Maker Pro
Maker Pro

868MHz Propagation problem

D

dgleeson422111

Jan 1, 1970
0
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.

Denis
_____________________
http://www.CentronSolutions.com
 
O

oopere

Jan 1, 1970
0
dgleeson422111 said:
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.

Denis
_____________________
http://www.CentronSolutions.com

You may experience receiver desensitization from a strong out-of-band
interferer which is not penetrating well into your office and hence not
interferring there.

Pere
 
D

dgleeson422111

Jan 1, 1970
0
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
 
higher antennas = longer range. Thats your issue.
Also lack of error correction, poor antenna pattern to meet the low
power regulations, and a less then robust protocol, ie your probably
using some form of on/off modulation where the receiver does not have
a continious wave signal to locl on and track. Try sending a preamble
character that is 10101010 a few times, assuming your receiver is
sophisticated enough to use AGC. This sets the DC level in the
discriminator more accurately and reduces errors.

your using a radio with intentionally designed in limited range to
allow reuse of the frquency.

Steve
 
1

1hogrider

Jan 1, 1970
0
dgleeson422111 said:
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.)

I see you have other responses. One test that would establish that is
environmental (adjacent signal, receiver desense, etc), if BOTH units
could be moved to a relative RF free environment such as a rural setting
and then test the range.

If this eliminates the problem, then its probably time to break out a
spectrum analyzer and take a look at what is around your operating
frequency.

Andy
 
J

Jon Slaughter

Jan 1, 1970
0
It's because the paths are not the same. Think of a silvered mirror. It can
transmit light much easier in one direction than the other. Take both
outside and see if it has the same problem. (extremely doubtful because the
path's is virtually identical)
 
E

EE123

Jan 1, 1970
0
It's because the paths are not the same. Think of a silvered mirror. It can
transmit light much easier in one direction than the other. Take both
outside and see if it has the same problem. (extremely doubtful because the
path's is virtually identical)

Did you design the receive software?
Or did buy the receiver "off the shelve"?

Perhaps there is a problem
with the receivers and how they are
receiving the data.
 
J

Jon Slaughter

Jan 1, 1970
0
It's because the paths are not the same. Think of a silvered mirror. It
can
transmit light much easier in one direction than the other. Take both
outside and see if it has the same problem. (extremely doubtful because
the
path's is virtually identical)

Did you design the receive software?
Or did buy the receiver "off the shelve"?

Perhaps there is a problem
with the receivers and how they are
receiving the data.

um, did you read his message? He said that it was independent of the
receiver because they exchanged them and the same problem. i.e., it helps to
read the original post.
 
D

Dave

Jan 1, 1970
0
dgleeson422111 said:
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?
 
J

JB

Jan 1, 1970
0
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.)

You don't say if you are using the same antenna on both ends, but the
environment is very different at each end. Consider that Transmitting is
about putting signal into an area, while receiving is about capturing the
signals in near field of the antenna, then discerning the intellegence. You
would have to analyze the output to be sure, but it may be localized
reflections, shading or noise at the street level. Because of this it is so
difficult to quantify range. There is equipment and software specifically
designed to do what you are trying to do but it is expensive. The result is
highly detailed signal strength graphic output overlaid on 3d mapping for
each fixed site at various frequencies. Most commercial antenna sites have
gone to great expense to generate those maps in order to show what their
tower does because it is very different for each location.
 
P

Paul Keinanen

Jan 1, 1970
0
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.)

Is the moving station manually portable or is it mounted on a vehicle?

If the moving operator is bored and continuously talks on a GSM phone
or if the (vehicle) generates some other kind of interference, this
would reduce the SNR at the moving station.
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.

Does the transmitter have some kind of SWR protection ?

If the Tx antenna is close to the wall, it might detune the antenna,
increasing the SWR and the SWR protection drops the transmitter power.

Paul OH3LWR
 
E

Ed Cregger

Jan 1, 1970
0
oopere said:
You may experience receiver desensitization from a strong out-of-band
interferer which is not penetrating well into your office and hence not
interferring there.

Pere


Such as traffic light electronics.

Without error correction protocols being employed, you are also at the mercy
of multipath interference. You're QRMing your own signal. 8>)

Ed, NM2K
 
H

Helmut Wabnig

Jan 1, 1970
0
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.)

Polarization could be a factor.
Reflections change the polarization appr. 90 degrees.

Rotate the antennas left & right to find out.

(you did not mention your type of antennas)

w.
 
H

Helmut Wabnig

Jan 1, 1970
0
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.)


This is typical for groundplane antennas.
They radiate upwards.


Polarization could be a factor.
Reflections change the polarization appr. 90 degrees.

Rotate the antennas left & right to find out.

(you did not mention your type of antennas)

w.
 
M

Mark

Jan 1, 1970
0
Have you tried relocating the office transceiver? There might be
something about the office environment that is interfering with its
transmission.



wow so many of these replies are wrong..

in almost all cases, RF path losses are RECIPROCAL.

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

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

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

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

Mark
 
This is normal when antenna "theory" comes up.


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


Well that guy is usually on a differet page.


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


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.




- 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
 
R

raypsi

Jan 1, 1970
0
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
 
T

Tio Pedro

Jan 1, 1970
0
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
 
J

Joerg

Jan 1, 1970
0
W3CQH wrote:

[...]
its interesting that a commercial company would bring a problem such as this
to a bunch of amateur radio operators... true some of the amateurs may be
professional engineers, but the remainder are not! I guess that in today's
market place we try to gain information on the dirt and cheap way.

Nah, that's ok. Every one of us occasionally runs into an unusual
situation. Then one can either stew on it for days without getting to
the ground of it, or ask. IMHO it is a sign of maturity if somebody has
the guts to say "Hey, guys, this problem here really has us puzzled".

That is especially true for EMI cases and this sure looks like one.
Sometimes a brief hint by someone who has been in the trenches for
decades can get things going again. And most older ham radio operators
had that exposure, fixing the neighbor's electronics because their
manufacturers messed up.
 
Top