Coaxial Cable Selection Guide

Contents

1. Introduction
2. Low Loss Communication Cable
    2.1. CFD Cable
    2.2. HDF Cable
    2.3. BT3002 Coaxial Cable
    2.4. LMR Cable
3. Cable numbering
4. Cable selection
5. Some of the reputed brands of cable manufacturers
6. Conclusion
7. Annexure

1. Introduction

This article talks about the following topics: 

• Low loss communication cable 
• Cable numbering system 
• Cable selection criteria 
• Some of the reputed brands of cable manufacturers

2. Low Loss Communication Cable

In the next section, we discuss some of the specially designed low loss coaxial cables designed, LMR, CFD and HDF, for communications systems such as wireless LAN, GSM and GPS applications.

2.1. CFD Cable

CFD or Celled Foam Dielectric is a series of low loss cable, which are functionally similar to the LMR and HDF series cable. CFD cable is ideally suitable for 2.4 GHz WiFi antenna cabling outdoor installations. The physical and electrical properties are comparable with the RG58/U and RG8 cable. These 50-ohms cables exhibit good attenuation property and at the same time are rugged to be used in outdoor applications.

CFD construction	Properties		CFD 200	CFD 400
	Mechanically similar to		RG58/U	RG8/U JIS 8D
	Nom impedance		50 Ω	50 Ω
	Velocity of propagation		83%	85%
	Capacitance		80 pF/m	78 pF/m
	DC resistance Conductors	Inner	0.0196Ω/m	0.00456Ω/m
		Outer	0.016Ω/m	0.00541Ω/m
	Attenuation	MHz	dB/m	dB/m
	Attenuation	2400	0.54	0.22
	Attenuation	2000	0.49>	0.19
	Attenuation	1500	0.42	0.16
	Attenuation	900	0.32	0.12
	Attenuation	450	0.22	0.08
	Attenuation	220	0.15	0.06
	Attenuation	150	0.13	0.05
	Attenuation	50	0.07	0.02
	Attenuation	30	0.05	0.02
	Outer diameter		4.95 mm	10.3 mm

2.2. HDF Cable

HDF is a closed cell polyethylene dielectric low loss flexible cable, very similar in functionality as the CFD and LMR cables. It is ideally suitable for WiFi applications at 2.5 GHz and 5.8 GHz frequency bands. It uses a center conductor made of solid copper or copper clad aluminium. HDF construction details and its electrical characteristics are given in Annexure B.

2.3. BT3002 Coaxial Cable

BT3002 is a 75-ohm coaxial cable. It has a solid center core (1×0.31mm) and a double braid shield with a PVC Jacket. The outer diameter is 3.55mm. BT3002 coaxial cable is commonly used for interconnection of data or telecommunication equipment with 75ohm connector like SMB RF connector, 1.0/2.3 RF connector, and 1.6/5.6 RF connector.

BT construction	Properties		BT3002
	Nom impedance		75 Ω
	Capacitance		66 pF/m
	Attenuation	MHz	dB/100 m
		100	20.9
		50	14.5
		20	9.3
		10	6.6
		5	4.6
		2	2.9>
		1	2.1

2.4. LMR cable

LMR cables are high performance broadband, flexible, low loss 50 Ohm coaxial communication cables designed for use in wireless applications such as: 2-way land mobile, IEEE 802.11a & 802.11b, Cellular, Wireless local loop, PCS, LMDS, Wireless Internet (WISP), MMDS, Broadband wireless data, CLEC, Telemetry and Paging. LMR cables are useful in connections such as internal component and equipment wiring, inter/intra cabinet jumper cabling, Base station and antenna jumpers, tower and pole feeder runs, in-building runs, including riser runs and air-handling plenums and rooftop installations.

Fig. LMR cable

LMR cable sizes range from LMR-100 (2.54 mm) to LMR-1700-DB (31.75 mm). Because of its flexibility it can be used as small size feeder cable in place of larger corrugated feeder. They exhibit better attenuation property compared to RG cables for wireless applications.

Features of LMR cable (from Times Microwave Systems):

Polyethylene Foam Dielectric

• Closed cell
• Dry nitrogen gas injected- no moisture to degrade performance
• High velocity
• Low loss

High Performance Flexible Shielding System

• Multi-laminar aluminium composite tape bonded to the dielectric provides > 90dB isolation shielding (180dB cross talk)

• Bonded construction ensures 100% effective shielding acts as a second moisture barrier

• Outer Braid of tinned copper provides positive means for grounding and connector attachment

Polyethylene Outer Jacket

• Heavy duty UV, sunlight and weather resistant, extended shelf life

LMR cables have lower loss and better shielding than comparably sized braided cables. Polyethylene jacket, closed cell foam poly dielectric and bonded tape conductor contributes to the better weather resistance as compared to braided cables and 9913.

A wide variety of connector interfaces with the LMR cable is used such as:

o N o MUHF o F
o BNC o 716DIN o LC
o TNC o SMA o HN
o UHF QDS (quick disconnect)
o Reverse polarity
o QMA

3. Cable numbering

The history has it that the use of coaxial cables has been made popular by the military during the World War II. They were the ones to come out with commonly used coaxial cable such as the RG cable. The name RG stands for Radio Grade. The military brought out a handbook, named MIL-HDBK-216 (published in 1962) containing reference material for the coaxial cables. The handbook designated the cables in the form RG-"cable number" or RG-"cable number/U". For example, RG-6/U means RG-6 type of coaxial cable for 'Universal' (U) applications. These designations, since then have been made obsolete, and the Military Handbook withdrawn. However, for historical reasons (such as familiarity with the name RG and its usage over the generations), these designations continue to be used till today. In practice, the cable manufacturers list their cable products (in the form of a catalogue) with their own designations and provide cross-reference to the equivalent RG number.

Part or Identifying Number (PIN): The PIN consists of the letter "M" followed by the specification number, the associated slash sheet number and the sequentially assigned dash number or "RG" number. (Reference: MIL-C-17 spec document)

The military standard specification being used today is called the MIL-SPEC standard, MIL-C-17. The specification enlists all types of coaxial cable being designed and manufactured today. It designates the cable with MIL-C-17 numbers for a cable, for example, M17/75-RG214. It is to be noted that, since there is no standard applicable to RG-'number' type of cable being manufactured today, there is no guarantee that the cable will conform to the electrical and physical characteristics. It is left to the users to verify these characteristics by referring to the manufacturer's catalogue data or through test results data published by them. The RG designators are generally used to identify a compatible connector that will fit the inner conductor, dielectric and the jacket dimension and geometry of the old RG series cable. Apart from the RG designation, some of the other designations used for coaxial cable in the industry are the URM, CT, BT, RA, PSF and WF series. Other popular cable type name includes LMR, HDF, CFD, and H series.

4. Cable selection

Cable selection criterion is driven by the end application from the system design perspective. Sometimes, success or failure of a system performance has a direct bearing on the cable design. In today's applications, cables are exposed to conditions such as extreme temperatures, chemicals, abrasion, and extensive flexing. These variables have a direct impact on the materials used for cable insulation and jacketing as well as the construction of the cable. Using a systematic approach will help ensure that we select the right cable for our application - an approach that includes the following:

• List the constraints that will affect performance, including electrical, mechanical, environmental, and application-specific factors.
• Share this list with the cable manufacturers so they can select the best materials and construction; through testing and data analysis, the manufacturer should demonstrate that the cable would, in fact, perform in the specified environment.
• Understand the total cost of ownership. What is the cost of a failure?

Electrical performance is the first criteria in selecting a cable system. Factors that effect should be explored in detail, such as, the ones that compromise signal integrity:

• Electromagnetic interference (EMI) from internal and external sources;
• Crosstalk, due to undesired coupling of signals between two transmission circuits;
• Attenuation, which ultimately determines the maximum length of a signal cable;
• Conductor resistance, which affects voltage drop over a power line.

In general, while selecting a cable or designing a cable system the following six important electrical requirements should be considered:

• Cable Impedance
• Power Handling
• Frequency of Operation
• Signal Attenuation
• Phase/Group Delay
• Noise or cross coupling

Mechanical stress of a cable system happens when it is exposed to high-speed motion in restricted space, such as in hand-held devices or automation and aerospace applications. Movement like random, rolling, and torsion types of motion cause flexing; or when they rub against each other, the cable chain, or other hardware in the system, generating friction that can result in jacket abrasion. Mechanical stress also happens when a cable is manually operated, which can result in kink, pinch, or crush due to improper handling. Hence, tensile strength of the cable is essential in overcoming mechanical stress. Also, in harsh environment, a cable can come into contact with sharp surfaces that can cut or expose it to severe abrasion. Extreme acceleration and vibration can lead to mechanical stress that can compromise the signal integrity and cause failure of a cable.

Environmental stress results from the physical area and climatic conditions in which the cables are exposed. Extreme temperatures and pressures (vacuum) affect cable materials, with low temperatures making them brittle and high temperatures causing them to become soft. Like extreme temperatures, extreme pressures (like in vacuum or hydrostatic pressure) can have a significant impact on cables in which the insulation or the jacket gets affected. Radiation can damage both insulation and jacket materials, depending on the type and dosage level. As environmental stress can shorten the life of a cable, these issues must be taken into account when designing a cable system.

Application-specific stress results from constraints that are unique to the application. For example, aerospace applications require lightweight and small size cables to minimize mass during take-off. Network routers in high-speed computing applications require a long cable that has high bandwidth and high data rates, so cable size and attenuation are important considerations for these applications. There is also a need to check the connector compatibility, which is application specific (audio, video, high speed data etc). Cables used in public places (transportation and automation applications) should additionally take into account the safety issues such as flammability, voltage, and the use of halogens.

5. Some of the reputed brands of cable manufacturers

• Belden
• Commscope/Andrew
• Gore
• Canare
• Gepco
• Times Microwave Systems
• Meggit

6. Conclusion

To summarize the contents of this cable discussion; we have covered different aspects of RF and microwave coaxial cable, from audio-video to RF and to speciality cables used in submarine and aerospace application. In conclusion, we would like to note that with the advent of new methods of technology the material and construction process has improved. Cable industry is a well spread-out and established one and serves a vital role in the telecommunication industry. Research community and industry together, have evolved newer transmission line concepts, gearing the cable industry to meet future challenges. Certain aspects of a cable; like cable testing and assembly has been covered in a separate article, which can be found at www.rfcables.org.

7. Annexure

Annexure A

The table* below shows a comparison of electrical and physical properties of different types of cables and referred to by their RG and MIL-W-17 type names.

* Reference material collected from RF Café website

Annexure B

Source: www.mmtmachrone.com