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Coax Attenuation


I wasn't going to post this concept, but since there
seems to be some discussion today about coax attenuation,
I think I will anyway.

I tried last week to develop (since I couldn't find one)
a formula for calculating the attenuation of a known
length of cable given the attenuation per 100 ft.

For example, given an attenuation of 3dB per 100 feet,
what is the attenuation of 50 feet of said cable in dB?

Given 10 watts into 100 ft of this 3dB line, the ratio
of input power to output power is 10/5 or 2 (3dB).

For 50 ft, the power loss is half what it was before or
2.5W lost rather than 5W lost.  The I/O power ratio of
50ft is 10/7.5 or 1.25dB.

The scaling must be done with power ratios, not dB.  I have
seen, however, articles saying to halve the 3dB factor which
gives 1.5dB not 1.25dB.

This may seem a small difference, but for critical calculations
it becomes important.

I put the calculations into a spreadsheet but still would like
to have an explicit equation for this calculation.  If anybody
knows of such a formula please post it.

Bill, WA6QGR

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