Handbook for Sound Engineers

420 Chapter 14

14.10.3.2 Outdoor Installation

Outdoor installations require special installation tech-
niques that will enable the cable to withstand harsh envi-
ronments. When using cable in an aerial application,
lash the cable to a steel messenger, or buy cable with a
built-in steel messenger. This will help support the cable
and reduce the stress on the cable during wind, snow,
and ice storms. When direct burying a cable, lay the
cable without tension so it will not be stressed when
earth is packed around it. When burying in rocky soil,
fill the trench with sand. Lay the cable and then place
pressure-treated wood or metal plates over the cable.
This will prevent damage to the cable from rocky soil
settling; in cold climate areas, bury the cable below the
frost line. Buy direct burial cable designed to be buried.

14.10.4 Coaxial Cable Termination Techniques

14.10.4.1 Soldering

Soldering offers several advantages as it can be used on
solid or stranded conductors and it creates both a solid
mechanical and electrical connection. The disadvantage
is that it takes more time to terminate than other methods
and cold solder joints can cause problems if the connec-
tor is not soldered to the cable properly. The use of
lead-based solder might also be a consideration if RoHS
(reduction of hazardous substances) requirements are
part of the installation. Soldering is not recommended
for high-frequency applications, such as HD-SDI or
1080p/60 as the variations in dimensions will show up
as variations in impedance and contribute to return loss
(see Section 14.10.5).

14.10.4.2 Crimping

Crimping is probably the most popular method of termi-
nating BNC and F connectors on coax cable. Like the
solder method, it can be used on solid or stranded con-
ductors and provides a good mechanical and electrical
connection. This method is the most popular because
there is no need for soldering so installation time is
reduced. It is very important to use the proper size con-
nector for a tight fit on the cable. Always use the proper
tool. Never use pliers as they are not designed to place
the pressure of the crimp evenly around the connector.
Pliers will crush the cable and can degrade the electrical
properties of the cable.

14.10.4.3 Twist-On Connectors

Twist-on connectors are the quickest way of terminating

a coaxial cable; however, they do have some draw-

backs. When terminating the cable with this type of con-

nector, the center conductor is scored by the center pin

on the connector, thus too much twisting can cause dam-

age to the center conductor. It is not recommended for

pan and tilt installations as the constant movement of the

cable may work the connector loose. Because there is no

mechanical or electrical crimp or solder connection, this

connector is not as reliable as the other methods.

14.10.4.4 Compression Connectors

There are connectors, often a one-piece connector, that

fit over the stripped cable and fasten by having two parts

squeeze or compress together. This is a very simple and

reliable way of connecting cable. However, the very

high-frequency performance (beyond 500 MHz) has yet

to be proven and so these connectors are not recom-

mended for professional digital applications. A compres-

sion connector that is measured with a return loss of

20 dB at 2 GHz would be acceptable for professional

broadcast HD applications.

14.10.5 Return Loss

At high frequencies, where cable and connectors are a

significant percentage of a wavelength, the impedance

variation of cable and components can be a significant

source of signal loss. When the signal sees something

other than 75: , a portion of the signal is reflected back

to the source. Table 14-22 shows the wavelength and

quarter wavelength at various frequencies. One can see

that this was a minor problem with analog video (quarter

wave 59 ft) since the distances are so long. However,

with HD-SDI and higher signals, the quarter wave can

be 1 inch or less, meaning that everything in the line is

critical: cable connectors, patch panels, patch cords,

adaptors, bulkhead/feedthrough connectors, etc.

Table 14-22. Wavelength and Quarter Wavelength

of Various Signals at Various Frequencies

Signal Clock

Frequency

Third

Harmonic

Wavelength Quarter

Wavelength

Analog video

(4.2 MHz)

analog analog 234 ft 59 ft