Antennas come in many different types, and it can sometimes be overwhelming to decide which one fits the need the best. In the case of immunity antennas, the correct selection may be the difference of reaching field and not. The antenna is the most important part of the system. Given that the amplifiers can be the costliest part of the system, one needs to take the time to research and find the correct antenna to make the best use of the available power. Always look at what antennas are available for your application. Just because you already own an antenna, it does not make it the best option in all cases. Getting a new antenna might allow a smaller amplifier to be purchased or give you more margin so as not to saturate the power output of the amplifier, which improves the overall testing quality.
Following are questions about your testing that you need to know before we can start looking at antennas.
What test standards are being tested?
There are many test standards for testing Radiated immunity. Some of the more common ones are:
| Standard | Frequency Range | Possible Levels V/m | Test Distance |
| IEC 61000-4-3 | 80MHz – 6GHz | 1, 3, 10, 30 | 3 m |
| MIL-STD-461 | 2MHz – 18GHz sometimes 40GHz | 5, 10, 20, 50, 200 | 1 m |
| RTCA DO-160 | 200MHz – 18GHz | 20-300 CW, 7200 Pulse | 1 m |
| ISO 11452-2 | 80MHz – 18GHz | 20, 50, 75, 100 | 1 m |
Does the standard require specific antenna types, or give example suggestions?
Test standards, in many cases, do not control or require specific antennas to be used for immunity testing. They may state that it must be a linearly polarized antenna (not circular polarized), and there may be parameters as to how close it is allowed to the floor/ground plane and walls/absorbers. This may affect the size of the antenna that can fit in your chamber/setup. Other than this, the user has the freedom to choose the best match for their needs. Even if the standards do list examples, it is not required to use these types if not specifically indicated.
What is the frequency range of testing?
The frequency range required is stated by the standard and product. But when looking for an antenna, you may break this range down into sub-bands to find better-performing antennas. For example, in the 30MHz to 200MHz, it can be very difficult to produce 200V/m for MIL-STD, and a specialized high-power antenna may work best. For example, a Tuned Yagi antenna that changes element length with every frequency step can produce a 2.5:1 VSWR or less, as the alternative Biconical antenna has a VSWR >20:1 at 30MHz. With less VSWR, more power is delivered to generate field and not lost to reflections. For the same MIL-STD-461 standard, if you are testing at lower field levels such as <50V/m, not so much power is required. It may be cost-effective to use a wider frequency range antenna. To reduce setup changes. For high-power applications, it is common to match an antenna to each amplifier band.
What is the Field level/s and test distance?
The standards will give you this information. It can be used to calculate what kind of gain you need from an antenna with a given power or what power you need from a given antenna gain.
What is the illumination size (DUT/EUT size) your testing must cover?
Making sure the whole DUT/EUT is illuminated by the RF energy is required. Therefore, at the proper test distance, the antenna needs to have sufficient beamwidth to cover the EUT. Trig equations can be used to find spot size from the 3dB beamwidth given in degrees and the test distance. If the antenna has a high gain and a small beamwidth, some standards allow you to “window” the EUT. This method moves the antenna into different positions so the whole EUT is covered with multiple sweeps. This method does increase test time, but in some cases, it is necessary.
How much power do you have, or much do you need to purchase?
Power, gain, or field can be calculated if you know the other parameters. The gain equations work well in the far field of the antenna. The far field is the distance where the antenna exhibits predictable results. In the near field, there is much more error in the calculation. Having experience in test setups and direct measurements is required. This is especially true in low frequencies < 100 MHz. Other adverse effects on the field are related to the setup, chamber, and use of a tabletop ground plane.
Do you have cable losses to consider?
Always remember to consider your coax losses in your calculations. This becomes very critical at higher frequencies. When in the 6GHz range, it is not unheard of to have losses greater than 6dB. Using high-quality coax with low losses is best as not to lose your power before it gets to the antenna.
Once the above is known, we can start looking at antennas and comparing the specifications to match your needs.1
Power input – the antenna input power for immunity testing needs to be known. Antennas will have a maximum power input. It is best to oversize the antenna and not push it past the published rating. Using an antenna beyond its own specifications can have unknown results, even though you may not notice. The antenna can saturate or have corona effects which reduce the quality of the test signal. This would not be known unless you measured the RF spectrum directly.
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. Other effects beyond the antenna might increase VSWR, such as reflections in the chamber.
Antenna Factor – is a comparison of how efficient the antenna is, also related to Gain. Gain is the parameter most useful for immunity testing.
Gain – is listed in dB and is the gain of the antenna over an isotropic ideal radiator. The higher the gain, the more directional the antenna. Also, the higher the gain, the smaller the beamwidth becomes.
Beamwidth – is given in degrees or may be shown as polar plots over the frequency range. The 3dB beamwidth is where the field falls off by 3dB from the center. There are values for E (same direction as polarity) and H (perpendicular to polarity). Both E & H parameters are critical since the EUT/DUT is a 3d object. It is possible for an antenna not to work well over its whole frequency range for your application. This specification is important in your selection to make sure that your test spot size meets your needs or if you must implement a “window” test method discussed earlier.
In short, the antenna with the best gain and beamwidth that covers the test area and can handle the required power to reach the field is what you are looking to find. Selecting the antenna might take a few steps to find the one that fits best. Rely on experts in the field to help guide you and make suggestions. This may help you save valuable time in the selection. As always, verify your selection because what works well for someone else may not be the best fit for you.
When selecting antennas for reverberation chambers, the gain is not an important factor anymore. In this case, the field is purposely dispersed and bounced around in a volume, and then having low gain and very wide beamwidths are more desirable.
Reference
- See required equations and calculators to find field levels, antenna gain, losses, beamwidths, spot sizes, and convert units.
