http://www.neltronics.com.au/belFAQ.htm
Q: What is a radar detector?
A:Until now all radar detectors leaked a small amount of signal that police can detect by using a "Radar Detector Detector" (RDD).
They scan for common IF (intermediate frequency) emissions given off by the radar detector. Early RDD’s used VG2 Technology, which concentrated on IF emissions at 11.550 GHz, but radar detector manufactures soon moved their emissions to different frequencies to eliminate its effectiveness. Along came VG3! VG3 technology works in the same manner but scans for multiple IF emissions. RDD’s are of no concern to drivers in states where radar detectors are legal.
The VG2 Interceptor has been used in some states of Australia since 1991. It comes from Canada where a company called Techsonic Industries manufactures it. Radar detector companies struck back by moving their tuning frequency outside this scanned area, making their units "VG-2 Proof" or Stealth.
A newer device named Stalcar (Spectre in the USA) seems to have given the enforcers the upper hand for the time being. It comes from a Queensland company – Stealth Micro Systems Pty Ltd. The Stalcar is now used in parts of North America where it is called the Spectre. The Australian built Spectre and Stalcar RDD were designed to sweep a larger spectrum and a higher frequency than previously possible.
The threat from the Stalcar (Spectre) RDD varies in different areas. Country roads and highways are likely to be the areas where the RDD’s are used most often. In the city and suburbs, false alarms render any RDD far less accurate.
Several Radar manufacturers moved their oscillators outside of the range detected by the Spectre and Stalcar. The RDD manufacturers therefore redesigned them to sweep even larger bands and they were detected. The Spectre / Stalcar cannot detect a radar detector that is not powered up. They also cannot detect a Radar Detector with NO EMISSIONS. The Beltronics XR and USA model Sti are the first radar detector to have no emissions and therefore be undetectable.
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well there you go. there are undetectable detectors. that word has lost all meaning to me.