Yes, log periodic antennas are not just suitable; they are one of the most common and reliable tools for radiated emissions testing in EMC laboratories worldwide. Their unique design provides a consistent performance over a very wide frequency range, which is absolutely critical for efficiently scanning for potential electromagnetic interference across the broad spectrum mandated by standards like CISPR, FCC, and MIL-STD.
To understand why they’re so effective, let’s look at how they work. Unlike a simple dipole antenna that resonates at a specific frequency, a Log periodic antenna is a multi-element array where the lengths and spacings of the dipole elements increase logarithmically from the front (shortest elements) to the back (longest elements). The clever part is that only a small group of elements—the ones that are approximately half a wavelength long for the frequency being measured—are actively radiating or receiving at any given moment. This “active region” moves along the antenna structure as the frequency changes. This design delivers a stable gain and a consistent radiation pattern across its entire operating bandwidth, which for a typical EMC model might be from 200 MHz all the way up to 10 GHz or more. This eliminates the need to constantly swap antennas during a test, saving a massive amount of time and reducing potential setup errors.
When you’re in an EMC lab, you’re often dealing with frequencies from 30 MHz to 1 GHz, and sometimes up to 18 GHz or even 40 GHz for newer standards. For the core 30 MHz to 1 GHz range, the log periodic is the workhorse. Its performance characteristics align almost perfectly with the requirements of the test standards. Let’s break down the key specifications that make it so indispensable.
Gain and Directivity: A typical EMC log periodic antenna has a moderate gain, usually between 5 to 10 dBi. This is a sweet spot—it’s sensitive enough to pick up weak emissions from your equipment under test (EUT) without being so sensitive that it overloads the measurement receiver or spectrum analyzer with ambient signals. Its directional nature (it receives best from the direction it’s pointing) is a huge advantage. You can rotate the antenna during testing to find the polarization (horizontal or vertical) and the orientation that yields the maximum emission from the EUT, which is exactly what the standards require you to do to ensure a worst-case measurement.
Voltage Standing Wave Ratio (VSWR): This is a measure of how well the antenna is matched to the coaxial cable and the receiver. A poor VSWR means signal reflections and inaccurate readings. High-quality log periodic antennas are designed to maintain a low VSWR, typically below 2:1, across their entire frequency range. This consistency is vital for making accurate, repeatable measurements because it means the antenna’s efficiency isn’t swinging wildly with frequency.
The table below shows a simplified comparison of typical performance between a standard log periodic antenna and a biconical antenna, which is its common partner in EMC testing for the lower frequency range (30 MHz – 200/300 MHz).
| Parameter | Log Periodic Antenna (e.g., 200 MHz – 10 GHz) | Biconical Antenna (e.g., 30 MHz – 300 MHz) |
|---|---|---|
| Frequency Range | Wide, multi-octave (e.g., 200 MHz – 10 GHz) | Wide, but typically lower frequency (e.g., 30 MHz – 300 MHz) |
| Typical Gain | 5 – 10 dBi (relatively stable) | 0 – 5 dBi (can vary more with frequency) |
| Polarization | Linear | Linear |
| Primary Use in EMC | Radiated Emissions, 200 MHz+ | Radiated Emissions, 30-300 MHz |
| Key Advantage | Stable performance over ultra-wide bandwidth | Excellent performance at lower frequencies where log periodics are physically large |
This partnership is key. A standard EMC setup for radiated emissions from 30 MHz to 1 GHz uses a biconical antenna for the lower band (30-200/300 MHz) and a log periodic antenna for the upper band (200/300 MHz to 1 GHz). Above 1 GHz, you might continue with a high-frequency log periodic or switch to a double-ridged guide horn antenna for even higher gain and directivity.
Now, the performance of these antennas isn’t just theoretical. It’s rigorously defined by their antenna factors (AF). The antenna factor is a calibration constant that allows you to convert the voltage measured at the receiver’s input (from the antenna) into the actual field strength (in Volts per meter or dBµV/m) at the antenna’s location. For every frequency, there is a specific AF. The stability of the log periodic’s design translates into a smooth, predictable antenna factor curve. This is provided by the manufacturer on a calibration certificate, often traceable to national standards like NIST. When you’re running your EMC scan, the test software uses these AF values to automatically correct the readings, giving you the true field strength. Without this stable AF, your measurements would be meaningless.
But it’s not all sunshine and perfect signals. Log periodic antennas have a few practical considerations. Their directional nature is a double-edged sword. While great for finding maximum emissions, it means they must be precisely aligned and rotated during testing, which adds complexity to the automated antenna mast systems in semi-anechoic chambers. Also, to achieve wide bandwidth, the antenna has a physical size that is inversely proportional to its lowest operating frequency. A log periodic that works down to 80 MHz is significantly larger and heavier than one that starts at 200 MHz. This can be a challenge for antenna mast positioning and can potentially affect the measurement of very large EUTs due to the antenna’s size relative to the EUT.
Furthermore, at the very high end of their frequency range (say, above 6 GHz), the gain of a standard log periodic might start to fall off compared to a purpose-built horn antenna. For ultra-high-frequency measurements where every dB of sensitivity counts, an engineer might opt for a horn antenna despite the need for more frequent band changes. The choice always comes down to the specific standard you’re testing to and the required measurement uncertainty budget. The decision to use a log periodic is also influenced by the test environment. In a fully accredited lab with a pristine semi-anechoic chamber, its performance is predictable. But in a less ideal environment, like a screen room or for pre-compliance testing on a bench, its directivity might pick up more reflected signals, complicating the results.
Ultimately, the suitability of the log periodic antenna is proven by its endorsement in international standards. Bodies like the International Electrotechnical Commission (IEC) and the American National Standards Institute (ANSI) specify the use of antennas with known characteristics for compliance testing. The log periodic’s ability to maintain a consistent 50-ohm impedance, stable gain, and well-defined radiation pattern across decades of bandwidth makes it a reference instrument in this context. It provides the repeatability and accuracy that manufacturers and certification bodies rely on to ensure that electronic devices can operate without causing or succumbing to interference in our increasingly crowded electromagnetic spectrum. Its design, while conceptually simple, solves a complex measurement problem with an elegance that has yet to be surpassed for general-purpose wideband EMC testing.