dgleeson422111 wrote:

Hi Guys

Thanks for your input on this.

The idea of height being a problem is interesting. The radio in the
office is on the 1st floor (i.e above the ground by one floor).

If the antenna were at the same height would the problem go away?

We will test and see

If I remember my antenna/propagation theory correctly, such a change
should affect both the transmit and receive gains equally. That is to
say, if moving the office antenna lower will reduce its transmit range,
it will also reduce its receive range.

What you are looking for is something that is reducing the office end
transmit power while leaving its receive sensitivity intact. Someone
else suggested an SWR problem due to near field interference with the
antenna.


--
Paul Hovnanian
------------------------------------------------------------------
Don't anthropomorphize computers. They hate that.

Dave wrote:
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.

We are testing to find the range of communication. We are testing them
as follows. One remains in our office while the other is taken to the
street outside. we are communicating charecter strings such as "Im at
the lamp post number 1" etc. There is somebody at each end sending
text strings in a continuous conversation. There is no protocol, error
checking or error recovery.

Do you have a license?

Not required in EU for 868 MHz

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

Anthony Fremont wrote:
dgleeson422111 wrote:
\ 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 radio outside is likely being desensitized by some other signal.



Most likely scenario.

Also make sure that in your design you are powering down the transmitter
fully in the receive mode. In some radios there can be a residual
carrier from the PLL causing self interference ar interference to other
nearby nodes.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

Mark wrote:
On Oct 24, 2:00 pm, "Paul Hovnanian P.E." wrote:

wow so many of these replies are wrong..

This is normal when antenna "theory" comes up.

in almost all cases, RF path losses are RECIPROCAL.

I agree, including the antenna gains (meaning that the antennas at each end
don't have to match).

(even a 1/2 silvered mirror is reciprocal to light)

Well that guy is usually on a differet page.

As some have said, the cause of your issue is probably interference to
the outside receiver.

That's what I think is the most likely cause. The guy outside probably has
his Nextel phone with him. ;-)

Or maybe something about the indoor setup is causing the indoor Tx to
operate at lower power.

I suppose that's possible too. The OP should make sure the power supplies
are equivalent (meaning that the output power would suffer more than the
receiver gain with a drooping supply). So check the indoor supply.

It is very unusual for RF path loss to be non-reciprocal.

On Oct 24, 2:26*pm, "Anthony Fremont" wrote:
Mark wrote:
On Oct 24, 2:00 pm, "Paul Hovnanian P.E." wrote:
wow so many of these replies are wrong..

This is normal when antenna "theory" comes up.

in almost all cases, *RF path losses are RECIPROCAL.

I agree, including the antenna gains (meaning that the antennas at each end
don't have to match).

(even a 1/2 silvered mirror is reciprocal to light)

Well that guy is usually on a differet page.

As some have said, the cause of your issue is probably interference to
the outside receiver.

That's what I think is the most likely cause. *The guy outside probably has
his Nextel phone with him. *;-)

Or maybe something about the indoor setup is causing the indoor Tx to
operate at lower power.

I suppose that's possible too. *The OP should make sure the power supplies
are equivalent (meaning that the output power would suffer more than the
receiver gain with a drooping supply). *So check the indoor supply.



It is very unusual for RF path loss to be non-reciprocal.- Hide quoted text -

- Show quoted text -

It is assumed that the path to and from the office should be inferior
to the line of sight path out on the street. This may or may not be
true. The building may actually be offering gain in certain
directions. I would start by seeing if the same condition existed at
many more locations. If it does then other ideas need to be examined,
I would start by examining the RF environment outside the building.

Jimmie

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

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.

We are testing to find the range of communication. We are testing them
as follows. One remains in our office while the other is taken to the
street outside. we are communicating charecter strings such as "Im at
the lamp post number 1" etc. There is somebody at each end sending
text strings in a continuous conversation. There is no protocol, error
checking or error recovery.

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.

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.

Are we getting interfeerance from GSM, ISM or Paging?

As far as GSM bands go in Europe 890–915 MHz OR 1710–1785 MHz So these
shouldnt be our problem.

ISM is where we intend to operate and the greatest band occupancy
should be 10%. So we should get communication even though we would get
some interfeerance.

Paging - well this has very much gone out of fashion in Europe. But
even if it was the cause then wouldnt it be intermittant
communication. We have a constant problem.

Can anyone shed light on this phenomonon? Is it an issue of wireless
propagation that Im not familiar with or is it Interfeerance?

Best regards, and thanks for any input.


I'll second Pere and Anthony. Strong out-of-band signals is what I found
mostly in situations where path losses seemed strangely non-reciprocal.

Post a schematic. One possibility is that your receive input filter is
wide open like a barn door and GSM swamps the receiver.

Another option if you have a spectrum analyzer: Hang a large near-field
probe into a long cable, tape it to a broomstick and hold that out the
window for a while. See what pops up on the screen. If this is in a city
be prepared for some commotion among the pedestrians down there :-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Hey:

My crystal balls are saying:

Multipath where a reflected signal from the source
is being cancelled out in the zone sort of like
standing waves. It's like waves in a bath tub
they bounce of the sides and produce more inter-
ference waves, and so on. Or FM broadcast stations
signal in the big city with tall buildings, all of a sudden
the signal just goes puff while you're driving along down-
town.

I'd say gits your antenna near a window or put up an
outdoor antenna.

73 OM

n8zu

"Mark" wrote in message news:89e3cfad-c877-4be6-8eca-
wow so many of these replies are wrong..

in almost all cases, RF path losses are RECIPROCAL.

( It is very unusual for RF path loss to be non-reciprocal.

Mark


I agree. But, one thing that should be proven is that both modules
are working as intended. The one being used for the remote
application *could* be defective. Swap modules and see if the
receiving problem also switches locations, or remains the
same.

Pete

Hello,

There should be a reciprocal relationship between the modules
regardless of height, antennas etc. as long as TX power and RX
sensitivity of the modules are the same (which they appear to be).

The modules are unlikely to have very good input filters at 868MHz, so
any adjacent channel interference (GSM etc.) is likely to cause the
out of balance behaviour.

Try heading in a different direction outside, or go somewhere else
with your experiment.

For your real world application, are you likely to have any site close
to adjacent channel transmitters (cell or broadcast) ? If so, think
about a decant front end filters. This could be costly, but might save
you...

Regards,

Mark



On Oct 25, 9:25*am, wrote:
On Oct 24, 2:26*pm, "Anthony Fremont" wrote:





Mark wrote:
On Oct 24, 2:00 pm, "Paul Hovnanian P.E." wrote:
wow so many of these replies are wrong..

This is normal when antenna "theory" comes up.

in almost all cases, *RF path losses are RECIPROCAL.

I agree, including the antenna gains (meaning that the antennas at each end
don't have to match).

(even a 1/2 silvered mirror is reciprocal to light)

Well that guy is usually on a differet page.

As some have said, the cause of your issue is probably interference to
the outside receiver.

That's what I think is the most likely cause. *The guy outside probably has
his Nextel phone with him. *;-)

Or maybe something about the indoor setup is causing the indoor Tx to
operate at lower power.

I suppose that's possible too. *The OP should make sure the power supplies
are equivalent (meaning that the output power would suffer more than the
receiver gain with a drooping supply). *So check the indoor supply.

It is very unusual for RF path loss to be non-reciprocal.- Hide quoted text -

- Show quoted text -

It is assumed that the path to and from the office should be inferior
to the line of sight path out on the street. This may or may not be
true. The building may actually be offering gain in certain
directions. I would start by seeing if the same condition existed at
many more locations. If it does then other ideas need to be examined,
I would start by examining the RF environment outside the building.

Jimmie- Hide quoted text -

- Show quoted text -