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Original question:

868MHz PCB/planar antenna design - resonance and return loss quite off

868MHz transmission under water

Markovicz
Prodigy 220 points

Dear TI-Team,

For a project with a university, I would like to design an antenna that transmits at 868MHz under water. Froma related work:

Taylor, H. D., Anton Kruger, and J. J. Niemeier. "Embedded electronics for a mussel-based biological sensor." 2013 IEEE Sensors Applications Symposium Proceedings. IEEE, 2013.

I found following details:

- The wavelength of radio waves is around 1/9 in water compared to the one in air

The group has just used a wire with 2.6cm for their 315MHz antenna, see below:

For 868MHz, the wavelenght is around 34.5, a standard 1/4 wavelenght antenna would be 8.62cm long. Since the wavelength in water is reduced by 1/9, I could simply go for 34.5cm/9=3.8cm

For a simple wire antenna, what should be considered?

- How large should the ground plane be?

- Is a matching circuit recommended?

- Is a 50Ohm microstrip line recommended?

thank you a lot for your help!

  • 0
    TI__Expert 4560 points

    Hi

    Water has a high attenuation of RF signals in the MHz range, so evaluate first if RF is a viable option for transmitting your data under water. As far as I know, there are not many commercial uses for RF signals under water. 

    Regards
    Vegard

  • 0 in reply to Vegard H
    Prodigy 220 points

    Hi Vegard,

    Thanks a lot :). That's for sure a good question but I just need to bridge 1-3m in water and it's a research project. RF is preffered compared to ulrasound or light since there are more convenient modules etc.

    Best,

    Marko

  • 0 in reply to Markovicz
    TI__Expert 4560 points

    Hi Marko.

    Ideally you should have a matching circuit, but i think it will be very difficult to connect a network analyzer to properly tune it with the correct conditions under water with epoxy casing.  The ground plane should be at least a quarter wave in each direction from the antenna. Try to keep everything as close to 50 ohm as possible. 

    Vegard