Choosing the Correct Cable Type


I've already mentioned the big three components in an RFID network: the reader (and antenna), tag, and software. However one of the most overlooked components of the entire system is the cabling. Cabling is a vital aspect of designing a quality RFID network and yet is the least understood topic in the system design. Even after having the highest-quality RFID components, if the RF signal is not transmitted by using a proper medium, the entire system will suffer. By using a proper cable and installation technique, you can improve the performance of the entire RFID system.

Figure 5.3 shows the cutaway view of a coaxial (or coax) cable. A coaxial cable is one that consists of two conductors that share a common axis. The inner conductor is typically a straight wire, either solid or stranded, and the outer conductor is typically a shield that might be braided or foil.

image from book
Figure 5.3: Cutaway view of coax cable

In a coaxial cable, the RF signal is carried between the cable shield and the center conductor. Coaxial cables are typically characterized by the impedance and cable loss. The length has nothing to do with coaxial cable impedance. Characteristic impedance is determined by the size and spacing of the conductors and the type of dielectric used between them. For ordinary coaxial cable used at reasonable frequency, the characteristic impedance depends on the dimensions of the inner and outer conductors. Most common coaxial cable impedances in use in various applications are 50 ohms and 75 ohms, but 50 ohms is predominantly used in RFID systems.

If you select an incorrect coaxial cable, that mistake can degrade the overall signal transmission and/or allow outside electromagnetic or radio frequency interference to be introduced into the main signal. This causes high noise levels, which in turn can result in poor read rates. Using the wrong coaxial cable is common with poorly trained RFID technicians. In an RFID system, the most important factor to be considered is that there is proper impedance match between the reader, coaxial cable, and the antenna for maximum power delivery. If this is not done, there will be signal loss and reflection resulting in short distance transmission and poor RF signal quality. Figure 5.4 shows a well-matched RFID system.

image from book
Figure 5.4: A well-matched RFID system

If the cable is such an important part of an overall RFID system, how do you go about making sure you have the right one for the job? Like everything else in RFID, you can use science and a strong foundation in physics to plan for the optimal RFID network. The parameters to consider while selecting a cable are as follows:

  • Mechanical Characteristics These can include center conductor material, dielectric material, shield type, and jacket material.

  • Electrical Characteristics These include resistance, capacitance, impedance, and attenuation.

The center conductor, dielectric, type of shielding, and jacket all play important roles in deciding the preceding parameters.

image from book
Is What You Have What You Want?

Often when you order an RFID reader and antenna combination from a hardware manufacturer, you will be given the option, and often the recommendation, to purchase their cables. This is not always the best option. At ODIN Technologies, after setting up and deploying thousands of antennas and cables, we've taken a different approach to cabling. We have in-house teams that make up our cables, or for very large orders we use a specific outsourcer who builds just the cables for us. Often we'll use three or four types of cables on a deployment, and sometimes two types of cable on the same reader.

image from book

The higher the frequency of the RF signal, the more the signal travels on the outer surface of the conductor. This phenomenon is known as the skin effect. Hence, depending on the frequency of use, a certain type of center conductor needs to be selected. Dielectric material and its physical dimensions and composition decide the electrical characteristics such as capacitance, impedance, attenuation, and velocity of propagation of the cable. It is essential to also consider the type of shielding the cable has, because it determines how well the center conductor carrying the RF signal is isolated from the ambient environmental noise (AEN). The shielding also needs to act as a low-resistance ground path. Depending on the environment where the cable needs to be installed, the cable with appropriate jacket needs to be selected.

LMR are the letters that proceed a number (like LMR-400) when ordering a thickness of cable. The higher the LMR number the thicker the cable and the lower the loss. LMR doesn't seem to be an acronym for anything, rather it seems to have cropped up as an industry way of describing cable types.

The first step in selecting a superior-performing RFID system is getting the tags, readers, and antennas correct based on what you learned from your site survey and tag testing. Depending on your frequency selection and system requirements, you should choose the right type of cable. Choosing the right cable is only as good as choosing the right reader-if not deployed properly, then it can ruin what might be a perfectly designed RFID network.

Note 

Cable manufacturers have a very specific measurement when it comes to laying out cable. This is true not just for RFID, but also Internet networks, fiberoptic, and other similar transmission mediums. The manufacturer will give you the radius measurement, which cannot be exceeded. A team of ODIN engineers went on-site to a third-party logistics provider (3PL) to perform an RFID Rescue. The 3PL was hamstrung with low read rates and wanted to improve them. One of the first things our team noticed was cable stuffed into racks at very tight radii to accommodate all the equipment. This tight bend was interrupting the flow of RF energy and a partial cause for the poor read rates.

Our team at ODIN has a checklist a mile long of things we are careful to avoid when it comes to deployments, but here are the highlights for installing the cable for the system:

  1. Evenly distribute the pulling tension over the cable and never exceed the minimum bend radius or the radius that you can bend the cable around and still get maximum performance. Exceeding these factors can result in permanent mechanical and electrical damage to the cable.

  2. When laying out cable, try to protect it from harsh environments by installing the cable in conduits. When pulling the cable from the conduits, try to lubricate the cable before pulling and also deburr the ends of the conduit. Failing to do so can damage the external jacket and shielding.

  3. Always lay the cable without any tension and leave some extra cable to incorporate any changes or correction.

Appropriate end connectors are required to connect the reader and antennas to the cables. The required connector types will depend on the reader and antenna being used. The cable selection (thickness) can be limited by the connector. Sometimes longer cable lengths cannot be implemented since the thicker low-loss cables are unusable. The thicker cable may be unusable because of the connector types being used by the reader or the thickness of the cable housing not working with a bigger cable. There are several methods to make the cable and connector connection, but the most common are as follows:

  • Solder Method This method provides the most robust mechanical and electrical connection. The technician needs to have extensive soldering experience to make a good connection. The main disadvantage of this method is that there can be a dry or cold solder joint which requires more time to make a solder joint.

  • Crimp Method This crimping method provides an average mechanical and electrical connection by squeezing the connectors onto the end of the cable. The main advantage is that it requires less time to make a termination compared to the solder method. A tight fit on the cable is important; hence proper crimping tools are a must. Doing it incorrectly can damage the connector or the cable, resulting in degraded electrical properties.

Standard coax cable connectors used in RFID systems are as follows:

Series

Frequency GHz

Power Watts [*]

Typ. Diameter Inches

Relative Cost

BNC

0–4

80

0.6

Low

N

0–11

300

0.8

Moderate

SMA

0–18

100

0.4

Moderate

TNC

0–11

100

0.6

Low

[*]At 1 GHz

Some commonly used 50-ohm cables are as follows:

Type

Frequency MHz

Diameter inches

Relative cost

RG-58

0–3000

0.2˝

Low

LMR-240

0–2000

0.4˝

Moderate

LMR-400

0–2000

0.4˝

Moderate

Cable attenuation at various frequencies are as follows:

Attenuation (dB per 100 feet)

MHz

30

50

100

146

150

440

450

1000

2400

LMR-100A

3.9

5.1

 

8.8

8.9

15.6

15.8

  

RG-58A/U

2.5

4.1

5.3

6.1

6.1

10.4

10.6

24.0

38.9

MHz

30

50

100

146

150

440

450

1000

2400

LMR-200

1.8

2.3

 

3.9

4.0

6.9

7.0

 

16.5

LMR-240

1.3

1.7

 

3.0

3.0

5.2

5.3

 

12.7

LMR-400

0.7

0.9

 

1.5

1.5

2.7

2.7

 

6.6

           

Approximate values shown for comparison purposes only.

The following are some important definitions and formulas you need to be familiar with:

  • Attenuation Attenuation is the decrease of a signal over distance in the direction of propagation. Attenuation may be expressed as the scalar ratio of the input magnitude to the output magnitude:

    image from book

  • Capacitance Capacitance is the ability of a dielectric material between two conductors to store electric energy. For coaxial cables, capacitance per meter is as follows:

    image from book

    where, er = dielectric constant, and Zo = impedance

  • Impedance (Zo in ohms) Impedance is the characteristic property of a transmission line describing the ration between electric and magnetic fields. For coaxial cable, the characteristic impedance is basically given by the permittivity of the dielectric and the dimension of the conductors:

    image from book

    where, er = dielectric constant




CompTIA RFID+ Study Guide Exam RF0-101, includes CD-ROM
CompTIA RFID+ Study Guide Exam RF0-101, includes CD-ROM
ISBN: N/A
EAN: N/A
Year: 2006
Pages: 136

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