Editor’s note: This question was asked in response to Interference Technology’s recent webinar by Keith Armstrong. To view the webinar, click here.
Question: What are best practices for qualification of homemade antennae to ‘calibrate’ pre-compliance measurements?
Answer: Apart from ‘sniffer’ probe antennas, I have always used antennas that I have purchased, complete with calibration charts from laboratories that are accredited to calibrate antennas.
One antenna that I use a lot is the York EMC “Active Receive Antenna, model number ARA01” – which covers the same frequency range as a traditional ‘bilog’ (30 to 1000MHz) with comparable antenna factors, but (unlike a bilog) is small, lightweight, and much more portable and easy to use, visit http://www.yorkemc.co.uk/instrumentation/ara01.
However, it is possible to purchase calibrated “RF noise generators” – that go by a variety of marketing names – and use them as “transfer calibrators” for your own antennas. They are all small and portable, and battery-powered, and have been tested on a proper antenna calibration site to give a graph of their emissions versus frequency when measured perfectly.
Examples include:
“Comparison Noise Emitters” up to 40GHz, from York EMC: http://www.yorkemc.co.uk/instrumentation/cneiii
“Reference Sources” from Laplace Instruments: http://www.laplace.co.uk/products/15/
“Universal Spherical Dipole Source (USDS)” from Applied Electromagnetic Technology LLC:
http://appliedemtech.com/usdsmain.html
“Comb Generators” from Com-Power Corp.: http://www.com-power.com/comb_generators.html
Of course, an antenna doesn’t exist in isolation – it is always affected by its surroundings, the emissions test site.
So we should calibrate our antennas in the exact site where we intend to use them to make emissions measurements – ideally making the structures of those sites as close as we can to the design of the test site that is specified in the relevant emissions test standard.
Another use for these portable RF noise generators is when testing a large system or installation on the factory floor, or in-situ after commissioning. They allow us to compensate not only for the deficiencies in our homemade antennas, but also for the deficiencies in the site itself.
This question addresses the issue that the sensitivity of our home-made antennas varies with frequency, so must be compensated for by calibrating them to obtain the antenna correction factors (usually just called Antenna Factors) that we have to add to our ‘raw’ emissions measurements to be able to compare them with the limits in the standards.
But we must beware – a problem with home-made antennas can be very poor sensitivity at some frequencies, requiring large antenna factors that can make the measurement noise floor get very close to, or even exceed the limit line at those frequencies!
So perhaps it is best to purchase low-cost antennas specifically designed for pre-compliance testing. I’m sure there are several companies offering such products, but the only one I know of is Laplace Instruments in the UK, visit http://www.laplace.co.uk, who have been adding to their excellent range of pre-compliance emissions and immunity products for 20 years.
-Keith Armstrong
