Tuesday, August 25, 2009

Why Ka-band is a Headache for Radar Detectors

MPH BEE III Ka-band radar gun with POP mode is one that doesn't use the 35.5 GHz frequency.

If you drive with a radar detector that digitally displays the radar gun's frequency, you may have noticed that one of the three Ka-band frequencies is particularly hard to detect.

Sitting at the very top of the super-wide Ka-band radar spectrum, 35.5 Gigahertz (GHz) is the toughest. There's a lot of these radars trolling the highways. Two of the four domestic radar manufacturers adopted 35.5 GHz and today there's about a 50-50 chance that when you encounter Ka-band radar, it will be one theirs.

The use of 35.5 GHz in traffic radar wasn't accidental. It was chosen by someone I've known for many years. I'll refer to him here only as Steve. No last names; he shuns publicity. Steve once confided that he knew this frequency would be a headache for radar detectors. And he was right.

Ka band runs from 33.4 GHz to 36.0 GHz, some 2,600 megahertz wide. In comparison, K band is 200 megahertz wide and X band only 50. Steve could have chosen any slice of that Ka-band real estate, and he purposely grabbed the highest available. He knew that countering it would require superior engineering and more-expensive components, making for a pricey radar detector.

To illustrate the conundrum this frequency poses to detectors, visualize standing before a trio of identical, half-mile-long railroad tunnels. The left tunnel represents 33.8 GHz; to the right is 35.5. In the center is 34.7 GHz.

Your job is to spot an approaching train and warn others. (No, you can't rely on your auditory senses; this is a government job. You're wearing OSHA-approved hearing protection.)

The train will be traveling at 300 mph (pretty fast for Amtrak but hey, it's a Mag-Lev) and there's an equal chance it can emerge from any of the three tunnels. Taking up station in front of the center tunnel might seem sensible but by standing there, you can't see far enough into the other two. So you sprint back and forth between the three tunnels, conscientiously keeping watch on each. It's exhausting work, for there's a very real risk that a train in one tunnel will get the drop on you while you're busy peering into another.

This illustrates the task facing a radar detector on Ka band. For the past decade few manufacturers have even attempted to monitor all three frequencies with equal facility. Most chose the midpoint, 34.7 GHz, and accepted a steep performance roll-off on both sides. This practice has been nearly universal among detectors priced under $200.

This has changed recently. For the first time, mass-marketers Cobra Electronics and Whistler offer several attractively-priced models with class-competitive Ka-band performance, including against that elusive 35.5 GHz frequency.

I recently tested several models from each manufacturer, plus a BEL (Beltronics) Vector 955, the perennial class leader. Reviews of these radar detectors and the test results are on Radartest.com.