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How Is RF Cable Signal Loss Calculated?
3 CommentsI am a regular reader of RFID Journal, which I find very useful. The content is amazing indeed for RFID professionals. I'd like to seek your expertise with regard to a question concerning RF cable loss encountered in an overall RFID system.
I saw some information online that said a 2 dB cable loss results in 37 percent loss in signal when it reaches the other end. Similarly, a 20 dB loss in a cable renders 99 percent loss of signal by the time it reaches the other end. The typical length of cable with regard to dB loss is not specified.
How is the loss calculated mathematically and logically in the above scenario (without knowing cable length), and how were the above examples (37 percent and 99 percent for losses of 2 dB and 20 dB, respectively) arrived at? I'd be grateful to you for your reply.
Best wishes,
Sharath
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Dear Sharath,
I am not an engineer, but to my knowledge, you cannot calculate the signal lost as it travels through a coax cable without knowing the cable length. For a 900 MHz signal traveling through a 100-foot RG-11 coax cable, signal loss is 5.4 dB. If the signal travels over 50 feet of cable, the loss is 50 percent less. If the cable length is 200 feet, the signal loss doubles. Different types of coax have different loss rates. I don't know if there is a formula for calculating loss per 100 feet; perhaps one of our readers can chime in with additional information.
—Mark Roberti, Editor, RFID Journal
I saw some information online that said a 2 dB cable loss results in 37 percent loss in signal when it reaches the other end. Similarly, a 20 dB loss in a cable renders 99 percent loss of signal by the time it reaches the other end. The typical length of cable with regard to dB loss is not specified.
How is the loss calculated mathematically and logically in the above scenario (without knowing cable length), and how were the above examples (37 percent and 99 percent for losses of 2 dB and 20 dB, respectively) arrived at? I'd be grateful to you for your reply.
Best wishes,
Sharath
———
Dear Sharath,
I am not an engineer, but to my knowledge, you cannot calculate the signal lost as it travels through a coax cable without knowing the cable length. For a 900 MHz signal traveling through a 100-foot RG-11 coax cable, signal loss is 5.4 dB. If the signal travels over 50 feet of cable, the loss is 50 percent less. If the cable length is 200 feet, the signal loss doubles. Different types of coax have different loss rates. I don't know if there is a formula for calculating loss per 100 feet; perhaps one of our readers can chime in with additional information.
—Mark Roberti, Editor, RFID Journal
READERS' COMMENTS
Derar Sharath
Cable loss is usually given in dB/m or dB/ft. You just have to multiply this value by the length of the cable to get the total loss in dB. Then, you might want to convert this value into a percentage. The exact formula for the amount of remaining signal is 10^(-value/10). A 3dB loss means you get half the power, a 6dB loss means quarter the power, and 10dB means one tenth of the power. RF engineers prefer to use dB, because calculating the total loss requires a multiplication when counting in dB, but requires an exponentiation when in percentage. Best regards
Posted By: Laviale Anthony 6/11/09 at 8:54 AM
example of cable loss calculation
Cable are specified as to loss (in dB) per unit distance, so knowing the length, one can calculate the loss for a given cable. Once you know the loss for your given cable, apply the formula dB=10 log(P1/P2). The % loss is the resultant ratio between P1 and P2. For example, 2 dB = 10 log P1/P2, so P1/P2 = 10^.2, and P1/P2 = 1.58, and P2 = .63 P1, or P2 is 37% less than P1.
Posted By: 6/11/09 at 4:00 PM
RFID
Thanx for the solution on RF cable signal loss(%) calculation which was hard to comprehend without knowing cable length & I/P, O/P power levels. Best Wishes, Sharath
Posted By: Sharath Nandalike 6/16/09 at 12:33 AM