Common Mode Choke for VLF

Adding a common mode choke to the Ground Probe Antenna. I don't think it will help with the noise on 474 KHz as I don't think it is coming out of my house.  Won't hurt to add it. The purpose is to keep noise signals that propagate along the transmission line from getting to the antenna. I wound this using the same core I used for the matching transformer on the Ground Probe Antenna.  It was wound with insulated #18 wire I happened to have.  Around 23 turns, two conductors twisted together. 

I use a drill to twist the wires before winding the core.  Clamp one end of two wires into a vise.  Clamp the other end into the drill chuck and let her wind away. 

Differential mode loss is less than 1 DB.  Common Mode loss was taken with my MDO3000 scope using the function generator to generate flat broadband noise from a few Hertz to past 2 MHz.  This image stops at 2 MHz.

Common Mode Response

The spikes are local AM broadcast transmitters.  The input noise level to the choke was -59 DBm. A table of the data is listed below

Frequency KHz          Loss (DB)

25                                  -13
50                                  -20
100                                -27
250                                -33
350                                -37
450                                -39
500                                -40
700                                -42
1000                              -42
1200                              -41
1500                              -39
2000                              -37

 This will be installed in a junction box that is near the feed point of the antenna. It is about 5 feet away from the connection point. If there are any noise signals on the coax line this should reduce them.