Wireless bridge between two buildings in a forest

[Joze]

We have a specific problem where we need to establish a wireless bridge between two buildings that are located in a forest and don’t have a clear line of site, the forest and also the terrain obstruct the view. Distance between the two buildings is around 800m, we would need to establish a wireless RS485 Modbus RTU connection between the two systems.

We already got the opportunity to test Eaton ELPRO 915U-2 devices that work at 800-900 MHz, I think we had two dipole antennas, the two devices managed to establish a connection but they lost it soon and so it wasn’t reliable at all.

So we see that you sell some different devices for that kind of communication e.g. Radiant 868, DL Series Radiomodem, Radiomodem VHF 169MHz, XStream™ 2.4GHz radiomodem,… What kind of package would you recommend ? Is there any way that we could use some test equipment ? We are thinking that using something at a lower frequency would be better, something like Radiomodem VHF 169MHz ?

[Sergio Bertana]

Considering the obstruction made by the trees and specially the obstruction of the line of sight made by the terrain to have the maximum signal propagation we suggest to use the lowest frequency available.In our range of products, the two models that we can propose are which that operate on 169 MHz VHF frequency. You can refer to the D51x series at 169MHz that has an output power up to 500mW.

[Joze]

We decided that we will try your hardware and will make an order in the coming days, We just need to clarify a few things before. So here are the questions:

Do you recommend 1/4 Lambda, Yagi 3-elements or dipole antenna for communication through a 800m long forest (the one mentioned before) ?

We are using Eaton XV102 with Codesys as our master device and want to communicate with a remote PLC over a wireless connection. We are currently also using Eaton Easy821 PLC’s but they are limited to CAN communication and therefore we can’t use them with DL169 as it is limited to Modbus RS485 (communication XV102-DL169-Easy821 is not possible). So we need a replacement PLC and are looking at your SlimLine PLC’s with RS485. Now what we are wondering is, can a SlimLine PLC programmed in LogicLab be used as a Modbus RTU slave device ? So we can use the Eaton XV102 PLC as a Modbus master and the SlimLine PLC as a Modbus slave and so we can make a wireless Mobus communication between the XV and the SlimLine PLC over the DL169 (XV102DL169SlimLinePLC). Is that possible ?

ERE Wireless is offering IP66/68 certified Radiomodes, what are the prices compared to the regular DL169 ?

[Sergio Bertana]

Considering the obstruction I suggest to take all the possible improvements to make the connection between radiomodems more stable as possible. So the Yagi 3-elements antenna is suggested. Please also to consider to use the VHF169 instead of the DL due its more output power. All our SlimLine family products can manage the Modbus communication (They can act as a master or slave). So there is no problems to establish a communication between the Eaton and our SlimLine. Moreover also the communication between LogicLab programming environment and the SlimLine is using the Modbus protocol, so it’s also possible to program and debug the program over the radiomodem connection.The ERE Water Proof IP66/68 radiomodems (Datasheet) have the same DL series characteristics. The only advantage is that they are in mounted in a waterproof case. If you need that kind of case, you can use them. Please remind to the output power difference against the VHF169 serie.

[Joze]

Regarding the 155-175MHz Yagi 3-el. antenna pole mount, we are in a situation where we would need 40m long cable to the antenna, would this work or is 10m the maximum allowed length ? If it is ok, do you happen to stock antenna cable that long ?

[Sergio Bertana]

Usually it’s recommended to keep the antenna cable as short as possible but due the low frequencies 155-175Mhz, the cable loss is not so high so it’s reasonable to have a cable 40 m long.

If you contact our sales department they can send to you an offer for an antenna with a 40 m cable.

[Joze]

Thank you, we already received the cable.Additional question regarding over voltage protection that you recommend with the antenna, would the combination of 40m cable and the surge protection lead to excessive loss or it shouldn’t be a problem ?

[Sergio Bertana]

The cable we supplied to you is a CO100, this cable has a low signal loss (See the datasheet). Considering that you use a low frequency 169 Mhz I suppose that the signal loss in the over voltage protection is trascurable.

[Joze]

Thank you. One more question, this should also work, right ?

It would be placed before the radiomodem. At 169MHz loss is still quite low as shown in the graph in the above document.

[Sergio Bertana]

I suppose yes, but it has different type of connectors, and if you use any kind of adapters you must consider that you lose signal on any connection.

Unfortunately there is no specification for the voltage protector we provide, but I suppose that all the protectors should have similar signal loss characteristics.

[Joze]

I think the connectors are OK. The protectos is suppose to have “BNC”, which is the same that radiomodem uses.

The “device” side is where the radiomode is connected – BNC male on protector and BNC female on radiomodem. While on the “line” side its the opposite – BNC female on the protector and BNC male on the antenna cable.

So it seems to be ok ?

[Sergio Bertana]

It seems ok…

[Autore]

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