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. 

630 Meter Interference

I had a chance tonight to examine the interference that is blocking reception of the 630 meter band in the evening.  Last night the interference stopped abruptly around 11:15pm local time.  As I suspected the noise returned tonight.  I was out when it started but it was in full force when I returned at 7:30pm.  This gave me a chance to examine the noise in more detail.  First was to look at the audio coming from the receiver.  What this shows is a series of pulses space approximately 8.2 ms apart.  The inverse of this is the frequency which is around 120 hz.  That is why I hear the low frequency buzz in this interference.  I have observed a similar pattern with arching of high voltage distribution hardware.  In that case the noise was very broad band.  Covering from HF into UHF.  In this case the noise is limited to a much narrower bandwidth.  Considering the bandwidth and the way it shuts off I am sure this is a device and not power line hardware related. 

 

Looking at the FFT signature of this same audio sample I see a square pulse that starts around 115 hz and ends around  3.488 KHz.  I think that is why, on Spectrum Lab, I see flat audio response out of the receiver for its full 3100 Hz bandwidth.  The FFT image is shown below:

Looking at a spectrum analyzer from 410 KHz to 500 KHz I see the following.  This essentially wipes out the 630 meter band. 

It will be interesting to find out what this is.  I suspect this is a television or something in the room with a television that household turns on in the evening and turns it off when they go to bed.  So far I have not heard this noise during the day (Monday - Friday). 

To track this down I plan on taking a ferrite rod and build a loop stick antenna. Those have a sharp null off of the end of the rod.  Walking around the neighborhood I hope to be able to use that null to at least locate the house where it is coming from.   Of course I need to make sure it is not something within my own home.  That would be an easy fix if that was the case.  Since this shut down last night around 11:15pm I can not think of anything in the house that would have done that last night.