iPhone app to calculate Tee Pi Balanced and Pad Attenuation Networks
One downside to an SDR is that you more easily notice the mysterious carriers and other local noise/RFI signals. After reading this article on Common Mode Chokes, I decided to see what I could do to improve my situation.
As a first step, I captured this baseline of the 6500-7000 kHz range, where I am most interested in listening (click to enlarge):
I then added a choke on the antenna input to the SDR, right where it enters the radio. It is 9 turns of the coax on a large toroid core, probably type 37 or 43 material, possibly a Fair-rite 5943003801:
The other coax cable next to the antenna input is the reference signal from the 10 MHz GPS reference. Adding ferrite to it had no effect. I’ll get to the orange toroid next. Here is the result (click to enlarge):
As you can see, there was a significant reduction in the number of carriers and other noise signals.
Next I added the orange toroid also pictured (unfortunately I have no idea what type of ferrite it was, it was from the junkbox), as well as two of the clamp on ferrites you often see on AC power or video cables to the ethernet cable that runs from the SDR to the computer, here are the results: (click to enlarge):
This got rid of a few more. Pretty much, what is left is an actual signal. I was able to identify these:
6604 New York Radio
6519 A voice transmission, perhaps another VOLMET
6660 The second harmonic of CHU 3330
6725 An RTTY transmission
6885 Israel fading in
6970 faded in and out, so it seemed to be a legit DX station
All in all, a significant improvement, for a few minutes worth of work!