| This section is for those of you who need the fundamentals to
build an indoor (or outdoor) RF distribution system. If you are an "Old
Pro" in the cable television industry, this will be a boring review. For the "first timer" this will open your eyes to the reasons
why your TV signals have snow or lines in the pictures. This article is just
the "fundamentals" of cable television (CATV) and master antenna television
(MATV) signal distribution. |
| If your RF distribution network requires more than one amplifier
in series, the number one cable TV rule applies. Cable
television systems consist of UNITY GAIN building blocks. What that means is
every length of cable (and the passive devices in that length of cable) is followed by an
amplifier with an equal amount of RF gain and equalization to overcome the differing
losses at all desired frequencies. |
| A piece of coaxial cable is nothing more than a piece of round
wire surrounded by an insulating material (usually foam) that is then surrounded with an
outer metal jacket which may consist of overlapping vinyl/foil layers, braided wire, or
extruded (or welded) aluminum tubing. The object of coaxial cable is to keep
electromagnetic RF carrier waves inside the cable from getting out and similar waves from
the outside world from getting inside the cable. And by the way, the term COAXIAL means the center conductor must stay in the CENTER. If you kink, or flatten the cable, it does not work
as well. Damaged cable creates reflections and it will radiate and absorb signals to
and from the outside world. DO NOT DAMAGE THE CABLE. |
| Electromagnetic waves of different wavelengths (frequencies) can
flow through a single cable at the same time. The higher the frequency of a wave,
the greater the insertion loss of the cable for a given length of cable. This loss
is usually expressed in decibels (dB) per 100 feet of cable here in the United States.
Common cable television test equipment and signal level meters are calibrated with
decibel ranges. A table of losses for popular cable types is included in this
article for quick reference. The signal level losses will vary with the temperature
of the cable. These data tables are published for ambient cable temperature of 70
degrees Fahrenheit. For cables exposed to the weather, when
the temperature is COLDER, the losses of the cable are LESS. On the hottest day in the summer, the cable losses are the greatest.
Cable that is buried in the ground, or indoors, will usually remain at the same
temperature year round. |
| For ease of use and recognition, each of the different television
frequencies has been assigned a channel number. The channel numbering scheme applied
to these frequencies is different in over-the-air TV transmissions as received with an
antenna when compared to the channel numbers and frequencies used in a cable television
system. There are several different frequency plans used in cable. We will
only be concerned with "standard CATV" channels in this article and not the IRC
or HRC CATV RF frequency plans. For more information of
channel numbers versus actual frequencies, CLICK HERE. |
| The passive splitters and directional
couplers used in cable systems are called flat loss devices. Below 450 MHz
(CATV channel 61) they are essentially flat in their loss characteristics. With the newer 750 to 860 MHz RF distribution systems, the high-frequency
insertion losses of these devices needs to be addressed when totaling the signal losses
between amplifiers. Passive losses for splitters, taps and directional
couplers are published by the various manufacturers. Pocket-sized reference books
are published by the major CATV manufacturing companies containing this type of
data. A brief excerpt of this type of data is included in this article. |
| A splitter is a passive device with from 2
to 8 ports with equal loss to each and every port. The most common type of
splitter has one input port and two output ports. It is commonly referred to as a
"two-way splitter" - - meaning it has two output ports. In a perfect world
of 100% efficient devices, an equal split of RF energy would result in a signal loss of 3
dB to each output port. The "real world" loss of the splitter is typically
3.5 dB to 3.8 dB at lower frequencies to as much as 5 to 5.5 dB at the highest frequencies
used in CATV / MATV systems. For practical purposes, the loss of a 2-way splitter
may be assumed to be 4 dB, a 4-way splitter loss can be 7.5 to 8 dB, and an 8-way splitter
can be from 11 to 12 dB insertion loss to each port. |
| A directional coupler is a three-port or
four-port passive device that does NOT evenly divide the RF signals at the output
ports. Depending on the RF bandwidth of these devices, they typically have
tap loss values of 8 dB, 12 dB, or 16 dB. The higher the loss to the "tap
port", the lower the insertion loss to the "through port". When a directional coupler is connected directly to a splitter in a single
housing, it is called a directional tap or multi-tap. Common tap loss values
for these devices vary from 8 to 32 dB depending on configuration and number of tap ports
(up to 8). |