How to perform a radome transmissivity test
In the aviation industry, the nose cone of an aircraft is not just an aerodynamic feature. It often functions as a radome (radar dome) which protects the antenna of the aircraft’s weather radar system. Naturally, an important trait of the radome is that it is transparent to the frequency at which the weather radar operates. Transmissivity is a measure of how well the radome transmits electromagnetic energy.
The purpose behind testing radomes
Aircraft radomes are subject to damage from a number of common sources including bird-strikes, hail damage, and bumping hangar doors or maintenance equipment. Because the radome is located on the end of the plane it has a higher tendency for such injury. Once a radome is damaged, it immediately becomes susceptible to water ingress, which severely impedes the weather radar’s performance.
Consider for a moment the purpose of a weather radar system: identifying hazardous weather conditions in an aircraft’s path. It is searching for water in the air ahead. A compromised radome will trap water inside of the aircraft’s nose cone, essentially creating a layer of water between the antenna and the outside environment. To the radar, this will appear as exactly what it is, water right in front of the plane. This also creates an effect called a radar shadow.
Because most of the energy of the radar antenna signal is reflected back by the water in the damaged radome, no energy passes through, effectively rendering the weather radar blind to anything that might be hiding in the shadow of that patch of water. Therefore, in addition to showing a false positive storm directly in front of the aircraft, the pilot is unable to see if any real storms exist beyond that-rendering the radar to be useless.
A radome appears opaque to the human eye because the human eye operates in the visible light spectrum. Water appears clear at these frequencies. At the weather radar frequency, the situation is reversed, with the radome becoming clear and the water becoming opaque. Water trapped inside the radome then becomes like dark black paint smeared across a window.
After repairs are made to a radome, it is essential to test that it is properly transparent for the weather antenna. This must be done with antennas simulating real-world signal conditions because a physically-repaired radome may look entirely whole, but still have water trapped inside. To ensure that a radome is repaired properly, a transmissivity test must be conducted.
Test system setup
To perform a transmissivity test, a two-axis gimbal was constructed, large enough to support an aircraft radome. The test system has three degrees of motion, allowing the test software to change the radome’s orientation with the respect to the antenna along two axes (representing the azimuth and elevation). A third degree of motion allows the test software to reposition the test antenna.
Motion control hardware and software was used to control stepper motors responsible for the motion of all three axes and continuously track the position of the gimbal. Home switches were placed on all paths of motion, allowing the system to self-calibrate by running the gimbal rings to their extreme top-left position and by lowering the test antenna to its lowest posture.
To reduce signal reflections, radar-absorbing material was used to cover the gimbal wherever possible. Additionally, the transmit antenna was suspended from a rod dropped from the ceiling of the test facility. This was done because the test facility ceiling was adequately high to operate in the antenna’s far-field region and to save valuable floor space. This required additional radar-absorbing material to be placed beneath the test gimbal.
A 10-in. X-band antenna was used for the receiving antenna, while a horn antenna was used for the transmit antenna. A vector network analyzer (VNA) served as the system’s primary measurement device.
The goal of the measurement is to assess the radome’s transmissivity at the same frequency the weather radar system normally operates. To achieve this, a VNA is used to assess the S21 parameter (from ceiling to receiving antenna) of the test system with and without a radome present. A baseline measurement is collected without the radome present. This allows measurement of the total loss in the system from all contributors, including cabling and free space losses. The radome is then introduced to the system and more measurements are taken. By this practice, the radome’s appearance is the only variable that has changed in the environment. Thus, losses attributed to the radome can be determined by comparing the S21 parameter with the radome present to the S21 parameter without the radome present.
The software prompts the operator to provide some information about the radome to be tested. The test system then automatically returns to the home position and begins the calibration sweep. In ideal environments with less noise and reflections, a measurement can be performed with the gimbal placed at the (0, 0) position or 0 degrees in azimuthal and elevation axis (indicating that both axes are in the level posture).
The calibration sweep operates in a similar same way as a measurement sweep, only no radome is loaded into the test system at this time. This creates a baseline measurement of power received in the antenna with the gimbal in every posture exactly as it will be when the measurement sweeps are conducted. For example, in a system that is used to test repaired radomes, the specifications require testing along the azimuthal axis at -80, -60, -40, -20, 0, +20, +40, +60, and +80 degrees. Along the elevation the specifications require -20, -10, 0, +10, and +20 degrees. The test moves to one azimuth position first (i.e., -80 to start) and then moves along the elevation positions (i.e., -20, -10, etc.), taking a measurement at each position. For the -80 azimuth position the following points would be measured: (-80, -20), (-80, -10), (-80, 0), (-80, 10), and (-80, 20).
Since there are nine azimuth positions and five elevation positions there are a total of 45 points measured for a complete sweep. It is easiest to think of this in terms of how the radar would be used for its intended purpose. The azimuthal axis is essentially the horizon. The aircraft needs to be able to sweep its radar across the entirety of the horizon it is chasing. The elevation axis then follows logically as the region above and below the level of the plane. Naturally, this axis is the smaller of the two, as a consequence of the nature of flight.
Aircraft radomes may be attached to a skirt, ergo some mathematics must be done to scale the motion of the attached gimbal in such a manner that the angles of interest prescribed above serve as the angles illuminated by the antenna system through the radome’s viewing window. Regions outside of the viewing window are not relevant for this test.
At each point along the viewing window, the motion sweep rests briefly and takes a measurement. The measurement includes some averaging of sweeps to reduce noise, and for this test was centered around 9.4 GHz. Once a measurement is taken it is stored to be used later.
At the end of the calibration sweep, the gimbal returns to its (0, 0) position and the operator is prompted to mount the radome into the gimbal. After the operator confirms that the radome is mounted and ready to continue with the test, the software starts its first measurement sweep.
Measurement sweeps are conducted in the same manner that calibration sweeps are conducted, only with the radome attached. Measurements are collected at each point using calls to the same software functions. In our case this was the S21 response of the VNA with connections such that the antenna mounted on the ceiling was the transmitting antenna and the signal was received in the far field by the antenna beneath the radome.
As the test proceeds, the operator is provided a constant stream of information showing the present orientation of the test gimbal, progress through a calibration or measurement sweep, and the phase of the overall test, shown in Figure 5.
At the end of the first measurement sweep, the test system adjusts the position of the receiving antenna one-quarter wavelength using a stepper motor attached to a translation stage. Once the antenna is at its next position, the second measurement sweep proceeds the same as the first, stopping at all 45 points and measuring the S21 parameter.
Once the second measurement sweep is completed, the gimbal returns to the (0, 0) position to facilitate easy unloading. A report is generated at this time which can be printed and included with the radome for traceability.
Results of the measurement sweep
Data gathered from the first and second measurement sweep are averaged together on a positional basis, meaning that the dB loss at (-80, +10) for the first measurement sweep is averaged with the dB loss at (-80, +10) for the second measurement sweep, and so on. These averaged values are then compared to the corresponding calibration sweep values to calculate the dB loss from including the radome in the system. This resulting number is then converted to a percentage of power transmitted through the radome (where 100% is no loss, no gain). These values are then displayed to the operator and saved to the report.
Radomes are rated on a letter scale: A, B, C, D, and E. Score is determined by the average of all collected percentages, as well as the lowest percentage of any individual location. In the results shown in Figure 5, areas marked in blue are above the minimum threshold for an "A" grade performance of 85%. The scoring criteria is shown in Table 1.
Table 1: Scoring criteria
The areas marked in green represent a "B" grade performance range and are enough to cause this particular radome to be rated as class B. The color-coded display allows for repair technicians to easily identify areas where the radome performance is degraded, improving the ability to make quick repairs.
For the radome (test article) shown in Figure 5, the test results are:
- Average transmissivity: 89.8
- Minimum transmissivity: 83.5
- Repaired radome class: B
Current testing has shown that the test system exhibits +/- 2% stability for test durations over four hours. Stability was assessed by taking free-space measurements once every 30 seconds for extended periods of time and comparing the extreme low and high values. Additional stability testing was conducted by running the full test cycle with no radome attached at any point. This simulates an "ideal" radome with 0% loss. This test typically showed an error rate of +/- 0.1%.
The first of these test systems has been in operation for over six months, testing a variety of radome shapes and sizes for X-band frequency systems. This has allowed the customer, a facility that repairs radomes and other aircraft parts, to begin testing their repaired radomes in-house, saving them time and money. This has increased their ability to make faster repairs for radomes where the initial test showed that additional repairs were required.
The RTCA DO-213A document outlines the requirements that were followed for this test system. The document is available at www.rtca.org. While this particular system was designed for testing aircraft radomes, the basic methodology can be applied to automotive radomes and other radomes or materials in which transmissivity testing should be conducted.
James Duvall is a senior project Engineer at G Systems, L.P. Edited by Emily Guenther, associate content manager, Control Engineering, CFE Media, firstname.lastname@example.org.
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