>>Message imported from previous forum - Category:Accutech<<
User: mchartrand, originally posted: 2018-10-17 19:19:19 Id:93
This is a re-posting from the obsoleted (October 2018) "Schneider Electric Telemetry & SCADA" forum.
What are the transmit distances for the Omni antennas for the BR 20 radios? We are dropping out comms to the AM 20's on various wells and just wondering if it could be interference between different sites._**
It can be very challenging to come up with exact maximum distances for the BR20 radios. This is because of the variability between systems. Eg one system might have a clear line of sight to all of the wireless sensors, while another might have a lot of buildings and/or vehicles/people in the paths, and possibly even heavy tree cover on longer paths. All of these can greatly affect a path.
Having said that, it is possible to state typical distances for Accutech systems. I will have to make certain assumptions...
We don't know the over-the-air baud rate used (19.2 or 76.8 kbps) so will assume 19.2 kbps. The receiver sensitivity drops 3 dB at the faster speed, so 19.2 kbps is recommended on longer paths. I will presume a 6 dBd omni antenna, though a 3 dB version is also available. And because an external omni is used in this application I will also assume the 7 dBd yagi antennas are used at the wireless sensors.
(A 3 dB reduction in transmit power or receiver sensitivity drops the maximum distance to 70% of its original value. A 6 dB difference drops it to 50%)
We don't know the height of the antennas at each end of this path. Antenna height is important as it can get the radio signal above obstacles. I will presume 15 feet at the BR20 end and 8 feet at the wireless sensor end. (high enough so nobody will hit their head on the antenna) Lower antennas would mean a shorter maximum distance, especially if there are obstacles in the path.
On a long path the earth itself can become an obstacle. Whether a small hill or a steep slope, it can have an effect. If the earth isn't relatively flat on the path in question, an on-site test may need to be done. Alternately a software path study can be done to simulate the terrain and other obstacles.
One other issue that can have a strong effect on a system is called Fade Margin. While the radios have an approximate -110 dBm receiver sensitivity at 19.2 kbps, it is best to design the system so that the actual receiver signal levels are significantly higher, to account for intermittent losses due to bad weather, people or vehicles in the path, tree growth or maintenance problems. Typically a 20 dB fade margin is assumed. So we really want to see a -90 dBm or better Rx signal level.
With all of the above assumptions, and presuming there are no significant obstacles, an Accutech sensor should be able to communicate reliably over a distance of about 1.2 miles. (6400 feet)
If just the internal antenna is used at the sensors, along with a 6 dBd omni at the BR20, there would be about a a 9 dB reduction in the path. As a result the maximum distance the signal could travel would be only about 1/3 as far as with the 7 dBd yagi antenna. (about 0.4 mile or about 2100 feet)
If the BR10 were used instead, with its internal antenna, connecting to wireless sensors also using just their internal antennas, the likely maximum distance would be about 1000 feet. (presuming no substantial obstacles)
All of the above stated distances will be reduced if there are significant obstacles which are taller than the antennas. In such cases a field study will need to be done to measure actual receive signal levels.
It's also possible that there is an interference source somewhere in the vicinity. The Accutech radios use the 900 MHz unlicensed band. If any other transmitters are nearby in the same band this could have an effect. Most important in such cases is to ensure that the antennas for the systems are separated as much as possible.
Where separation of antennas is necessary, it has been found that vertical separation is about 10x more effective than horizontal separation. Eg if 10 feet of vertical separation were required to avoid interference, about 100 feet of horizontal separation would be required to do the same thing.
TSN03896 is available which talks about the required antenna separation between the Accutech base radio antenna used by a BR20 and the Trio K Series antenna which may also be used.
If interference is suspected, a Trio K Series radio may be used to look for it. Do a Read from the radio, then go to the Modem menu, then select Spectrum Analyzer. Perform a Continuous sweep. This will show the signal strength of any signals heard on the channels used by the radio. Try this with the Accutech BR20 both on and off.
Look for interference near the bottom of the 900 MHz band which might be caused by a nearby cellular tower. Or near the top of the band, which might be caused by a paging transmitter. If either of these exist, a bandpass filter may be added.
Here is TSN03896 as previously quoted by Joel.
Attached file: (editor/cb/gj5m1jmwkoc4.pdf), TSN03896.pdf File size: 354504