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868MHz Propagation problem
On Fri, 24 Oct 2008 02:14:24 -0700 (PDT), dgleeson422111
wrote: Hello All We have a propagation issue that is confusing us. We have two 868MHz modules on test, each capable of 500mW transmission. Hi Denis, That is a lot of power for a problem of losing contact in the space of blocks. What we have repetadly found is that the radio in the office can receive long after the radio in the street has stopped receiving. (The person with the radio in the street is moving away from the office.) The radios are the same and the power supplies to the radios are the same. Indeed switching the radios gives the same effect. The propagation paths between the two radios are the same in terms of distance. However the radio signal transmitted from the office travels through walls first before then traveling through open space. Its the opposite for the radio in the street, firstly traveling through space and then through the walls in the office. A very good description of the symptoms and relations. We did think we had identified an interfeering source in one direction (when moving away from the office) so we started moving in the opposite direction. We found exactly the same effect. A good test for reducing variables. If you identified an interference, I would presume you could do that again in the other direction. Are we getting interfeerance from GSM, ISM or Paging? Only if the remote set was nearer to an undiscovered interferer. You seemed to have resolved that once, and you should be able to detect the similar occurrence again. As far as GSM bands go in Europe 890–915 MHz OR 1710–1785 MHz So these shouldnt be our problem. This is a flaw in the logic. Your frequency of operation is very close to these bands - unless your sets have been designed with elaborate front end tuning (which seems unlikely). Can anyone shed light on this phenomonon? Is it an issue of wireless propagation that Im not familiar with or is it Interfeerance? One component not discussed is the remote set's proximity to the operator. The operator could disturb the local field (aka shield). Going beyond is the effect of a Fresnel Zone, but your descriptions describe a complete black-out. To test this requires you to go further away to see if you pick up the signal again. This is a long shot, however, as the effect would be distinct within the space of a meter or so. For mobile operation, the Fresnel Zone creates what is called "Picket Fencing" as the signal comes and goes quickly with distance traveled. Perhaps it is the proximity of a Pub, and your operator stopped in for a Guinness. 73's Richard Clark, KB7QHC |
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