For the Over 50 (GHz) Crowd:
DC/NoVa EMCS Chapter Raises the Frequency Bar
Once upon a time it was hard to get much higher than “Utra High” frequencies with a top end of 3000 “Megacycles”. I recall the tuner on my parents’ television and the funky dial that tuned the UHF band TV stations north of Channel 13. It didn’t ‘clunk’ with the detent like the VHF knob, but spun with the smooth tuning of a variable-capacitor arrangement buried behind the wooden cabinet in the warm heart of the tube-driven TV.
Now that all the “can’t-miss” over-the-air entertainment has gone digital and the “TV White Spaces” are being carved up for other uses, one doesn't hold the same view of the UHF “band”, especially when considering the mysterious spectrum in the Extremely High (what’s next: Crazy High?) Frequencies EHF that defines millimeter waves (1mm being about the width of the character M in ten-point font). Anything above 30 GHz (10mm wavelength) is assumed to be a millimeter wave.
Rhein Tech Laboratories in Herndon, VA was the venue of our latest EMC Society Washington DC/Northern Virginia Chapter meeting. Mr. Mitchell Lazarus of Fletcher, Heald & Hildreth, PLC held forth on a balmy January 31st evening. Mr. Lazarus’ presentation Millimeter Wave Radiation gave insight on the FCC’s view on the upper edges of measurable RF energy. His presentation can be found here: http://wll.com/inprint.html - other. Mitchell writes on FCC proceedings involving emerging technologies at his blog at http://www.commlawblog.com.
Mitchell Lazarus Addresses the EMC Washington/NoVA Chapter
There are several interesting applications for the 50+ GHz frequency band. For communications, the bands around 57-64, 92-95 GHz have unlicensed applications. For licensed operations, there is available spectrum in the 71-76, 81-86 and 92-95 GHz bands. Radars are allowed in the following bands: 57-64 GHz and 76-77 GHz. There are also proposals and outstanding waivers for other radars in the 70 GHz bands.
View of Spectrum analyzer: Second Harmonic of Gunn Diode output (with image)
The promise of these bands is that they can have tremendously wide bandwidths; seven gigahertz can host a lot of bits, allowing the transmission of uncompressed video, for example—albeit at short distances.
An interesting historical perspective that was shared during the meeting was the birth and evolution of the FCC’s “Restricted Bands” of operation, set up in the early 1980s to protect government, GPS and astronomy applications. The authors of the rules threw everything above 38.6 GHz into the “Restricted” bin. This was (ostensibly) not an intentional banning of the use in these high frequencies, rather a “safe” approach to sequester these bands until uses could be developed and their properties better understood.
Since then, the various chunks of spectrum that have been carved out have had to endure a waiver/rulemaking process that has encumbered the adoption of these bands. (Author’s Opinion: slowing the development and rollout of promising technologies).
One of the characteristics of the operation at these frequencies is the physical limitations on propagation, most notably at oxygen resonances, one of which is squarely in the middle of the 57-64 GHz band. This reduces the propagation “as if through a fog” says Mitchell. The half-centimeter wavelength (c/f = 3E8/f) wiggles the O2 molecules, which sucks energy from the field. The O2 resonance alone contributes 16 dB per kilometer of loss.
Oxygen Absorption at 60 GHz
The positive aspect of this absorption is that the interference potential is reduced. The other aspect of short wavelength signals is that a very sharp focus that can be achieved using physically small, high-gain antennas. This also reduces interference possibilities because the narrow beamwidths that are achievable reduces the spatial coverage and potential overlap into other systems signal paths.
On the regulatory side, there are several areas where industry has successfully worked to loosen up the allocations, namely in the bands mentioned above. In addition, there are promising technologies for imaging and sensing. One notable technology is to use a radar application to spot “Foreign Object Debris” on airport runways. Apparently, there is a lot of junk on the tarmac out there and 78-81 GHz is a good frequency for spotting this stuff (an NPRM was pending as of December 2011). Other applications that have been implemented and are being wrangled around the FCC include tank level sensing.
After Mitchell’s discussion, Desmond Fraser of Rhein Tech Laboratories, our gracious host, gave a presentation on the challenges of sensing these teeny-tiny waves. EM fields just don’t behave as we are accustomed and require serious concentration and focus on connectors, cables and positioning.
Desmond Fraser Delivers the EHF Goods
Desmond’s presentation can be found here: LINK
Among the challenges in making millimeter measurements is calibration. Traceability is a serious problem, not to mention the difficulty in repeatability and measurement uncertainty.
Desmond gave an overview of mixer technologies. Spectrum analyzers, operating up to 50 GHz, use a variety of different mixers to extend the range. The mixers are particular to the SA model and have different LO and IF frequencies. Thus, an Agilent mixer cannot be interchanged with a mixer from R&S.
The latest innovations in mixers employ USB-enabled data files that can be used to upload correction factors. The latest algorithms allow for spurious image rejection, which speeds measurement. This all comes, naturally, at a price.
Rick McLay, Desmond Fraser and Dan Baltzell Share a Spurious Laugh
The evening wrapped up with a demonstration of the measurement of a gunn diode coupled to a horn antenna. The audience was able to get the hands-on treatment of the funny ways the millimeter wave flow, including detuning, reflections from nearby objects that the strong polarization effects that must be accounted for while making the measurements.
The next meeting of the WDC/NoVA chapter is March 15 at Muldoon’s Restaurant in Gaithersburg, MD. Steve Ferguson of Washington Labs is arranging for a MIL-STD presentation.
Contact: email@example.com for more information and “stay tuned”.
Pictured (L-R) Rick McMurray, Desmond Fraser, Elmer Rodriguez, Marshall Pye, Steve Ferguson, Jeff Silberberg, Steve Koster, Rick McLay, Dan Baltzell, Rik Smith, James, Ritter, Mike Violette
Submitted by: Mike Violette, Washington DC/Northern Virginia IEEE EMC Society Chapter Chair (Acting)