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OT: Glass Antenna?

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  • OT: Glass Antenna?

    Just getting a new Pike Pass here and it must be RFID since it has no battery and is a flat credit card sized thing that's just a little lumpy in the middle.

    Anyway, they've got instructions for installation which are basically just peel the adhesive and stick it to the windshield below the rear view mirror mount. All straightforward and just about what I'd expect except for one comment that it uses the windshield as an antenna. Really? Is there something I don't know about propogating radio waves with glass or does the instruction writer have NF idea about the technology or is just making it up as he goes along?
    .
    "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

  • #2
    Maybe they were talking about windscreens that have the antenna built into them? Both of my cars are like that.

    Phil.

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    • #3
      They are making it up. It's easier than trying to explain to the great unwashed that it needs to have a clear view of the transceiver. The antenna is in the card.

      The more we depend on technology, by default the less the general population knows about how anything works.
      Last edited by Evan; 06-09-2011, 12:24 AM.
      Free software for calculating bolt circles and similar: Click Here

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      • #4
        +1 Evan

        They have no idea of how it works. The antenna is in the card and the system reads it as you go by.
        Robin

        Happily working on my second million Gave up on the first

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        • #5
          I wonder if it will work on those super expensive windshields that have a transparent coating of conductive tin oxide for defrosting?
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            Glass is an excellent insulator and dielectric up to very high frequencies, so it is virtually invisible at any normal radio frequency. It could act as an antenna at audio frequencies, where it is physically vibrated by an actuator of some sort, but that's about it. It begins to interfere with electromagnetic frequencies somewhere in the sub-visible light range. That's a lot higher frequency than is used in any radio communications devices that I know of.

            If we're talking about glass that has a bit of a conducting component in its makeup, such as microwave oven plates, then it does change the story. I can't even speculate as to the degree that this would make an effect- it's probably measurable, but not significant.

            This 'glass antenna' story reminds me of a commercial I saw the other day. I don't recall the product, or the 'information' being peddled, but basically there's a woman representing the interests of the public, looking at these two flunkies who are giving her the 'story', which they couldn't possibly have any real knowledge of, and she looks at them and says 'you two sure are smart'. Made me really proud to be a north american citizen -NOT!
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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            • #7
              Our toll roads have been using these for several years. Sure beats having to line up and wait at the toll both. We pull a trailer a lot and they charge by the axle. Dont't understand how they do that.
              Byron Boucher
              Burnet, TX

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              • #8
                Originally posted by Boucher
                Our toll roads have been using these for several years. Sure beats having to line up and wait at the toll both. We pull a trailer a lot and they charge by the axle. Dont't understand how they do that.
                Guessing the same system as traffic calculation uses: Capacitive/magnetic sensing of the vehicle passing by and detecting the type of pulse it sends out to distinguish between autos, trucks and other vehicles.
                Amount of experience is in direct proportion to the value of broken equipment.

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                • #9
                  If we're talking about glass that has a bit of a conducting component in its makeup, such as microwave oven plates, then it does change the story.
                  Not mixed in the glass but a coating deposited on the glass. Tin Oxide is completely transparent but a very good conductor. It is one of the few materials with this combination of properties. I suspect it would reflect the interrogation signal.

                  An RFID chip gets its power from the interrogation signal. It's a case of using RF to transmit a small amount of power that is used by the chip after rectification and filtering to then switch the tuned circuit (antenna) on and off with a bit pattern. That modulates the reflected signal similar to how moving turbine blades modulate radar.

                  I have had an idea to make a passive radar jammer for both speed radar and lasers for a long time. By making a corner cube reflector from light weight material that is also a mirror such as very thin polished aluminum and then mounting it on a small loudspeaker cone that is playing music it should completely scramble the radar/Lidar gun.
                  Last edited by Evan; 06-09-2011, 05:42 AM.
                  Free software for calculating bolt circles and similar: Click Here

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                  • #10
                    There are two styles available. The conventional one that mounts to the windshield and another that mounts to the front license plate mount for those windshields that are coated. You can check with the vendor for which will work with your vehicle.

                    At least that is the way it works with EZ Pass.
                    Jim H.

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                    • #11
                      Antenna

                      My RFID Sun Pass in Florida will not work through the coating at the top of my windshield.
                      The instructions said to place it under the rear view mirror bracket.
                      I was sure it would be blocked by the coating but did try it in that location.
                      Work's fine at the bottom of the windshield.
                      The instructions must have been written by the sales department instead of engineering. LOL

                      Evan - That's a clever idea for a jammer.
                      I wonder if the reflected signal from a small corner reflector would be strong enough to overcome the vehicle reflection.
                      I know that some early radar guns needed a fairly stable Doppler offset to work on the theory that a vehicle could not change speed quickly.

                      The first radar gun was developed by an engineer at Naval Avionics Facility Indianapolis (NAFI).
                      We were working on Doppler radars that measured the velocity of bullets fired from gun turrets on Navy aircraft and corrected the gun laying computer parameters.
                      The ammunition speeds varied by 15% from batch to batch.
                      That's where the technology for police radar guns came from.
                      Bill
                      I cut it off twice and it's still too short!

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                      • #12
                        On a rental car in Florida there was a gadget that mounted in the hood crack on the passenger side. If a windshield wouldn't work, that would be a good location.

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                        • #13
                          If its like here in Illinois if you pass and it doesn't read your pass it takes your picture and your license is compared to the database of pass owners. If you have a valid pass you don't get a ticket.

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                          • #14
                            Originally posted by Rustybolt
                            If its like here in Illinois if you pass and it doesn't read your pass it takes your picture and your license is compared to the database of pass owners. If you have a valid pass you don't get a ticket.
                            Yup, sam ting here. They wouldn't have a chance otherwise with people flouting the system.

                            Reminds of a not so great idea quite a few years ago. They'd set up an automated radar speed check over the interstate just outside of Des Moines that would flash your speed and a warning to slow down if you were over the limit. This turned out to be a challenge to see what speed you could post going through it. After a fairly short time they reprogrammed it to put a limit on the speed display and just tell you that you were going over the limit. Not nearly as much fun.
                            .
                            "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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                            • #15
                              Originally posted by Evan
                              I have had an idea to make a passive radar jammer for both speed radar and lasers for a long time. By making a corner cube reflector from light weight material that is also a mirror such as very thin polished aluminum and then mounting it on a small loudspeaker cone that is playing music it should completely scramble the radar/Lidar gun.
                              Cool!
                              Then you could speed too Evan!
                              Mike

                              My Dad always said, "If you want people to do things for you on the farm, you have to buy a machine they can sit on that does most of the work."

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