DW,

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You can confirm what the expected total power of multiple signals is by the usual formula:? P_total ?= ?P _{of one signal} + 10Log N ?where N = number of identical powered signals.?

In your example, 10 signals of -30dBm each produce a total combined power of -20dBm.?

However, there’s a ‘gotcha’ lurking here for the unwary.? As @nj902 indirectly alluded, there may be other (larger) signals present outside the frequency band that your instrument is observing.? Observing off-the-air signals, this is likely the case.? The only way to be sure is to use a broadband power meter and then tuneable filters to only pass the signal band of interest to your SpecAn’s input.

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A quick and ‘accurate’ way to confirm ?that your instrument’s front-end is not suffering from overload (and IM issues) is to increase the input attenuation 10dB and observe that ALL signals reduce by 10dB. (You should insert another 10dB attenuation to be sure.)? If some, or all of the signals drop by more that 10dB – you’ve got IM and overload present !

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As an aside, measuring the total power of the 3-30MHz HF spectrum ?with even a half-decent antenna is very instructive.?? With some Asian HF broadcast stations having ERP’s in the order of many megawatts using directional arrays, it’s no wonder many receivers suffer overload and image problems. ?

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Cheers,

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Kit