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