Looking for a radio friendly ATX PC case - Page 1
Forum section: electronics / rf-microwave

author: deephaven
author_rank: Frequent Contributor
timestamp: November 13, 2021, 09:15:44 pm
content: I recently constructed an ATX PC and am using (ironically) a case by Be Quiet. Yes, it is very quiet acoustic wise but not so RF wise. I get a burst of carriers from it on my SDR from 430 to 436 MHz, evenly spaced at around 31.25 KHz. They are very stable, not drifiting and also are a pure note when listening on SSB.Can someone recommend an ATX tower case which has decent EMC shielding?Thanks,Nigel.

author: wraper
author_rank: Unknown
timestamp: November 13, 2021, 09:22:08 pm
content: The question is if the case is really at fault. Does it have any transparent windows? Did you try placing ferrite rings on the cables, disconnecting them? IMHO you should try changing a video cable to the monitor or try changing resolution/refresh rate and see if it causes any changes.

author: deephaven
author_rank: Frequent Contributor
timestamp: November 14, 2021, 12:24:09 pm
content: Quote from: wraper on November 13, 2021, 09:22:08 pmThere are no transparent windows and I've tried disconnecting everything except the mains lead to no effect. If I use my SDR as an RF sniffer, it doesn't see anything near the mains lead, it seems to peak at the front of the PC.

author: deephaven
author_rank: Frequent Contributor
timestamp: November 14, 2021, 12:25:48 pm
content: Quote from: evb149 on November 13, 2021, 09:55:38 pmThanks for all your suggestions. I really want a case which will house my existing ATX board. I will look into what you mentioned.

author: wraper
author_rank: Unknown
timestamp: November 14, 2021, 12:29:55 pm
content: Quote from: deephaven on November 14, 2021, 12:24:09 pmUnder the front plastic cover should be USB, audio jack, power/reset and probably fan cables coming out of the metal cage. They may be what conducts the noise from the inside and acts as antenna. Try disconnecting them from the motherboard and see if it helps. If you use the same hardware inside a different case, you probably will have the same issue since on the front there will be similar cables coming out from the metal cage.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 14, 2021, 12:59:35 pm
content: +1I'd bet some externally connected cable is spreading noise, sourced from the internal wiring in the environment. If you have a full metal case, it's not very likely that direct radiation from the interior is the problem. Possible suspects will also be PSU and display an their wiring.

author: deephaven
author_rank: Frequent Contributor
timestamp: November 14, 2021, 03:51:51 pm
content: Quote from: HB9EVI on November 14, 2021, 12:59:35 pmI've unplugged all cable except the mains and also tried unplugging the front panel cable but this had no effect on the interference.Although the case is full metal, there has been no provision for earthing the sides to the main chassis, so there are gaps everywhere.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 14, 2021, 04:04:16 pm
content: the case is already on earth level via the power supply; but earth can also introduce noise in a system; and noise elimination over earthing/grounding is no certaintywhat really surprises me is the fact that you experience noise on the 70cm band; normally PC/domestic electronics are prone to cause interference on shortwave but not on UHF

author: wraper
author_rank: Unknown
timestamp: November 14, 2021, 04:18:44 pm
content: Quote from: deephaven on November 14, 2021, 03:51:51 pmYou could attach conductive foam gaskets to fix that.https://www.aliexpress.com/item/4000933306259.html

author: MartinL
author_rank: Regular Contributor
timestamp: November 16, 2021, 12:33:23 am
content: Quote from: deephaven on November 14, 2021, 03:51:51 pmThen I'd guess the mains cable is what it's mostly radiating from. Here's a design for a mains choke you could put close to the PSU input:http://www.m0pzt.com/blog/mains-filter/

author: deephaven
author_rank: Frequent Contributor
timestamp: November 16, 2021, 10:31:21 am
content: Quote from: MartinL on November 16, 2021, 12:33:23 amThanks, I'll look at that. I did rig up an SDR to my laptop with a short antenna to sniff around and it didn't seem to pick anything up from the mains cable. The interference was mainly coming from the front.

author: A.Z.
author_rank: Super Contributor
timestamp: November 16, 2021, 01:13:59 pm
content: Given that you performed quite a number of checks, here are my 2 centsFirst of all if you are feeding your antenna(s) with coax, ensure to add some good chokes along the coax to take care of CMC noise, a good design is winding about 17/18 turns of RG174 over an FT240-43 toroid core, the reason is explained herehttp://www.karinya.net/g3txq/chokes/ensure to add at least two chokes to your coax, one near the antenna feedpoint and a second one where the coax enters your building (add more chokes if you can, they won't hurt)If, after choking the power lines and the feedline you still face the issue, do NOT remove the chokes, since they'll still be useful, instead, consider the idea of using a separate RX only antenna; a good one, which in my experience will help A LOT is the one described herehttp://www.kk5jy.net/LoG/I know, it may seem crazy, but believe me, it works; just ensure to build it "by the book" (feeding transformer, wire len and so on) just, to stay on the safe side, add a couple of chokes to its feeder, that antenna WILL make a difference

author: MartinL
author_rank: Regular Contributor
timestamp: November 18, 2021, 02:40:58 pm
content: Quote from: deephaven on November 16, 2021, 10:31:21 amIf you want to narrow down where it's coming from more specifically, you could build yourself a near field probe. Dave did a video on how to make one a while back:Another thing that's easy to make is an RF current probe. Just take a clip-on ferrite and wind a few turns around it, then connect the ends to centre and shield of a bit of coax. Clip on to any suspect cable and use your SDR to look at the common mode RF currents on it.The latter is usually a lower frequency tool (HF or so) but it's quite likely that the interference you're seeing around the 70cm band is harmonics of something lower down anyway.

author: deephaven
author_rank: Frequent Contributor
timestamp: November 18, 2021, 04:28:14 pm
content: Quote from: A.Z. on November 16, 2021, 01:13:59 pmThanks for taking the trouble with that extensive reply. The stuff you mentioned I think is more applicable to the HF bands. My main interference problem is on the 70cm band. I have a 21 element yagi on a mast and the interference gets a lot worse when I beam it towards my upstairs shack.

author: deephaven
author_rank: Frequent Contributor
timestamp: November 18, 2021, 04:28:54 pm
content: Quote from: MartinL on November 18, 2021, 02:40:58 pm