Overview: The antenna is one of the least understood components in an EMC test instrumentation set-up, and a poor choice of antenna(s) can lead to multiple problems including erroneous test data, failed tests, lengthy or difficult test procedures, and challenges with repeatability. Useful to beginners and the more experienced alike, this webinar, presented by AH Systems, guides you through the key parameters that need to be considered when selecting an EMC test antenna. Attendees will also receive a handy check list for practical future reference.

Speaker:
Tom Mullineaux
An author and RF Engineer, Mullineaux has been in the EMC industry for 20 years, both as a supplier to the industry, and as a hands on program manager, achieving EMC compliance for new products. Mullineaux received his degree in electrical and electronic engineering from Portsmouth University, England, and is a prolific writer of EMC related articles, with all articles having a strong slant towards the engineering basics behind the tests. He has given many IEEE society presentations, most looking at the physics behind today’s commercial, automotive and MIL-STD tests.

The following are questions presented to the speaker by the attendees during the webinar, along with answers to each.

What is a safe distance (in wavelengths) between an antenna and a wall to assure that the latter makes no impact on the antenna performance?
Answer: The simple answer is as much distance as possible but obviously this is limited by the available space inside your chamber. If we assume a centrally mounted (gives balance) horizontal polarized (reduces the impact of the fully reflecting floor) antenna, and the ferrite / absorber lining is doing its job of attenuating normal-incident free-space EM waves by 20 dB, and attenuating non-normal incident waves by a similar amount, and we further assume that near-field fields are similarly attenuated (a lot of assumptions), then the impact of the walls should be minimal. In the real world the way to answer your question is to gauge for yourself how sensitive the performance of the antenna is to the proximity of the lined walls. This can be done by comparing the supplier’s VSWR data with your own gathered data. A fair assumption is the lower the frequency, the greater the impact.

Using a directional position for emissions testing…Was this an FCC or other requirement again?
Answer: Good question. If you had asked about immunity testing, the answer would be the practicalities / economics of generating a test field dictated the use of directional antennas, as the vast majority of standards require immunity testing.

However, bodies such as the FCC do not require proof that a product is immune to the emissions of another nearby product, but concentrate instead on limiting the emissions of all products to protect allocated communication bands. The most practical way to get a clean reading of a product’s emissions is to point a directional antenna at it as this creates a far stronger signal (compared to non-directional) at the antenna’s connector making measurement above the system noise floor easier.

For reasons of accuracy, practicality and repeatability, bodies such as CISPR stipulate emissions-test set-ups and define antenna performance where applicable. For the same reason given above, wherever practical, the stipulated antenna will be a directional one.