[TowerTalk] Metal Roof

[Jeff DePolo]

> 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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