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A SINGLE CABLE FOR BASE STATION MAIN FEEDER AND JUMPER APPLICATIONS

A 7/8 inch cable that has the efficiency of a main feeder cable but the bending and flexing capabilities of a typical jumper cable

June 1, 2000
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A SINGLE CABLE FOR BASE STATION MAIN FEEDER AND JUMPER APPLICATIONS

ANDREW CORP.

Orland Park, IL

Wireless communications rely on coaxial cable transmission lines to link tower-mounted antennas to base station radio equipment. The traditional way to form this link is to use a main feeder cable and jumper cables. The main feeder is normally a 7/8-inch (dia) cable or larger. This size cable has good transmission efficiency to minimize signal loss. Jumper cables, at each end of the feeder, are 1/2-inch (dia) or smaller. They are typically five to 10 feet in length and have a smaller bend radius than the main feeder to ease attachment of the transmission line to the equipment and antenna.

With each extra cable installed in a feeder system, some performance is sacrificed and installation time and cost increase. To address this issue, a single cable has been developed that performs both main feeder and jumper functions. This new type VXL5-50 7/8-inch cable offers the efficiency of a main feeder cable but is lighter in weight and has the bending and flexing capabilities of a typical jumper cable. In wireless applications, the VXL5 cable can be a single-cable solution for the transmission line function. Performance benefits from using a single cable are realized for both return loss and insertion loss. In addition, a single cable offers site cost savings because lighter weight and extra flexibility aid the installation process. Fewer connectors contribute to lower insertion loss and, along with requiring less weatherproofing, reduce installation time and cost at each site. The VXL5 cable may be the lowest cost solution for today's design requirements.

Fig. 1 Link loss comparison.t

INSERTION LOSS

From the viewpoint of insertion loss, the selection of cabling for a given application is based on the overall length of transmission line run and the transmission efficiency required to meet the allowable link loss budget. Cabling system options are compared by adding up the total link loss evaluated at the maximum operating frequency of the system.

Link losses are caused by cable and additional transmission line components, such as connectors and surge arrestors. The benefits of a single cable installed directly into an antenna can be compared with a traditional installation of standard 7/8-inch feeder with a 1/2-inch jumper (10 foot) at the antenna end. Figure 1 shows the link losses of the two systems as a function of installed cabling length.

Although slightly higher in attenuation than the standard feeder, the site utilizing a VXL5 cable solution has a lower total insertion loss. Shorter installation lengths, which include those having an efficiency target of 71 percent (1.5 dB of total loss) between the radio and antenna, compare even more favorably. The link loss contributions for components of each installation type are listed in Table 1. For the calculations, a 100-foot installation with a maximum operating frequency of 2 GHz is assumed.

TABLE I
INSERTION LOSS FOR 100-FOOT SITE INSTALLATION

Component

Standard Site IL (dB)

Site with VXL5 IL (dB)

10-foot jumper

.033

__

90-foot feeder

1.69

__

100-foot feeder

__

2.03

Two connectors

0.14

0.14

Surge arrestor

0.10

0.10

Total link budget

2.40

2.27

Calculation references:

Connector loss at 2 GHz = (0.05 ´ Îãf ) dB = 0.07 dB

 

LDF4* attenuation at 2 GHz = 3.27 dB/100-foot

 

LDF5* attenuation at 2 GHz = 1.88 dB/100-foot

 

VXL5 attenuation at 2 GHz = 2.03 dB/100-foot

*LDF4 is Andrew HELIAX¨ 1/2" and LDF5 is HELIAX 7/8" foam dielectric coaxial cable

 

TABLE II
SWR FOR A 100-FOOT SITE INSTALLATION

Component

Standard Site IL (dB)

Site with VXL5 IL (dB)

Jumper

.033

__

SWR

1.10

__

Reflection coefficient

0.0476

__

Attenuation factor

0.667

__

Ref(sub)1

0.032

__

Feeder

 

 

SWR

1.13

1.13

Ref(s)2

0.0610

0.0610

Surge arrestor

 

 

SWR

1.07

1.07

Ref3

0.077

0.070

SUM (max reflection)

0.127

0.095

System SWR

1.17

1.15

Max SWR

1.29

1.21

Calculation references:

 

Attenuation factor = exp[Ð(feeder attenuation (dB/100 ft) ´ length (ft)/434.3)]

RSS = square root (Ref12 + Ref22 + É)

SUM = Ref1 + Ref2 + É

SWR PERFORMANCE

The same installations can be compared for system SWR. With a maximum SWR of 1.13 for the feeder and 1.10 for the jumper, the expected performance for system SWR is listed in Table 2. Here, all connector SWR contributions are included within the feeder and jumper specifications. (This is not an important distinction for evaluating the effect of the jumper.) The specified SWR for each component is listed as well as its equivalent reflection coefficient, Refx, at the radio end of the feeder cable. These values are direct conversions of the SWR values, except for the jumper. The equivalent reflection coefficient of the jumper is obtained by multiplying the converted value by an attenuation factor, which takes into account the loss of the feeder cable.

Fig. 2 SWR comparison.

The measured system SWR depends on the phase relationships of the individual reflections and is normally estimated in one of two ways. The first method assumes random phase relationships and uses a root sum of squares (RSS) estimate. The other method assumes all reflections add in phase (SUM) and provides the result for the worst case. The 1.17 SWR, estimated for the standard site RSS, represents a 25 percent increase in reflected power compared with the system RSS estimate of 1.15 for the site installation with the VXL5.

Figure 2 shows the system SWR behavior as a function of installed cable length. Again, the differences in performance are even more apparent as the installed length becomes shorter than the example system.

INSTALLATION COST

The VXL5 cable provides technological advantages through its simplicity and increased reliability and is a transmission line solution that has lower direct and indirect costs. Since the VXL5 cable does not require a separate jumper to the base station antenna, the costs of jumpers are saved in addition to the cost of secondary weatherproofing. Table 3 lists cost comparison details between a traditional system installation and one utilizing VXL5 cable.

TABLE III
INASTALLATION COST COMPARISON

 

Traditional System

System with VXL5 Cable

1/2", 10' Jumper

$  110.00

$      0.00

7/8" DIN Connectors (2)

$    98.00

$   98.00

7/8" Cable, 100' (110', VXL5)

$  585.00

$  583.00

7/8" Grounding Kit (3)

$    90.00

$    90.00

7/8" to 1/2" Weatherproofing*

$    38.00

$      0.00

7/8" Standard Hangers (30)

$  105.00

$  105.00

Total

$ 1026.00

$ 876.00

Savings

 

15%

*Weatherproofing is Andrew 3Mª ColdShrinkª self-shrinking tubing

In addition to the direct cost savings, installations are smoother and quicker, resulting in further cost savings. Less time is required because there are fewer jumpers and fewer connectors to fit. In addition, coordination is simpler because of fewer parts to order and track, and installation is simpler due to the use of more flexible feeder cable (five-inch bend radius) that is 15 percent lighter in weight and requires only one-step cable prep. Thus, the jumperless site installation results in lower costs, improved efficiency and reduced system noise. Additional information is available from the company's Web site at www.andrew.com.

Circle No. 321

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