Hardline is also a term used to describe a type of coaxial cable. Unlike flexible coaxial cable used to hook your TV to your cable box, or the patch cords running out of the back of your audio components, Hardline is a relatively large and rigid cable that is from 3/8 inch to over 3 inches in diameter. Instead of a stranded center conductor surrounded by an insulation and a braided shield, Hardline consists of solid center conductor, usually copper or copper-clad aluminum, surrounded by either foam or a spirally wound insulator, which is surrounded by either a solid shield of Aluminum or copper.
Why use Hardline?
Hardline and its connectors are much more expensive and hard to handle than the stuff behind your TV, but without it neither your Cable TV
, Cable Modem
or cell phone
would exist today. It would have been nearly impossible for the modern telephone network
to develop without hardine, though Fiber Optics
have supplanted it for the most part. Hardline can carry signals at UHF frequencies for long distances without excessive attentuation
. Its main uses are in commercial and military VHF
, and some lower microwave
radio installations. Hardline that is up to 6 inches in diameter is used to carry signals from high powered transmitters for commercial TV and radio stations to the antennas mounted hundreds of feet up. Somewhat smaller diameter hardlines are used for transceiver
s at cell tower
transmitters, and in a variety of commercial and amateur radio repeater
installations. Hardline is also the stuff that brings your Cable Modem and Cable TV signals to a utility pole
or box near you. The small diameter cable that feeds into your Cable Modem or TV is attached to a much larger cable, which is usually about 1 inch in diameter.
Types of Hardline
Hardline is rated by its outer diameter, type of jacket its characteristic impedance, insulation type or dielectric, and attentuation characteristics.
A hardline's outer diameter determines the amount of power it can carry, and to a great extent, the amount of attentuation a signal of a certain frequency will experience along a given length. Gigantic hardlines up to 6 inches can carry hundreds of thousands of watts generated by RADAR
and UHF Television station
s. The larger the diameter of the hardline, the lower the attentuation, and the more power the line can carry. The outer diameter is the diameter of outer shield, not including any armor or jacketing.
Type of Jacket
Hardline can come with a number of jacket types. Unjacketed hardline was used on utility poles to carry CATV signals, where adverse conditions are minimal. Aluminum is usually used for the shield material, because it forms a hard oxide coating that prevents further corrosion
. It is used primarily where cost is a primary concern and adverse conditions are not expected. Jacketed hardline can be for just weather protection, or incorporate armoring where direct burial or vandalism
may occur. Jacketing can also incorporate a secondary messenger cable
, such as a high-tensile steel wire to add mechanical strength, or a fiber optic cable.
Insulation Type or Dielectric
Hardline insulation or dielectric comes in mostly two varieties, either a foam, or a spirally wound ribbon of an insulating material such as polyethylene which leaves mostly an air gap between the center conductor and the shield. Better attentuation characteristics can be acheived with this type of insulation than foam, but this type of insulation is vulnerable to getting waterlogged should the jacket and shield be pierced. Often cables of this type are kept under pressure by tanks of dried nitrogen to force out any water that could infiltrate the line.
Hardline is rated by how much a signal of a given frequency is attentuated over a given length. A certain type of cable may have a chart drawn up which shows the attentuation in decibel
s of attentuation per 100 feet of line at various frequencies. All else being equal, larger lines have better attentuation characteristics than smaller lines. Attentuation increases with frequency due to the skin effect
, which means most of the RF current flowing through a cable is near the surface of the conductor. For this reason the surface of the center conductor is either copper
even if the the core is made out of aluminum. Rigid lines will even use hollow center conductors, which are hardlines in the extreme.
Characteristic Impedance Characteristic Impedance
is a function of the ratio of the diameter of the center conductor to the diameter of the shield. The larger the center conductor is relative to the diameter of the shield, the lower the impedance. This is important, because if the line is mismatched to the transmitter or the antenna or other termination, power will be wasted because it will not smoothly flow from the transmitter to the antenna
. Sort of like trying to drive from a stop in high gear, the torque is not well matched to the load. Most Hardline cable is rated at either 50 ohms or 75 ohms. 50 ohm hardline is primarily used in radio transmitter and receiving installations and data transmission, while 75 ohm hardline is primarily used in CATV and video applications. Note to hams:
Surplus 75 ohm hardline from CATV reel ends makes a great substitute for expensive LMR 600 or LDF4-50. Sure there is a bit of a mismatch, but it is not usually enough to worry about.