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Posted by on July 2, 2005, 11:30 am
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On Mon, 30 May 2005 02:08:40 +0200, -pull@shoot wrote:
>Regardless of some that try to flood the fish by mentioning unrealized
>performances of spread spectrum (SS) in wireless alarm systems,
>regardless the invalid comparisons with military equipment, everybody
>knows that wireless alarm systems are unreliable during Radio
>Frequency Interference (RFI).
>
>WHY?
>
>Does that Spread Spectrum (SS) do any good to interferences avoidance?
> Yes, of course "IF" it is applied the correct way like in military /
>space / ++ equipment's ($$$).
>
>What's the correct way to implement spread spectrum?
>1. The hopped frequencies should be "wide spread" over different
> frequencies, each frequency acting likes a separate transmitter /
> receiver link operating on a whole different frequency.
>2. The receiver should be tuned to each frequency in a "synchronous"
> manner with the transmitter otherwise it will capture even better
> RFI due to hiss wide spread capture range required when the
> receiver is not hopped.
>3. The receiver should be small band and receive only the requested
> hopped frequency and not capture at once the whole range of spread
> frequencies.
>
>First:
> How can the required synchronized transmission between sensor and
>control panel workout with "multi-sensor transmitters" operating
>"asynchronously" as required use in Wireless Alarm Systems?
> Impossible or with a high-speed multi frequency switching receiver,
>too expensive for the application here, to $$$.
>
>Second:
> FCC (and other country authorities) regs only allow spread-spectrum
>use on three bands: 902-928 MHz, 2.4 GHz, and 5.8 GHz. Those bands are
>not available for wireless alarm systems.
>
>3 th:
> Some systems implement small frequency hopping (very limited spread
>spectrum) feature, WHY do they let believe to sales people that there
>is SS in theyre system?
>
>
>Wireless Alarm Systems don't use the spread spectrum possibility as it
>should be in order to avoid interferences, they're allocated frequency
>spectrum is to restricted and the receiver is only tuned to the center
>frequency of the hopped frequencies (the large bandwidth cope with all
>hopped frequencies and as such is even more interference sensitive).
>
>The design purpose of this erroneously mentioned so called SS feature
>is to avoid sensor location black spot problems inherent to the higher
>frequencies used and not to avoid RFI like some tend to let you
>believe.
>
>Propagation problems and sensor location:
>The frequency hopping used in some wireless alarm systems is used to
>resolve the sensor propagation nodes problems at the receiver end when
>a metal objects is displaced from his initial installation setup
>position (car, statue, ++).
>
>Conclusion on SS frequency hopping:
>Why does installer and maintenance persons think (say) that it they
>have better systems now?
> Because they have the impression that they have a more solid (better)
>wireless data link connection in normal operating conditions because
>they find it not so critical by initial sensor position setup.
>
>That limited frequency feature, that manufacturers and sales people
>tend to let you believe as the latest miracle, is worse than more
>selective receivers designed to capturer one frequency.
> The wide receiver range included in so called SS systems capture all
>hopped frequencies simultaneously and as such open an even wider RFI
>susceptibility door.
>
>Again, where are the manufacturer technical specifications about the
>wireless data loop involved in theyre system???? Embarrassing to show
>to the general public the low quality of they're equipment in that
>area.
>
>Wireless alarm systems are UN RELIABLE during interference and don't
>warn you when they are MUZZLED.
>
>Paul
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