The Test Chamber at 5G and Microwave Frequencies

Overview

The latest generations of wireless devices like cellphones, routers, and now IoT devices keep inching up the frequency spectrum 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 cellphone 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 or RF shielded test enclosures. Using existing test chambers at the higher 5G and microwave frequencies led to leakage problems during test and in some cases even having the test chamber have leakage with gain! (Remember there is such a thing as a ‘slot’ radiator antenna where a small slot can actually perform as an excellent dipole antenna).

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 only being used by the newest Wi-Fi routers. 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 entry as well suitable Input and Output (I/O) connections in order to access the device contained within the enclosure - and this is where great lengths have been gone to, in order to provide such access and communication without violating the continuous shielding barrier. Various forms of gasketing, conductive ‘fingers’ and other mechanical means have been used to ‘seal’ the entry point and filtered I/O devices to pass desired data communication signals across the shielding barrier and into the test chamber. 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.

To allow use of JRE test chambers at these higher microwave frequencies, various techniques were used to seal up any small voids and leakage paths, an extensive redesign of the actual mechanical enclosure was not needed as the robust construction was 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 to the microwave region with excellent effect. 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 selected I/O filters may ordered with this microwave shielding upgrade at a nominal additional cost – the suffix, “uW” indicating microwave, is added to standard JRE part numbers to distinguish these products. One should also note that this upgraded “Microwave” shielding option will 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 higher 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-1812uW. The popular JRE Test TVK kit was used to perform the testing at 2.45 GHz and a Gunn source along with frequency converter for 28 GHz. 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.

Shown below is the test chamber leakage at 2.45 GHz and at 28 GHz, you can see that the leakage at 28 GHz is very pronounced, making the test enclosure virtually ‘transparent’ at microwave frequencies.

JRE Test RF chamber JRE1812 showing isolation at 2.45 GHz and 28 GHz
JRE 1812 RF test chamber with no I/O showing shielding isolation at 2.45 GHz and at 28 GHz

 

Here is the same test set up as before, but now with the added “5G Microwave” shielding upgrade, there is a substantial reduction of leakage at both 2.45 GHz and 28 GHz, in fact, the leakage is reduced below detection limits at 2.45 GHz and approaching 90 dB at 28 GHz.

JRE Test 5G enhanced RF chamber JRE1812uW showing isolation at 2.45 GHz and 28 GHz
JRE 1812uW 5G enhanced RF test chamber with no I/O showing shielding isolation at 2.45 GHz and at 28 GHz

 

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 the pictures below we see that adding the I/O filter only slightly affected the isolation at 2.45 GHz, but at 28 GHz, the isolation is severely degraded. When using a test chamber at 5G and microwave frequencies, 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.

JRE Test 5G enhanced RF chamber JRE1812uW with LAN-1 Ethernet filter showing isolation at 2.45 GHz and 28 GHz
JRE 1812uW 5G enhanced RF test chamber with with LAN-1 Ethernet filter showing shielding isolation at 2.45 GHz and at 28 GHz

 

Test results with JRE LAN-1uW 5G/Microwave enhanced Ethernet filter

Now we replace the standard LAN-1 I/O filter with the microwave enhanced version, LAN-1uW. Here we can see the isolation improve dramatically at both 2.45 GHz and 28 GHz, indicating excellent shielding isolation.

JRE Test 5G enhanced RF chamber JRE1812uW with LAN-1uW Ethernet filter showing isolation at 2.45 GHz and 28 GHz
JRE 1812uW 5G enhanced RF test chamber with matching 5G enhanced LAN-1uW Ethernet filter showing shielding isolation at 2.45 GHz and at 28 GHz

Conclusion

Not all RF wireless testing will need to be performed at such high microwave frequencies as 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 limits in isolation requirements or at frequencies above 6 GHz, JRE Test offers an excellent path to fulfilling your RF test chamber needs.