Questions to Ask When Looking for an Emissions Antenna for EMC Testing

Antennas come in all shapes and sizes and can be used for both radiating (Tx) and receiving (Rx). Selecting a good receiving (Rx) antenna for emissions testing is not difficult if you take some necessary items into account. Though many antennas can be used for both emissions (Rx) and immunity (Tx), it is best to separate these tests and use different antennas if possible. Later in the series of articles, we will tackle Immunity antennas.

What frequency range do you need to cover?

The standard you are testing will state the frequency range/s the test needs to cover. It may be broken into subranges with different parameters. For example, in CISPR 32 (EN 55032), the frequency range for radiated emissions is 30 MHz to 6 GHz; however, for the 30MHz to 1GHz range, there are requirements for resolution bandwidth and measurement detectors different from 1 – 6 GHz. It is possible to select a single antenna that covers this whole range, making setup easier. Or choosing two antennas to cover each band is another option. It is also acceptable to break the testing setup down more.

Example of 1 antenna:	Combination antenna VULB 9162 TriLOG covering 30MHz to 7GHz
Example of 2 antennas:	Combination antenna VULB 9168 TriLOG 30MHz to 1GHz - Partner Content - Shielding Effectiveness Test Guide Just as interference testing requires RF enclosures, isolation systems in turn need their own testing. This document reviews some of the issues and considerations in testing RF enclosures. Read more
	Double-Ridge Horn antenna BBHA 9120D 1-18GHz
Example of 3 antennas:	Biconical antenna HFBA 9122(balun)+BBA 9106(elements) 30-200MHz
	Log-Periodic Antenna VULP 9118 A 180MHz – 2GHz
	Double-Ridge Horn antenna BBHA 9120D 1-18GHz

All of these configurations work. The benefit of using fewer antennas is that you will have fewer test setup changes which may decrease test time. The advantage of using more antennas is that it may be easier to find antennas with better performance to match each band.

What type of antenna is required by the test standard?

Many standards limit what antenna can be used or have parameters that an antenna must meet. It is essential to be familiar with these requirements. Standards do change and are updated periodically. An antenna manufacturer may meet an older version of a standard and not update their marketing literature or website if their products fall out of compliance with new standards. Therefore, it is important to double-check the parameters.

The requirement may list the type and size of antenna needed for each frequency range, such as what is in MIL-STD-461, or it may list required parameters as what is listed in CISPR 32 that references CISPR 16. All these must be directly compared and understood before purchasing an antenna.

Once you know and understand the above, you can start to look at antennas.

What type of antenna is best?

There are many antennas on the market. For this article, we will limit it to some common choices. The most common options used for emissions testing are:

General Notes of Antenna Type
Biconical Antennas	Common Frequency: 30 MHz – 200 MHz Extended use: 20 MHz – 1000 MHz Uses a Balun to help match the impedance over the frequency range. Some Biconical antennas offer interchangeable elements for added versatility Non-directional has a very wide beamwidth Can show high VSWR. An attenuator can be used to match the receiver impedance better
Log-Periodic Antennas	Common Frequency: 200 MHz – 2 GHz Extended use: 30 MHz – 18 GHz Geometry of the antenna matches impedance over the frequency range. No balun is used At lower frequencies, the antenna can become very large, not fitting in most chambers Antenna is directional, maintains a wide beam width
Combination Antennas	Common Frequency: 30 MHz – 2 GHz Extended use: 30 MHz –7 GHz (1 antenna) Antenna combines a Biconical and LOG-Periodic antenna. Uses a balun to match the lower frequencies, and the geometry of the antenna matches higher frequencies. Can show high VSWR at the lower freq. An attenuator can be used to match the receiver impedance better
Double-Ridge Horn Antennas	Common Frequency: 200 MHz – 2 GHz Common Frequency: 700 MHz – 18 GHz Common Frequency: 14 GHz – 40 GHz Antenna Ridge allows the antenna to work over many frequency octaves. Geometry of the antenna matches impedance over the frequency range. No balun is used Antenna is Directional but maintains a wide beam width.

These antennas fit with most of today’s EMC Standards covering 30MHz up to 40GHz.

Parameters Listed in Antenna Specifications and How to Interpret Them

Power input – the antenna input power for emissions testing usually is not much of a concern. An antenna that can handle 5-10 watts is sufficient for most applications.

VSWR – Voltage Standing Wave Ratio is a measurement over frequency of the impedance match to 50 Ohms. A value closest to 1 is best. Levels of <2.5 are usually acceptable, and above this is considered a poor mismatch. High values are common with Biconical antennas. Using an attenuator placed on the antenna helps match the coax and receiver to the antenna.

Antenna Factor – a lower antenna factor is better as it allows for the ability to measure a smaller signal. Not in all cases, however, do you want a lower factor. Beamwidth and frequency coverage can be just as important. Antenna factors are usually given in the far field, but some data sheets will also list at a given distance.

Gain – is tied directly to antenna factor but is not very important for emissions testing. However, this value is helpful for immunity testing.

Beamwidth – In most cases, the antenna needs a wide beamwidth for emissions testing, so the whole test area is in the antenna’s field of view. In some cases, small signals need to be measured, and lowering the antenna factor is more critical, which generally reduces the antenna’s beamwidth.

Calibration

We can not discuss receiving (Rx) antennas without discussing calibration. First, each antenna must be calibrated. From the calibration, the user gets a table and chart of antenna factors used in the measurement. The antenna factor dB/m is an attenuation factor for the measured frequency. Calibrations are done in a prescribed way, generally at the same test distance as during emissions testing. This can be set at 1m, 3m, 10m, or even up to 30m. The calibration is essential and needs to be done by a competent calibration facility since all measurements are tied back to this correction number. Some manufacturers can offer fully accredited calibrations; if you plan to use the manufacturer for all future calibrations, this should be utilized. If you have a calibration facility in your country or locally, it is best to calibrate new antennas before use by this lab.

Since the geometry (physical attributes) of the antenna dictate how the antenna will perform, little should change over time unless something physically changes on the antenna. For this reason, it is important to protect the antenna from physical damage, dropping, or being bent. If mishandling does occur, one can perform an RF check but re-run a previous scan/test and compare the results. This is where a comb generator would come in handy. The antenna should be sent out immediately for recalibration if a large difference is seen.

Using Antennas for Both Emissions and Immunity

As stated earlier, it is best to separate out the use of the antenna when making your selections to find the best option for emissions testing. However, there are instances where an antenna does fit well for both setups and can be used interchangeably. An emissions antenna’s calibration and antenna factors are a critical part of the measurement, and one needs to consider this not to affect the emissions performance. Excess heating or overpowering the antenna in immunity testing can change performance; this is an aspect to keep in mind when using an antenna for multiple purposes.

The hope is that you are a little more educated on selecting the correct antenna for emissions testing.