COAXIAL CABLE, Jim Hadlock, K7WA, no. 51

COAXIAL CABLE
May 13, 2009 – Educational Radio Net
Jim Hadlock K7WA


Introduction:

Coaxial cable transmission line is commonly used to connect our transceivers to antennas. It is used for other purposes as well, such as Matching Sections, Baluns, Traps, and Stubs. Sooner or later, each of us will probably need to add coax to our home or mobile radio systems. Tonight’s session will cover the different types of coax and the criteria to consider when deciding what cable is best for a given situation. In the spirit of amateur radio, I am going to try to avoid using manufacturer and distributor names on the air, see the Educational Radio Blog for more information on specific products and suppliers.

Coaxial type cable was first used for transatlantic telegraph cable communication in the late 1800’s. These early cables were composed of a central conductor encased in a cylindrical insulating material, and were considered coaxial because the seawater that surrounded them completed their return circuits. Later developments led in 1929 to a patented design by two engineers who worked for AT&T for a coaxial cable system intended for transmission of television signals. During World War II the military accelerated the development and production of flexible, solid-dielectric coax. It was at this time that coax acquired its now-familiar RG/U (Radio Guide Utility) numbers. After the war, amateur radio operators began using the readily available surplus coaxial cable for their antenna feedline systems.

Coaxial cable consists of an inner conductor with an insulated covering (dielectric), which is then covered with a braided wire or foil sheathing (shield). The sheathing is covered with a flexible outer jacket. Although coax has greater loss than twin-lead or open-wire transmission lines, it has some important advantages: it can be buried underground, run inside a metal mast or taped to a tower without harmful effects. In addition, our modern radios are designed for unbalanced coaxial cable transmission line.

Specifications:

Looking at a table of transmission line characteristics (ARRL Handbook (2005): Nominal Characteristics of Commonly Used Transmission Lines (Table 21.1) page 21.2-21.3) we see several characteristics specified for each type of cable:

RG or TypeNumber           General type or Family-
Part Number                Manufacturer’s part number-
Impedance                  50 and 75 ohm are most common,
                           determined by the size and spacing
                           of the two conductors, and the
                           dielectric material between them -
Velocity Factor            rate of rf propagation in the cable
                           compared to free space -
Capacitance                two parallel conductors have capacitance -
Center Conductor AWG       center conductor size and construction -
Dielectric Type            dielectric material -
Shield Type                shield construction -
Jacket Material            jacket construction -
Jacket Outer Diameter      dimension of the outer jacket -
Maximum Voltage (RMS)      maximum voltage rating -
Matched Loss               loss in decibels for different frequencies -
Power Handling Capability  recommended maximum power -

Applications:

Consider the following factors when selecting coaxial cable:
Power
Highest frequency (HF, VHF, UHF)
Length
Loss (determined by frequency, cable type, and length)
Weight
Flexibility
Environment

Coaxial Cable Types (50 Ohm Impedance):
RG-174 used for internal connections -
RG-58 comes with commercial antennas (high loss) -
RG-8X low power HF, short VHF feedlines -
RG-8 higher power or VHF/UHF feedlines -
RG-213 high power HF, short VHF feedlines -
Heliax and Hardline long feedlines at VHF or UHF -

Examples: (remember, 3 dB is 50% power loss!)

Example 1: 144/440 mHz J-Pole 50 ft feedline: 100 ft feedline
RG-58 4.5 dB loss 9.7 dB loss
RG-8X 3.35 dB loss 6.7 dB loss
RG-213 2.35 dB loss 4.7 dB loss
RG-8 1.35 dB loss 2.7 dB loss

Example 2: HF Antenna (below 50 mHz)
RG-58 2.9 dB loss
RG-8X 1.5 dB loss
RG-213 1.3 dB loss
RG-8 0.8 dB loss

Connectors and Adaptors:
UHF (PL-259 type), most common general use -
Type N, low loss, used at VHF and UHF -
BNC, used on Hand-held radios and test equipment -

Conclusions:

Use the best coaxial cable and connectors for the requirements -
Used coaxial cable may not be a bargain -
Mostly, you get what you pay for -


References:

ARRL Handbook (2005): Nominal Characteristics of Commonly Used Transmission Lines (Table 21.1) page 21.2-21.3, also see the ARRL Antenna Book and other references

The Wireman, Inc. (coaxial cable, antenna wire, connectors, etc.), www.thewireman.com

Radioware (Amphenol PL-259 connectors, coaxial cable, etc), www.radio-ware.com

The Pacific Northwest VHF Society “Noise Floor” newsletter (Winter 2008 and Summer 2008) contains a two-part article on attaching Type N connectors to coax – must reading for anyone who wants to attach their own connnectors!: http://www.pnwvhfs.org/articles/noisefloor/noisefloor.htm