Radio frequency below ground?

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Hi,

I'm about to design a tracking device to be used by equipment located below ground. There will be a transmitter below ground and a handheld receiver. Where to find information about radio frequency attenuation in different media? Can I use 868Mhz or will more LF like 32768Hz work better?
 

868 mhz may penetrate a few feet into dry earth, much less in wet earth. LF will go much deeper--the trick is to make the antenna small enough to fit but still have some efficiency. Data communications typically need a lot of coding gain to get through.
 
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Ok, maybe I don't need to investigate 868MHz more deeply. If 32768Hz, what is the most efficient antenna principal to use. Antenna needs to fit within 6mm diameter and maximum 60mm long ?
 

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Definitely 32kHz will be better than 868MHz. For a similar application I know somebody that is using successfully a DCF77 receiver modified to work on 32kHz, and for TX a modified replica of a DCF77 transmitter. The antenna could be a challenge, but any long wire could be the starting point.
 

At 32 KHz, you would want an inductive coil, not a monopole. You can either resonate it with a capacitor, or drivie it unresonated with a current source.
 

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