Globalstar Product Management – Technical Support

RF Cable Line Loss Calculations

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The following technical note describes how to calculate RF attenuation for RF cables.

Line loss theory:

From http://www.TimesMicrowave.com and http://www.bestfit.com
Line loss is composed of conductor (wire) loss and insulation (dielectric) loss.
Conductor loss increases (very closely) in proportion (k1) to the square root of frequency, whereas insulation loss increases (very closely) in direct proportion (k2) to frequency.

The total matched line loss at any frequency is given by the formula
ML(dB) = k1 * SQRT(Freq) + k2 * Freq
where (in this case):

ML is in dB / 100 feet,
Freq is in MHz, and
k1,k2 depend on the characteristics of the line.

Connector Loss theory:

From http://www.Johnsoncomponents.com

Connector_Loss_Factor x SqRt (Fghz)
Loss factors for SMA connectors are:
0.06 for each straight connector
0.15 for each right-angle connector

Line loss of a cable includes connector loss at each end.  For example, connector loss (dB) for a cable at 1500MHZ with two straight SMA fittings will be:

0.06 SqRt(1.5) + 0.06 SqRt(1.5)

k1 and k2 values:

Times Microwave seems to be the only cable manufacturer which supplies k1 and k2 values (http://www.timesmicrowave.com/telecom/pdf/LMRGuide.pdf).  Other manufacturers only supply tables of values.  To obtain k1 and k2 values for these manufacturers, there is an Excel spreadsheet available on the internet (www.bestfit.com) which calculates best-fit k1 and k2 values from manufacturer’s tablular values.  These calculated coefficients allow you to predict cable losses accurately enough to predict cable losses, and thereby determine what size cable to use for a particular length.  However, predictions do not always match reality.  Therefore, since cable performance is critical to our application, we always measure the actual losses of the manufactured cables to see how closely they match the predicted losses.

Overall Cable Loss Calculations:

Finally, the following equation can therefore be used for cable calculations:

Cable Attenuation (dB per 100 ft) at Any Frequency = [ k1 x SqRt (Fmhz) ] + [ k2 x Fmhz ] + [Connector_Loss_Factor x SqRt (Fghz) ]

Since the whole cable is at the same frequency, you can add the connector loss factors for the two ends and use a combined value for the connector loss factor:

straight-to-straight ==> 0.12
straight-to-RightAngle ==> 0.21
RightAngle-to-RightAngle ==> 0.30

The k1 and k2 values calculated for Belden Cables are:

B7806A,  k1=0.301087757,  k2=0.001363041
B7808A,  k1=0.219552992,  k2=0.000442118
B7810A,  k1=0.116944336,  k2=0.000364839

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Written by Joseph Crowley

June 27, 2008 at 7:45 pm

Posted in RF Antenna Cables

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