The latest generations of wireless devices like cellphones, routers, and now IoT devices keep inching up in frequency, even into the microwave region in order to increase data speeds and performance. These increased operating frequencies have a great impact on testing such devices in the lab and engineering environment. Earlier generations of cellular and wireless technology operated in the UHF frequency range of 800 to 2600 MHz which fell neatly into the ‘up to 6 GHz’ operational spec of most RF test chambers. But, in using existing test chambers at the higher 5G and microwave frequencies, there was significant leakage beyond their rated 6 GHz limit.
First off, let’s discuss why most RF test chambers are specified to 6 GHz. This higher frequency limit falls nicely within the most used frequency range for the majority of present day wireless devices, with the 5 GHz frequency range being used by the latest Wi-Fi gear. An RF test chamber operates on the principle of a ‘Faraday Cage’ where a continuous conductive enclosure shields out all external RF radiation, or conversely, will contain any RF radiation from a device placed within the chamber. Of course such a continuous conductive enclosure needs some means of mechanical entry like a door, as well as suitable Input and Output (I/O) connections in order to connect to the device contained within the test chamber. Any connection through the shielding wall will interrupt this continuous conductive barrier and can be a source of RF leakage. This is where much thought and design effort has been made in order to provide easy access and communication without violating the continuous shielding barrier. Various forms of gasketing, conductive ‘fingers’ and other mechanical means have been used to ‘seal’ any entry points - and, in order to pass desired signals and power across the shielding barrier, the use of filtered I/O devices is required.
At frequencies below 6 GHz, the mechanical considerations for the construction of the test chamber are more easily accomplished due to only having to address shielding voids in the 1 cm range. As frequencies move up, their associated wavelengths get smaller, thus making the effect of small voids much more apparent. Considerable care must be taken to deal with even the most minute gaps or voids in the shielding barrier in order to maintain effective shielding at higher and higher frequencies.
Seeing the need for users wanting to operate their JRE test chambers at these higher microwave frequencies, JRE used various techniques to additionally ‘seal up’ tiny voids, leakage paths, and areas of potential ingress/egress. Extensive redesign of the actual mechanical enclosure was not needed since the robust construction was already very tight. Attention was focused on any discontinuities in the shielding walls, such as I/O plate, cover gasket seals and hardware attachment points. Additional gasketing and other engineering techniques were used to push the upper frequency limit into the microwave region with excellent effect, shielding isolation is better than 85 dB all the way to 28 GHz. Similar enhancements were performed on the line of JRE I/O filters to work at these higher microwave frequencies. Any of the standard JRE test chambers, and select I/O filters may ordered with this microwave shielding upgrade at a nominal additional cost – the suffix, “MW” indicating microwave, is added to standard JRE part numbers to distinguish these products. One should also note that this upgraded “Microwave” shielding option will also provide additional shielding isolation at conventional wireless frequencies too - important if your test needs require the utmost in isolation.
Lab tests and results
To explore the effects of microwave frequencies on the shielding isolation of an RF test chamber, tests were conducted at both 2.45 GHz and 28 GHz on a stock model JRE-1812 test enclosure and an upgraded “Microwave” version, JRE-1812MW. Two different I/O plate configurations were used, initially a completely blank plate to limit any potential I/O filter leakage and again with a single LAN-1 filter.
On the stock JRE-1812, shielding isolation was found to be greater than 100 dB at 2.45 GHz, but only 35 dB at 28 GHz. Clearly, the stock test chamber exhibited much more leakage at 28 GHz, and while there was reasonable isolation of 35 dB, for many tests, this leakage may be too much.
Testing the microwave version of the same sized test chamber, JRE-1812MW, showed greatly improved performance at 28 GHz. The test chamber now measured greater than 90 dB at 28 GHz, and as an added bonus, shielding isolation at 2.45 GHz had also improved to the point that it became unmeasurable at test equipment limits (-120 dB).
Test results with JRE LAN-1 Ethernet filter
The above tests show that the test chamber provides excellent shielding isolation with no I/O filter involved, this tells us that the inherent isolation of the test enclosure is good – now let’s add an I/O filter and see what happens. We will use the JRE LAN-1 filter, a popular interface for Ethernet signals at 10/100/1000 Mbps speeds. In this case we found that adding the I/O filter only slightly affected the isolation at 2.45 GHz, but at 28 GHz, the isolation was severely degraded, measuring just 30 dB near the filter. When using a test chamber, the test system is only as good as the weakest link and in this case, it is the standard 6 GHz rated LAN-1 Ethernet I/O filter. When using the enhanced microwave version of a test enclosure, one also has to be sure to use similarly rated I/O filters and accessories. Replacing the standard LAN-1 I/O filter with the microwave enhanced version, LAN-1MW, we saw the isolation improve dramatically at 28 GHz, measuring 80 dB, indicating excellent shielding isolation.
Not all RF wireless testing will need to be performed at high microwave frequencies up to 28 GHz, most wireless falls below 6 GHz and JRE’s broad equipment line up will fill most any need - but if your testing pushes the higher frequencies above 6 GHz, JRE Test offers an excellent path to fulfilling your RF test chamber needs.