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868MHz Propagation problem

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=96915 MHz OR 1710=961785 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 _____________________

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Reply to
dgleeson422111
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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

Reply to
oopere

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

ese

Reply to
dgleeson422111

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

Reply to
osr

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.

Is your office in a concrete building? Or one with any steel reinforcement or steel or aluminum studs? If so, it could well be that there is a standing wave issue, so that a portion of your transmitted signal is being reflected back, and so not making it to the receiver. This would not necessarily affect the reception on the radio in the office from the outside radio.

Reply to
Brenda Ann

dgleeson422111 wrote: \\> What we have repetadly found is that the radio in the office can

The radio outside is likely being desensitized by some other signal.

Reply to
Anthony Fremont

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

Reply to
1hogrider

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)

Reply to
Jon Slaughter

an

he

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.

Reply to
EE123

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.

Reply to
Jon Slaughter

Do you have a license?

Reply to
Dave

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.

Reply to
JB

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.

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

Reply to
Paul Keinanen

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

Reply to
Ed Cregger

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.

Reply to
Helmut Wabnig

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.

Reply to
Helmut Wabnig

uoted text -

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

Reply to
Mark

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

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
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A mathematician is a machine for converting coffee into theorems.
Reply to
Paul Hovnanian P.E.

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     mailto:Paul@Hovnanian.com
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Don\'t anthropomorphize computers. They hate that.
Reply to
Paul Hovnanian P.E.

Not required in EU for 868 MHz

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Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

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For if it prosper, none dare call it treason."

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RFI-EMI-GUY

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