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# EMI question: How to calculate an effected range by a transmitter

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#### IlanKupar

##### Newbie level 1
Hello,
I have an RF transmitter with an ERP of -10dBm, and a 0dB gain antenna.
Since I don't want to interfere to other receivers around me I want to calculate the maximum range I might effect.
A few assumptions:
1. The other receivers sensitivity is -174 dBm/Hz (a strict assumption, because I want to take the worst case).
2. ERP of transmitter (as mentioned above): -10dBm.
3. antenna gain= 0dB, no cable loss or polarization loss.

I used the sensitivity equation: S=ERP-22-20log(R)+20log(c/f), but for some reason I think I'm doing something wrong.
c=3*10^8
f=400*10^6 ,(400 MHz).

Am I using the right equation?
What is the maximum effected range?
Thank you,
IK

To calculate the range use the Friis equation, you can find a handy calculator here
https://www.random-science-tools.com/electronics/friis.htm
Your potential freee space interfering range will be quite considerable. You don't give a realistic receiver bandwidth, but assuming 10kHz your -10dBm could be easliy detected by an aircraft flyig overhead and with a small directional antenna by a spacecraft in low earth orbit. In a 1Hz bandwith it could be detected at almost 9500km. This is why EMC reguations are quite stringent on the allowable radiation from electronic apparatus.
I have in tha past been able to detect receiver local oscillators at over 5km and in one case a computer clock harmonic at about 20km. The latter was in an electricity meter installed bythe electricity company !!

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