Example Measured Emissions

An initial basic test was conducted using an iCom IC-A6E transceiver scanning the local Cambridge frequencies with the radio 2 metres (6 feet) from any of the test equipment or chargers, a squelch setting of 11 was OK to silence the background noise and permit the radio to scan the channels, only stopping when actual communications were in progress.  With a Charge2 unit powered up and charging an iPad, no adjustment was required on the radio, even if moved closer to the unit.  However, with the generic charger operational charging the same iPad, even when the squelch was increased to its maximum of 24, there was a constant hiss from the transceiver and it would not scan the channels.  

Given the level of interference seen, some more detailed tests were conducted to actually measure the difference between a generic USB charger and a Charge2.  Below are some measurements taken covering the aviation communication band of 118 MHz - 138 MHz. This will help to illustrate the level of RF interference some chargers generate.  This frequency range is the one used to communicate between pilots and air traffic services worldwide, it  also covers some of the Radio Navigation aids too.

Two types of test are shown here which are discussed at the end for those interested, but suffice to say that the lower the amount of RF interference generated, the less problems with radios etc.  

For all tests a generic USB charger and a Chargecharger are used to power an Apple iPad.  The iPad was drawing the maximum power it could, which was 12 Watts or 2.4 Amps.  This is a typical configuration with an iPad actively running SkyDemon and still charging its battery. 

The first test is looking at radiated emissions, the lower the amount generated the better, to prevent problems with radios etc.  

The Yellow trace is the general background noise, the trace was taken with the test equipment all connected up and power applied but no chargers connected.  This is the normal background noise that you would adjust the radio squelch to get rid of.


The Magenta trace is the level of interference generated by a Charge2 when powering the iPad.

The Cyan trace is the level of interference generated by a generic plug in USB charger when powering the iPad.  There is, very roughly, a ~440% increase in the dBuV level of radiated emissions over the whole of the aviation band radio spectrum.  In real terms, as dBuV is a logarithmic scale, the increase is nearer 3100%

The second test is looking at conducted emissions, as before, the lower the amount generated the better.  To show this below are some emissions tests conducted by an aviation shop when comparing Charge2 with some other chargers.  The frequencies shown are related to preventing interference in aircraft instrumentation rather than the radio directly.  The pre-compliance scans were in preparation for the formal tests for EASA compliance.

Plot from a Charge2

Plot from a Chinese unbranded dual port marine version

Plot from a Chinese unbranded mini

Plot from a USB Silvercrest SLM charger

These basic readings should help illustrate one of the problems with generic plug in chargers, they are RF noisy, and can easily cause problems not just with voice communication but the radio navigation systems too including GPS reception.

A bit of a more on the technical side, for those interested…

Whilst these results above are not from the certified equipment found in formal testing labs, they are taken using the same equipment, cables, power source and end devices to maintain the repeatability of the tests.  The whole area of EMC/EMI testing is huge and complex, this is not intended to in any way to try and explain it all.

As a little bit of background there are two main types of emissions which we need to be concerned about, Conducted and Radiated.  The aim is the same in both, to have a little as possible.  The various testing standards all lay down the limits which equipment must meet in order to pass.  

It also gets complicated as readings are taken in dB’s, which is a logarithmic scale, so a small change in a reading in dB can result in a large difference in the actual signal level.  Very roughly a for every 6 dB change in signal level the actual increase doubles. So a difference of 12 dB between two signals is 4 times, 18 db is 8 times etc.

Radiated Emissions

Radiated emissions are the RF signals directly generated by the device itself.  As always, the lower the level of emission the better.  Naturally there is an exception here, when the device is actually a radio transmitter.  So, for the likes of a radio transmitter like the iCom IC-A6E it is deemed an ‘intentional’ radiator when transmitting at the frequency it is set to and ‘unintentional’ at everything else.  As chargers are always unintentional radiators, then they should generate as little interference as possible at all frequencies.

To obtain a true radiated emissions footprint you need a Radio-Frequency Anechoic Chamber and/or open air test facility, some very expensive test equipment, multiple very expensive aerials, and plenty of time (which, guess what, is also expensive).  Typically testing can take several days to complete fully.  However, for pre-compliance testing, you can use ‘near field’ probes which are designed to help identify areas where equipment is ‘weak’ before being subject to the formal testing.  The radiated emissions test above were taken using a sensitive near field probe, the trace was set to hold the maximum levels recored and the probe moved all around the outside of the chargers looking for any interference. 

Conducted Emissions

These are RF signals emitted by a device back along its supply cables.  In effect turning the supply cables into a transmitter aerial and also feeding that signal into the power input of other devices.

As the power cables can be routed close to other cables, conducted emissions can cause all sorts of odd problems, not necessarily with equipment located physically close to the device generating all the noise.

To measure these emissions you, thankfully, do not need a full Radio-Frequency Anechoic Chamber and/or open air test facility, although it would help improve the readings.  What you need along with a spectrum analyser is a line impedance stabilisation network (LISN), this passes power to the device but enables the spectrum analyser to measure the emissions being sent back down the power line by the device under test.