EU CE-testing requirements for a 2.4GHz transmitter with an internal antenna? - Page 1
Forum section: electronics / rf-microwave

author: MaxWattage
author_rank: Newbie
timestamp: November 20, 2017, 01:15:41 am
content: Hi,I have a question about EMC testing in the EU of a 2.4GHz (ISM band) Short Range Radio (SRD) device, with an internal fixed antenna. (NB: EU, Not FCC)My understanding is that the device is limited to emitting 10mW EIRP, however, I am seeking clarification on how that is measured in practice if the antenna is integral and internal to the device.Real antennas have different gains in different directions, whereas a theoretical isotropic antenna's gain is even in all directions.So, how will my device be assessed for compliance?Will the test-house measure the maximum forward-gain output power, (which due to the antenna's forward-gain will make it seem like it is outputting a lot more than the isotropic average), or is there some method applied to calculate what the output would have been if it had a theoretical isotropic antenna attached to the transmitter?If so, should I be making an effort to modify a unit to replace the integral internal antenna with an SMA connector for the duration of the EMC tests, so that the test centre's own "antenna equivalent" load can be connected in place of the normal internal antenna?Thank you.

author: 2N3055
author_rank: Super Contributor
timestamp: November 20, 2017, 07:58:17 am
content: "Effective radiated power (ERP), synonymous with equivalent radiated power, is an IEEE standardized definition of directional radio frequency (RF) power, such as that emitted by a radio transmitter. It is the total power in watts that would have to be radiated by a half-wave dipole antenna to give the same radiation intensity (signal strength in watts per square meter) as the actual source at a distant receiver located in the direction of the antenna's strongest beam (main lobe). ERP measures the combination of the power emitted by the transmitter and the ability of the antenna to direct that power in a given direction. It is equal to the input power to the antenna multiplied by the gain of the antenna. It is used in electronics and telecommunications, particularly in broadcasting to quantify the apparent power of a broadcasting station experienced by listeners in its reception area." Wikipedia. They said it better than I would..You don't change anything.. You test device as is, transmitter and antenna, otherwise measurements don't apply.

author: MaxWattage
author_rank: Newbie
timestamp: November 21, 2017, 08:50:23 pm
content: Ok, that partially answers the question, I think.So, for an antenna with a highly-directional output, most of the transmitter's power goes in one direction, making the measured EIRP output higher.So to clarify; if my antenna happens to be highly directional, does that make it harder to pass the testing? (YES or NO?)Given that the transmitter outputs the same total power regardless of how directional my antenna is, it seems a little unfair to measure the main beam EIRP, instead of the actual transmitter's output power.

author: dmills
author_rank: Super Contributor
timestamp: November 22, 2017, 12:13:09 am
content: Why would that be unfair?If the objective is to limit the interference to some reasonable distance from the device then this is the only way to play.You just turn the TX power down by an amount equal to the antenna gain (Being careful of units here, dBi and dBd are not equivalent!).Note that a directional antenna on each end is still a win even at reduced power because the gain applies to both transmitting and receiving at each end.Regards, Dan.

author: RFDUK
author_rank: Regular Contributor
timestamp: November 22, 2017, 10:16:55 am
content: Like Dan said.The test house needs a represenative antenna attached during radiated testing.They will rotate your equipment and vary the polorisation of the test instrumentation antenna to find and record the maximum ERP available.Same methodolgy as stock EMC radiated testing for unintentional emissions.Cheers, Martyn.

author: TheDane
author_rank: Regular Contributor
timestamp: November 26, 2017, 01:21:04 pm
content: Quote from: MaxWattage on November 21, 2017, 08:50:23 pmA highly directional antenna WILL make the test harder to pass.Unfair. Not really!Photons carry energy, radio waves carry energy.The sun (energy source) radiates the same amount of energy - but a magnifying glass will focus the (fixed amount of) delivered energy, much like a directional antenna focuses energy.Is it fair to an ant, if you try to focus sunlight on it.Is it fair to your users, if/when they go blind because they look into your energy beam, and their eyes are heated instantly to a bit over 41 degrees C?https://en.wikipedia.org/wiki/Microwave_burn#Eyes