If you would also like to try a low pass filter between the output of your transmitter and the buried wire, here is a simple 100 kHz low pass filter that should do the job:
It only takes 0.022 uF capacitors (qty 5) and 1.2 mH inductors (qty 2) so it should be under $10 if you are buying everything new. The resistors shown are not part of the filter - they represent the source and load impedance. The 0.066 uF capacitor is three 0.022 uF capacitors in parallel. The 1.2 mH chokes can be something like this:http://www.digikey.com/product-detail/en/abracon-llc/AIAP-03-122K/AIAP-03-122K-ND/3059934
You can build it on perfboard or dead bug style on a piece of PCB. Mount it as close as you can to the transmitter and inside the Faraday cage if you go that route.
- Glenn W9IQ