[TowerTalk] Metal Roof
jd0 at broadsci.com
Fri Jan 12 01:13:40 EST 2018
> Another point. IM follows the square of the signal strength
> that excites
> it, so reducing TX power by 3 dB reduces the IM by 6 dB.
I believe you're speaking of a second-order mix. Third-order mixes tend to
be more problematic in a repeater installation as they yield "in-band" mix
products. Third-order mix amplitude varies as the cube of the fundamentals,
so a 1 dB change in the power of a mix contributor will result in a 3 dB
change in the third-order mix product amplitude.
Aside from potential PIM issues, the imperfect joint contact in a metal roof
is almost guaranteed to generate broadband noise when excited by a strong RF
source (such as a repeater transmitter). That is, it doesn't take a second
(or third or fourth) signal to cause problems.
--- Jeff WN3A
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