# Effect of a longer loopback cable on radiated EMI

We have a product that we successfully ran through radiated EMI testing (EN55022) with our highest peak approx. 13dB under the permissible limit.

Looking at our test setup and what is required to comply with the new EN55032 standard, I see our original test setup used loopback cables which were only 0.8 meters in length rather than 2 meters.

I'm looking for an equation to help calculate what would be the worst-case increase to our radiated EMI from loopback cables which are this much longer. I have a hard time believing a 2.5x longer cable would add 13db of noise, but I have no way to mathematically support that.

• What prevents you from using longer cables and test again? Jan 25, 2017 at 21:37
• Easy- They're called Maxwell's equations. (Which won't be easy to actually solve, so you are better off re-testing.) Jan 25, 2017 at 21:41
• It's approximately \$3000 of time at our local lab to run through all of EN550x2, not including my own time. The EN55022 and EN55032 setups are very similar with a few exceptions. I'm trying to determine if those differences, in the worst case, could add up to us violating the limits. Jan 25, 2017 at 21:43
• My hope is that there are a number of assumptions which can be made here (same distance to antenna, same termination impedance, same frequencies, etc.) which can be made here so that it boils down an equation to something pretty straightforward. Jan 25, 2017 at 21:50
• For clarity - I don't mind spending the money and time to re-test, as long as it makes sense to do so. If it turns out a common equation dictates a cable twice as long would only emit 3dB more of noise in this situation, then it doesn't make as much sense to put the time and money into re-testing just on account of the cable length. Jan 25, 2017 at 22:13