Any electronic device, system or subsystem generates EMI or ElectroMagnetic Interference and is susceptible to EMI generated by others. To allow them to coexist and cooperate, all such electronic devices, systems or subsystems must confirm to specific standards, which limit the amplitude and frequency range of EMI generated and tolerated by each of them.
Testing for such radiated emissions and immunity involves EMI chambers and OATS or Open Area Test Sites. To check for EMI generated, these chambers or OATS will have several types of antenna that can handle a wide range of frequencies. As visits to a full-compliance lab are expensive and time intensive, you may want to do pre-compliance tests, for which, it is a simple matter to set up a temporary antenna in a conference room or basement. This helps in troubleshooting and correcting EMI problems beforehand.
Several factors decide the nature of the antenna you should be using for your tests. The choice for the tests mostly ranges among radiated emissions, radiated immunity, pre-compliance, full compliance, frequency range, power and size of the antennas. The most common EMI test people perform is for checking radiated emissions. Here too, the antenna you use will depend on frequency, size, gain and your budget.
For pre-compliance tests, the most popular antenna is the hybrid. This is also called by names such as Combilog, Biconilog, Bi-log and others. Hybrids are so favored because of their wide frequency range, which easily covers different ranges from 30 MHz to 7 GHz, depending on the model. This is a very big advantage, as you do not need to switch antennas in between the tests, which you have to do if you were using log-periodic or biconical.
For a lab, where precision is more important, using multiple antennas gives an advantage in the performance. Typically, a lab might use a horn antenna for frequencies above 1 GHz, a log-antenna from 1 GHz to 200 MHz and a biconical antenna for frequencies below 200 MHz. However, for pre-compliance tests, hybrids or Bi-log antennas are adequate for makeshift labs.
The size of the antenna you can use depends on the space you have in your makeshift lab. Larger antennas cover a wider frequency range along with better sensitivities as compared to those offered by smaller antennas. Some designs of hybrid antennas come with bent elements, which help to fit them in limited spaces. In general, hybrid antennas are larger than most dedicated antennas.
Antennas are available that allow you to use them for both radiated immunity as well as radiated emission tests. However, for immunity tests, it is important to limit the power you drive into an antenna to get the required field strength. Typically, immunity testing requires larger antenna sizes as compared to those necessary for measurements of emissions alone.
Hybrid antennas usually combine a log-periodic element with a biconical element. This extends the frequency range the antenna covers as compared with that covered by single-type antenna. For example, one of the newest hybrid antennas covers the entire range of 26 MHz to 3 GHz, while being able to handle signal power up to 300 W for immunity tests.