868MHz Propagation problem

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 probably has 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.

nd

has

ies

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

Hi Denis,

That is a lot of power for a problem of losing contact in the space of blocks.

A very good description of the symptoms and relations.

A good test for reducing variables. If you identified an interference, I would presume you could do that again in the other direction.

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.

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).

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

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

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

end

o

ly has

plies

e

d text -

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 _____________________

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.

What is the modulation type and data rate? Could be a delay spread issue. Maybe the outside unit is receiving a reflected signal from the surface of a nearby structure and is experiencing inter-symbol interference that by chance the base unit is shielded from. This would occur only if a high symbol rate was being used.

--
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

[...]

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.

--
Regards, Joerg

http://www.analogconsultants.com/

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

The OP said that he did that, and the problem remained. It's certainly a desensing problem due to local interference at one end. That's the only possible cause.

what type of amateur radio application is this for? Or is it a commercial application posted in this amateur radio group? just wondering

e

Denis,

I agree with some of your previous posts - the outside receiver is likely being desensed by a strong signal, perhaps away from or a harmonic of the frequency you're using. I suggest trying the test away from town and at least 10km from strong transmitting towers. Let us know what you find out - may help those of us who do UHF/VHF mobile/ portable hamming.

As a ham I have NOT PROBLEM with the commercial community asking questions on a ham radio group - we can ALL learn from this.

Cheers,

Dick Post, N7EMW

i was thinking about that...but no....

all the multi-path paths are reciprocal as well,,, so whatever the delay spread is one way should be the same the other way.

Mark

Denis,

I agree with some of your previous posts - the outside receiver is likely being desensed by a strong signal, perhaps away from or a harmonic of the frequency you're using. I suggest trying the test away from town and at least 10km from strong transmitting towers. Let us know what you find out - may help those of us who do UHF/VHF mobile/ portable hamming.

As a ham I have NOT PROBLEM with the commercial community asking questions on a ham radio group - we can ALL learn from this.

Cheers,

Dick Post, N7EMW

*************************************************** I don't know why I don't get the > on your response, but whatever. He can get some feel for what is going on by taking *both* radios outside near his office, and walking around. He should be able to find a particular area that has poor reception. I don't buy the explanations for antenna gain and multipath, they should affect both directions of transmission equally.

Tam

Since most of the posts, in their "EXPERT OPINIONS", relate to your receiver being "de-sensed", do you have a way of negating/affirming this?

Begin there, and we can get down to "real problems!" :-) i.e., a wide-band field strength meter, freq counter, etc. will either make all this "right" or "wrong."

Regards, JS

And, I should have included, beg one, buy one, borrow one, steal one, etc. ...

Regards, JS

Desense is definately a possibility, but I still believe that it's at least AS good a chance that it's a standing wave issue on the office end. Others have said to take the 'office' radio outdoors and test the setup that way. That's a good (and simple) test.

I'm /still/ more inclined towards a desense problem - cities are astonishingly noisy these days throughout the RF spectrum. I've recently had 4.9 GHz data transmission problems that were eventually traced to a harmonic of a nearby surveillance TV transmitter!

Bob