What to Look for in an EMC Antenna

CISPR measurements officially require tuned dipoles, but a note in most standards allows the use of broadband antennas where they can be shown to give equivalent results. For this to be the case, the antenna must be calibrated to yield an antenna factor (AF).

The allowed system uncertainty in CISPR 16 is ±3 dB, comprising the receiver, antenna, cable loss, and mismatch uncertainties, but not including the test site. Directivity should be low so that signals from off-axis are not significantly attenuated. CISPR 16 mandates linear polarization. The VSWR (Voltage Standing Wave Ratio) should be less than 2:1, which, strictly speaking, demands an attenuator on the output of most biconical antennas.

Historically, the two types of antenna used for emissions measurement have been the biconical and the log-periodic. These are electric-field linear polarized and typically cover complementary frequency ranges from 30 to 300 MHz and 300 to 1000 MHz, respectively. Early biconical designs could reach only 200 MHz, but a modification to the structure has removed a resonance between 200 and 300 MHz, allowing that specification to be stretched.

The two types can also be combined into one device that will exhibit the characteristics of each of its components over the relevant frequency range (see Figure 1). Because these antennas are so commonly used, the correlation between measurements from different test facilities is generally quite good.

Figure 1. Common antenna types are a) BiLog, b) log-periodic, and c) biconical

Some other antenna types on the market are based on different principles than the biconical and log-periodic. One such antenna is the log spiral, which receives or generates fields with circular rather than linear polarization; it is widely utilized for military susceptibility tests but is unsuited to commercial IEC/CISPR testing. Other types of biconical are sometimes used, but the lower frequency limit of 30 MHz places a fundamental constraint on their size and construction, and more compact designs that do not obey this limit inevitably have compromised performance.

The maximum achievable field strength, as shown in Figure 2, is related to the power that can be radiated from the antenna: for a dipole, the field strength E (V/m) at a distance d (meters) with P (watts) radiated is calculated according to the formula

E=7

Power handling is principally a function of the balun design. Some power is dissipated in the balun, and the resulting temperature rise limits the power that can be applied. Good VSWR is essential to minimize the power reflected from the antenna and hence to make the best use of the power amplifier's capabilities. For these reasons, many antennas designed for radiated emissions testing cannot be used for radiated immunity.

Figure 2. Antenna as a source of RF immunity test field.

Low directivity maximizes the area that can be covered with a constant field strength. IEC 61000-4-3 mandates a uniform area of field at the position at which the EUT is located, a condition that is affected not only by reflections from the walls of the screened room but also by the uniformity of coverage of the antenna. To cover a given area, a narrow-beam-width antenna must be located farther away from the EUT. If even a broad-beam antenna is too close, different parts of the elements will be at significantly different distances from the EUT, and uniformity will suffer.

The field strength E is obtained by multiplying the voltage V at the receiver by the AF, allowing for attenuation in the connecting cable. The conversion can be expressed in dB by

E(dBµV/m) = V(dBµV) + AF(dB/m) + A(dB)

Since the noise floor of the measuring instrument (minimum measurable V) is fixed, a good system noise floor (minimum measurable E) requires a low antenna factor.

It has become common practice to use an antenna factor that applies to operation in free space with a 50 load. The AF varies with the antenna's proximity to the groundplane and the EUT. Errors due to mutual coupling cannot be avoided, but if all the test antennas used are based on a similar design, they will at least be consistent. With small EUTs, mutual coupling is unlikely even at 3 m to cause errors greater than 1 dB. For a biconical or combination antenna, variation of the AF according to the antenna's height above the groundplane is most pronounced in horizontal polarization and is typically 1 dB at resonance (around 70 MHz), falling to 0.5 dB above 300 MHz.

Variations in the slope of the AF curve due to resonances are undesirable. The AF values are generally programmed into test software that interpolates for a particular frequency; for minimum error, a smooth curve is essential.

When a single antenna factor is specified, an assumption has been made that the antenna will be used under conditions of maximum gain. For the log-periodic antenna, this is in the direction toward which the antenna is pointing, while for the biconical, it is perpendicular to the antenna axis. In all other directions, the response of the antenna falls off and the antenna factor becomes invalid. The polar-pattern response for a dipole is within 1 dB of the on-axis value over an azimuth variation of 45°; for a log-periodic array, the beam is narrower. This is particularly significant when the antenna is used at high frequencies with a height scan from 1 to 4 m and a close-in distance of 3 m. Under such conditions, the antenna will no longer be properly aligned with the EUT, and an error may result.

Polarization of the antenna refers to the plane of polarization of the electric-field component. CISPR 16 requires that the cross-polarization be better than 20 dB, which implies that the design of the antenna must ensure linear polarization.

EMC testing requires a fixed and known distance between the antenna and the EUT. In log-periodic or combination antennas, the active element (known as the phase center) shifts with frequency, and so the measuring distance must change. It is therefore a practical necessity to choose a specific point on the antenna boom against which the AF should be calibrated and to mark this permanently on the antenna itself.

Antenna VSWR affects the accuracy of emissions measurements and the power required to perform radiated-immunity tests. The mismatch uncertainty on receiver measurements is given by the formula

U(dB) = 20_log10 (1 ± |r_A| |r_R|)

where r_A and r_R are antenna and receiver reflection coefficients, given by the formula

|r| = (VSWR-1)/(VSWR+1)

The CISPR 16 requirement calls for a maximum antenna VSWR of 2:1. This is rarely met in practice below about 80 MHz, but with this figure for both antenna and receiver, the uncertainty from the above equations is +0.9/–1.0 dB.

Most EMC antennas employ similar mechanical designs and thus have many comparable properties. One parameter of which this is not true, however, is balun design. The antenna balun converts from the unbalanced coax feed to the balanced termination between the antenna elements; the resulting balance may be assessed by comparing signals received in the two possible vertical orientations. A well-designed balun will limit the difference to less than 1 dB, while a poor one may show differences greater than 10 dB.

Poor balance will evidence itself in several ways. The antenna factor clearly is going to depend on how the antenna is mounted and will be different for horizontal and vertical polarizations. Equally important is the fact that poor balance will severely affect the uncertainty due to proximity to the ground and to the antenna cable in vertical polarization. An unbalanced termination will result in appreciable common-mode RF currents flowing on the sheath of the antenna cable; these will in turn couple to the vertical antenna elements and cause a change in both the polar pattern and the antenna factor. This problem can be mitigated by locating the antenna cable far away from the antenna or applying ferrite sleeves to it, but good balance reduces the need for such measures.

Ground proximity, for its part, cannot be avoided; the only way to minimize error is thus to improve the antenna balance.