RF communication inside a chamber (silo)

[jportero]

Hello,

I have some questions about RF communication between two antennas inside a concrete or metallic storage silo. I would like to know the influence of walls and edges of the closed chamber (a cylindrical silo) in the RF signal.
Chamber's walls are connected to ground, and I have some antennas around two fixed vertical axis inside the chamber that transmit and receive at 868 MHz. Its position does not changes.



I know the power loss [attenuation in dB/m] in air and grain environments, and with that data theorically I can know the level of fill of the silo. But the rebounding signal can

What factors I have to consider that could affect communication? Like rebounding, walls absorption, etc..

Thank you,

Javier

 

[goldsmith]

I have some questions about RF communication between two antennas inside a concrete or metallic storage silo. I would like to know the influence of walls and edges of the closed chamber (a cylindrical silo) in the RF signal.
Chamber's walls are connected to ground, and I have some antennas around two fixed vertical axis inside the chamber that transmit and receive at 868 MHz. Its position does not changes.

Hi Javier
for such a high frequency ( 868 MHZ ) radiation will be like a line ! so , events depends on what material your chamber made from ? what about it's skeleton ?
Best Wishes
Goldsmith

 

[jiripolivka]

As the silo walls are metallic, locating ~800 MHz antennas inside means you will have a resonator. The wave mode depends on : resonator dimensions in wavelengths, locating the antennas can determine optimum points to operate the resonator in a "whispering-gallery mode", but the material in the silo and its electrical properties will affect the system quite significantly.

Take a textbook on microwave resonators to learn the details then try to figure out if it makes sense to go on.
Do you want only to communicate between selected points, or, do you want to use the UHF signal to indicate material height, or its properties?

You can theorize to start but experiments are needed to find out the results. A pretty interesting application!

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To find grain level in the silo, I would prefer to use weight sensors (tensometers) on silo external supports. You can use an ultrasonic sensor or a lossy cable hanging from the top, allowing to measure cable capacitance indicating the height.
Using your ~800 MHz probing is possible but a quite deep study is needed.

 

[jportero]

First, thank you for your reply, goldsmith and jiripolivka.
My environment is a cylindrical chamber with the aproximate next dimensions:

height: from 20 to 25 meters
diameter: from 7 to 10 meters
walls width: 10 cm
The chamber is totally closed, like a storage silo. And its external metallic skeleton is connected to ground.
If the communication is at 868Mhz, the wavelength of the signal is 34,5 cm. Is that important in order to fix the antennas at different distances? Will affect to signal the rebounding effect from the walls?
I'm going to make experimental tests at smaller scale. Could I approximate them to this real case?

Thank you,

Javier

 

[zorro]

HI Javier,

I guess that the attenuation is strongly affected by the moisture in the grain. Have you considered this factor?
Regards

Z

 

[vfone]

Yes, the moisture and humidity will be never the same (and hard to correlate with attenuation) to be able to do an accurate measurement.

 

[jportero]

Yes, I take in account grain moisture to aproximate the measurement. My question is if I connect metallic walls of silo to ground, signal rebounding effects will dissapear. I only want to know if the transmition of data is possible without rebounding effects in chamber walls.

Thanks!

Javier

 

[biff44]

There will be multipath, but the metal lining the silo is lossy, so I would not worry too much about "resonances". If you do not try a very high data rate, you should do fine.

There may be certain locations where there is a reception null, and might have to move the antenna over a few feet.

 

[volker_muehlhaus]

My question is if I connect metallic walls of silo to ground, signal rebounding effects will dissapear.

No, you can't avoid these multiple reflections.