Reducing HF Electrical Noise by Using a Faraday Cage for Switch-Mode Power Supplies
Over on his blog, DXer OH2-2192 was frustrated by lots of local electrical noise showing up on the HF bands on his Airspy + Spyverter SDR receiver. He discovered that the majority of the noise he was seeing was coming from the switch mode power supplies that power the electronic devices used in his setup. Switch mode power supplies are very common in the modern world, with almost every electronic device using one to efficiently convert wall AC into DC power. However, they convert power by rapidly switching on and off, and these on/off square wave pulses cause a lot of RF noise especially on the HF spectrum.
Instead of changing to noise free linear regulators which are expensive, very heavy and big, OH2-2192 decided that he'd try creating a Faraday cage shield out of metal mesh to enclose all his switch-mode power supplies. Using a simple AM loop antenna and Airspy's Spectrum Spy software he measured the amount of noise produced by a switch-mode supply placed inside and outside of the cage. The results he saw were very promising with the shielded supply eliminating the noise almost entirely.
The main point using such mesh was to have a safe, silent non-fan way to get rid of heat generated by the power units. Based on my measurements the temperature of the power units in full load is only 1-2 degrees higher than without the cage.
This experiment has grown to be my default for all power units and devices now and I do have tested it also with my AirSpy and RTL-SDR radios too with quite interesting results.
Wrapping RaspberryPi’s inside such mesh-cage was also working solution.
I do not know if all this was actually necessary as there are much bigger sources of HF-noise in my HAM-shack, but then again, this was fun project to do.
Br: Mac – OH2-2192
What use would the receiver be if you blocked all of its incoming signals?
It is much easier and more effective to install cheap ceramic capacitors at the radiation points on the supply board.
This may sometimes help, but is not a silver bullet.
The EMI can be (and frequently is) radiated by the wires connecting the power supply to the device. For that kind of EMI, a better remedy is a ferrite clamp on the offending wire.
Ferrite clamps work at UHF and to some extent at VHF but for HF you need many turns wound on large ferrite torroid(s).
Actually it might be enough to have four turn on torroid for significant noise canceling effect.
Using ferrite clamps and toroids are the basic thing to do, sometimes it is just not practical nor cheap as good quality toroids are quite expensive. With this kind of metallic mesh you can get outstanding results with just a few euros. The material I used cost 15 euros, but I used only like 1/10 of it so roughly this cost 1,5 euros to make – and some beer 🙂
Nothing is cheaper than .001uF ceramic caps applied where necessary, but it is up to you to do so. A dime a piece is hard to beat.
Very silly question; can you put the SDR receiver in the cage to shield out all the noise from your household appliances, LEDs, etc?
Sure, but it would benefit only if the case of your SDR receiver is made out of plastic. The metalllic casing usually used in good quality SDR devices is good enough to filter out such HF-noise caused by household appliances – and remember that the main “input” to your SDR receiver is the antenna itself, not the IC board or other electronics that the receiver is made from.
Wondering if I could achieve similar results by wrapping foil tape around all my power supplies, or by using an anti-static/shielded bag?
Good solution but wouldn’t be enough to make just the cover with the mesh and the rest with aluminum sheet?
any (well) conducting material will do. the easiest solution is propably to get a metal box at a hardware store.
A metal box would do fine, just make sure there’s some airflow so the power supply doesn’t overheat.
How would you keep the RF in or out of the box if it has ventilation holes?
If holes are small compared to the wavelength of the the signal you’re wanting to shield it still works.