How to calibrate a measurement system that uses a log periodic antenna?

How to calibrate a measurement system that uses a log periodic antenna

To calibrate a measurement system using a Log periodic antenna, you need a systematic approach that verifies the entire signal path, from the antenna’s input terminals to the final data output. This isn’t just about the antenna itself; it’s about ensuring the entire chain—cables, amplifiers, receivers, and software—is accurately characterized. The core process involves comparing your system’s readings against known, traceable standards, typically in a controlled environment like an anechoic chamber or an open-area test site (OATS). You’ll be measuring key parameters like antenna factor (AF), gain, return loss (VSWR), and phase center stability across the operating frequency band. The goal is to generate a calibration table or set of correction factors that your measurement software can apply to raw data, transforming it into accurate, real-world field strength or power density values.

Understanding the Components of Your System

Before you even switch on a signal generator, you must map out your entire measurement system. Each component introduces its own errors, and these errors compound. A typical setup for, say, EMC pre-compliance testing might look like this:

• Log Periodic Antenna: The device under test (DUT) in this calibration process. Its wide bandwidth is its strength, but its gain, impedance, and phase response vary with frequency.
• Coaxial Cables: These are not perfect conductors. They have insertion loss that increases with frequency and can be susceptible to movement (phase instability). You need to know the loss of each cable segment precisely.
• Preamplifier (if used): Boosts weak signals but adds its own noise figure (NF) and gain. Its linearity is critical; it must not compress or generate harmonics at your expected signal levels.
• Measurement Receiver/Spectrum Analyzer: This is your readout. Its absolute amplitude accuracy, frequency response, and input impedance must be calibrated separately by a certified lab before you start.
• Software: The brain that applies correction factors. It must be configured correctly with the calibration data you generate.

A common mistake is to assume the antenna is the only variable. If your 6-meter cable has a loss of 2.2 dB at 1 GHz and you forget to account for it, your field strength reading will be off by a factor of nearly 1.66. That’s a huge error.

Step-by-Step Calibration Procedure

This procedure is often called “Antenna Factor Calibration” and is best performed at an accredited calibration lab or a well-characterized test site. The standard method is the Standard Site Method (or Reference Antenna Method), as outlined in standards like ANSI C63.5 or CISPR 16-1-6.

Step 1: Pre-Calibration Equipment Check

First, ensure all your equipment is in good working order and has valid calibration certificates, especially for the measurement receiver and any signal generators. Perform a quick verification of your cables. Connect a signal generator directly to the receiver with a short cable and note the power level. Then, insert the cable you plan to use and measure the loss. This gives you a baseline cable loss table. For example:

Step 2: Setup in the Anechoic Chamber or OATS

You need a known, stable electromagnetic field. This is created by a transmitting antenna (the “reference antenna”) placed at a fixed distance from your antenna under test (AUT), which is the log periodic. The standard distance is 3 meters, 10 meters, or 30 meters, depending on the test standard. The key is to be in the far-field region, which for a log periodic antenna starts at a distance, R, where R > 2D²/λ (D is the largest antenna dimension, λ is the wavelength). Align the antennas for boresight (maximum gain) polarization. The height should be set to the standard height for your test, often 1.5 meters or 2 meters above the ground plane.

Step 3: The Substitution Measurement

This is the heart of the calibration. A known signal level is transmitted from the reference antenna. You measure the received power at your spectrum analyzer first with the reference antenna, and then with your log periodic antenna.

1. Reference Measurement: Connect the calibrated reference antenna to the receiver via your cable. Transmit a known power level, P_Tx, from the signal generator. Record the power received, P_Ref, at the receiver. The path loss includes the free-space loss and the gains of the antennas.
2. AUT Measurement: Without changing the transmitter setup, replace the reference antenna with your log periodic antenna. Record the new received power, P_AUT.

The difference between P_Ref and P_AUT, after accounting for the known gain of the reference antenna (G_Ref), directly relates to the gain of your log periodic antenna (G_AUT). The Antenna Factor (AF) is then calculated. AF is a more practical parameter for measurement; it describes how much the antenna converts a field strength (V/m) into a voltage at its terminals (V). The formula is:

AF (in dB/m) = 20 * log10(frequency in MHz) – G_dBi – 29.79

But from the measurement, you derive it as: AF_AUT = AF_Ref + (P_Ref – P_AUT) (all in dB).

Step 4: Sweeping the Frequency Band

A log periodic antenna’s characteristics are frequency-dependent. You cannot calibrate at one frequency and assume it’s correct for all. You must perform this substitution measurement at numerous discrete frequencies across the entire operating range of the antenna. A good practice is to use a step size no greater than 1% of the frequency. For an antenna covering 200 MHz to 2 GHz, you might take measurements at 200, 220, 240… up to 2000 MHz. This generates a table of AF values vs. Frequency.

Step 5: Validating Return Loss (VSWR)

While AF is critical for reception, the antenna’s impedance match is equally important. A poor match means signal reflections and inaccurate power transfer. Using a vector network analyzer (VNA), measure the S11 parameter (return loss) of the log periodic antenna across the same frequency range. A good log periodic should have a return loss better than 10 dB (VSWR less than 2:1) across most of its band. A sudden dip in return loss at a specific frequency could indicate damage or contamination.

Integrating Calibration Data into Your Measurement Software

The raw data from your spectrum analyzer is virtually useless without the calibration corrections. All professional EMC or antenna measurement software (e.g., EMC32, TILE, custom LabVIEW applications) has a section for importing antenna calibration files. This is typically a simple text file (CSV format) with two columns: Frequency and Antenna Factor. The software uses this to convert the measured voltage at the receiver input back to the field strength at the antenna’s location. You must also input the cable loss data for each cable in the system and the gain of any preamplifiers. The software then performs a calculation like this for every measurement point:

Field Strength (dBµV/m) = Receiver Reading (dBµV) + Cable Loss (dB) + Antenna Factor (dB/m) – Preamplifier Gain (dB)

Getting this data entry wrong is a common source of error. Double-check every value.

Maintaining Calibration and Performing Health Checks

Calibration isn’t a one-time event. Antennas can be physically damaged, cables can wear out, and connectors can become loose. A formal calibration at an accredited lab might be required annually to maintain ISO 17025 accreditation. However, you should perform a quick “health check” before every critical measurement campaign. This involves:

• Visual Inspection: Check for broken elements on the log periodic, damaged cables, and corroded connectors.
• Return Loss Check: Use a VNA to quickly measure the antenna’s VSWR. Compare it to the VSWR plot from its last full calibration. A significant change (e.g., a dip worsening by more than 3 dB) indicates a potential problem.
• System Verification with a Known Source: Use a calibrated, battery-powered field strength generator (a “dipole antenna in a box”) placed a short distance from your antenna. Measure the field it generates and compare it to the expected value. If the difference is within your measurement uncertainty budget (typically ±1.5 dB for a good system), your setup is likely still valid.

This proactive approach catches issues before they ruin days or weeks of valuable measurement data. It’s the difference between having confidence in your results and just hoping they’re correct. The process is detailed and requires attention to minutiae, but the payoff is measurement data you can truly trust, whether you’re certifying a new wireless device for market or mapping signal propagation for a network deployment.