At least once a year, you should do a complete cleanup to make sure you have not introduced new sources of RFI in the radio shack. Here’s how to sweep the shack noise floor.
For strange historical reasons, most of us refer to our technical listening posts as the radio shack. This terminology probably dates back a hundred years, when all the radio stuff was relegated to a small out building. Today, you will find the radio shack is usually a corner of a room, or even a complete dedicated room, often in the basement.
Your radio shack is probably like mine. Full of radios, plus all kinds of cables, computers and monitors. Often this equipment gives off radio frequency interference, RFI. At least once a year, you should do a complete cleanup to make sure you have not introduced new sources of RFI in the radio shack.
Many hams and SWL have published material on using ferrite chokes to reduce RFI that can flow along the outside of cables and interfere with your reception. Others have also stressed the importance of a good station ground for reducing noise. In this article, we will focus on a diagnostic approach for reducing RFI and the shack noise floor.
Here’s how you can do a diagnostic of local RFI in your shack. First, run your SDR without an antenna. You should see a jiggly line across the bottom of the spectrum display (white line above). Typically, you should see a level around -130 dBm, representing the internal noise floor inside your receiver. If you do see any signals displayed without an antenna, this means you have RFI entering your receiver through control or network cables, or the power supply. Apply filters!
With the antenna connected, you should see a higher level such as -120 dBm, which is decent for a quiet location like mine.
Shack Noise Floor – Cables and Grounds
In my shack, I have a bunch of antenna switches and cables. These are used to switch several antennas between several receivers. You should be aware that not all coaxial cables perform equally well. Also, not all jacks on your coax switch have the same level of isolation.
The test is to connect the antenna to the radio, but switch if off. Ideally, with the antenna switches off, you should get a spectrum display similar to “no antenna”. As you can see in the pink line above, I discovered that a lot of RFI was getting into the receiver even when the antenna was “switched off”. Even the router birdie signal level was almost as strong as when the antenna was switched on. Obviously, things had deteriorated over the year since my last spring cleaning.
After swapping cables for better performers, double checking ground connections and testing the isolation of different jacks on my relay switchers, I was able to reduce RFI bleed through with the antenna switched off. You can see the effect on the green line in the above picture.
Using this approach, you should be able to reduce the shack noise floor down to the level of “no antenna connected”, or at least pretty close.