribbon cable, poor
matching on the low channels) and yet is still one of the better indoor
antennas. UHF only.
An outdoor model, the DB-2, is sold by
antennasdirect.com . It is small
enough to use indoors, and as such is presently the highest performing indoor
antenna available.
4-Bay antenna - This is a stacked dipole reflector
antenna. It has a fan-shaped beam:
very broad horizontally, very narrow vertically.
An excellent antenna in the 20-30 mile range, a rotor is usually not
needed. It is a bad choice where
multi-path occurs. (Multi-path and
ghosting result when the direct signal path is blocked.) 4-Bays are made by Channel Master,
Winegard, Antennacraft, Wade-antennas.com , and Antennasdirect.com . This UHF only antenna is roughly
40”x18”.
Channel Master 4-Bay
4DTV - This is a digital satellite subscription
service requiring an 8-foot motorized C-band dish. About 10 HD channels and 150 CD-quality
SD channels are available.
8-Bay antenna - This is a stacked dipole reflector
antenna. Its searchlight-like beam
makes it very different from the 4-Bay.
This highly directional high gain antenna is commonly thought to be the
strongest overall UHF antenna.
8-Bays are made by Channel Master, Winegard, and Antennasdirect.com . The Winegard 8-Bay is skewed in favor of
the low channels, and is presently the best antenna available for channels
14-30. (If you want an antenna
skewed in favor of the high channels, select a big Yagi/Corner-reflector.) 8-Bays are roughly 40”x36”. 8-bays are intended only for UHF. However the Channel Master 8-bay will
pick up channels 7-13 quite well.
Channel master 8-Bay
This page is part of “An HDTV Primer”, which starts at www.hdtvprimer.com