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By: OLIVER P. FERRELL, Editor
RICHARD A. FLANAGAN, Associate Editor
 IF YOU were a reader of POPULAR ELECTRONICS in May, 1956, you may recall a story entitled "Radar on the Highway." It discussed radar speed meters, told how they operate, and explained why-at that time- it was practically impossible to escape detection. The story closed with the prediction that "a microwave detector (radar speed meters operate at microwave frequencies) could be built" to warn motorists as they approach a speed trap.
In the intervening five years, many microwave detectors have come and gone. POPULAR ELECTRONICS has investigated each new speed-trap detector and has rejected some because they violated government regulations; others because of size, weight, or impractical construction; and one or two because they were simply "wishful thinking." Within the past few months, we have spent many days testing a new unit called the "Radar Sentry." Made by Radatron, Inc., 232 Zimmerman St., N. Tonawanda, N. Y., it is available direct from the factory for $39.95.
Four Radar Sentry units have been tested by POPULAR ELECTRONICS, and on the following pages you will find our report on this ingenious device, phrased in an easy-to-follow, question-and-answer format.
What is the Radar Sentry?
The Sentry is a miniature (2" x 4" x 3"), self-contained, transistorized receiver, complete with a special built-in antenna and battery power supply. It will detect signals at about 2455 mc.- the frequency assigned by the Federal Communications Commission for radar speed meters and traffic signal controls.
How does the Sentry work-electronically speaking?
The Sentry is simply a receiver, using eight transistors and two diodes. A resonant-slot antenna (actually the back panel of the case) is fixed-tuned to the "S" band-2455 me. Whenever a radar signal is picked up, it is passed through a diode mixer and a diode detector, amplified, and fed to a miniature PM speaker. But since the speed meter signal is unmodulated, nothing would issue from the speaker were it not for an ingenious "flip-flop" audio-oscillator circuit. The diode mixer receives both the incoming signal and the output of the "flip-flop," and effectively "chops" the incoming signal at an audible rate. In actual use, therefore, the Sentry produces only a slight crackling sound until "triggered" by a radar signal, at which time it emits an audible (700-cycle) tone.
Power for the Sentry is supplied by two mercury cells, which, according to the manufacturer, have a life of 800 to 1000 hours; this corresponds to 35,000 miles of driving, assuming an average speed of 35 mph, battery life will vary somewhat with the setting of the volume control, but the batteries are readily replaceable and cost only 56 cents apiece.
How does the motorist use the Sentry?
The manufacturer recommends attaching the Sentry to the sun visor (a clip is supplied) on the driver's side of the car. The rear panel of the Sentry must have a clear, unobstructed "view" of the highway in front of the vehicle; in other words, the back plate "looks" out of the windshield and down the highway. Any object that interferes with this "view" -windshield wipers, for example-could scatter microwaves, reducing the Sentry's "sensitivity."
The volume control on the Sentry should be set so that a slight crackling sound is audible. To minimize battery drain, the volume should be kept as low as possible. When the beam of a radar speed meter is intercepted, the Sentry will emit a distinctive 700-cycle signal. The tone will get very loud very rapidly and will be heard only when a microwave signal is being detected by the Sentry.
How much warning does the Radar Sentry give the motorist?
Although there is no all-inclusive answer to this question, a motorist can expect to be warned about 1000 to 2000 feet away from an operating radar speed meter. The range of the warning zone depends upon the topography of the highway, traffic conditions, and, in particular, on just how the speed meter itself has been "aimed" to observe traffic flow.
Aren't all radar speed meters used the same way?
No. Since the majority of present-day radar speed meters are portable, each setup is slightly different. The beam from the speed meter is relatively narrow, and "aiming" it down the highway is a matter of experience on the part of the traffic officers.
If an officer is shooting for maximum range on a "clear" highway, the Radar Sentry will give a warning signal at least 1500 feet away.
On turnpikes and freeways where a portable speed meter is mounted on an overpass, the Sentry will give 1800- to 2200-foot warnings. However, if the traffic officer decides-to sacrifice range and maximum speed readings by pointing the speed meter into the highway at a 40-45° angle, the Sentry may not give 11 warning until you are 600 to 750 feet away.
Does the Sentry interfere with the speed meter, and can traffic officers tell that a Sentry is in use?
A double no. The Sentry is simply a receiver and does not radiate any signal of its own.
Can traffic officers' find ways to defeat the warning given by the Sentry?
Yes-there are several possibilities. Probably the one most advantageous to the traffic officer would be placing the speed meter so that it records departing vehicles rather than approaching or on-coming cars. It makes no difference to the speed meter which way it is aimed, since it responds only to velocity and not direction or distance.
It might also be possible for a traffic officer with a Sentry in his own car to aim the speed meter in such a way as to minimize warning time but still preserve good speed meter sensitivity.
Lastly, the writers foresee the possibility of radar speed meter manufacturers shifting "wave polarization" in future models. Such meters would feed cross-polarized waves to the Sentry, effectively reducing its sensitivity by 10-12 db.
Why don't traffic officers simply change the frequency of speed meters, leaving the Sentry sitting high and dry?
Unfortunately for the traffic officers, speed meters are licensed to be operated at about 2455 me. Also, to shift to a new frequency, a speed meter would need to be altered by the manufacturer-an expensive proposition.
Aren't radar speed meters being changed anyhow?
Not necessarily. But there is very limited production of an entirely new speed meter operating in the 10,000-mc. "K" band. It has improved circuitry with considerably more power output and much greater range than the present popular "S"-band models.
Will the Sentry detect the new meters?
No. The difference in frequency is too great, and this necessitates new equipment. However, the same principle of operation applies, and the manufacturer will undoubtedly introduce "K"-band speed meter detectors when they are needed. Radatron is also considering a composite model which would be designed to receive simultaneously on both frequencies.
Is the radar speed meter signal the only one a Sentry will pick up?
No. It will react to any signal on the frequency to which it is tuned-2455 me. In addition to traffic control devices, aeronautical and marine radars operate in the "S" band. But the latter do not offer much interference-in fact, they are a good check on whether the Sentry is operating properly.
How do you identify an aeronautical or marine radar?
Such radars "search" with rotating antennas and may sweep by the reception zone of the Sentry at any time. The signal from these transmitters is a short "swish" or "blurp," not the clean, clear-cut tone of the speed meter.
Why doesn't POPULAR ELECTRONICS publish plans on' building a detector similar to the Sentry?
The Sentry discussed in this article has a number of patent applications outstanding, and the manufacturer does not want to reveal the exact circuitry. In addition, the sensitivity of any such detector would depend upon its being tuned to 2455 me. This is not as easy as it sounds, and getting a "test signal" on this frequency would be a problem for most experimenters.
POPULAR ELECTRONICS is nevertheless currently investigating a slightly different "home-brew" speed meter detector and plans to report on it in the November issue. |
