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Here's a shout-out to Michael Black W9MDB for his RFI post here: /g/QRPLabs/message/97552 - his post help me solve my RFI problem.

This past weekend I went up to the Rocky Mountains to a place at 10,000 ft (3,048 meters) with my #QMX, an old MFJ 936B magnetic loop antenna, a 3S Lipo battery pack (@12.2V), and a MS Surface computer, all on battery power to see what my QMX could do from there. Tuned to the 20M WJST-X frequency, signals were booming in after a quick tune-up on the mag loop. My intention was to use CW from that location, but alas, I had forgotten to pack my paddle (I need to get a good portable one).

However, when I booted up the PC, I started hearing some dreaded, but light, computer noise coming in on the receiver. I'm far away from urban QRM, but now I'm bringing my own to an otherwise quiet place. Frustrating. But things got incredibly worse when I started the WJST-X program!! The computer generated interference was horrendous, blocking all but the strongest FT8 signals coming in. This doesn't happen at all with the same setup at home minus the magnetic loop antenna since there I use my vertical antenna out in the back yard while I'm in my basement office space. So why now? In the field I have no ground connection, and that mag-loop is about 2.5 meters away from the rest of my gear. The noise volume changed as I touched the PC or touched the metal enclosure of the mag loop tuner, but didn't go away.

After trying several different USB cables I happened to have in my PC bag, and finding none solved the problem, based on Michael Black's general RFI post, I ordered this cable from Amazon the following day. . It's a Digirig Shielded Short USB-A to USB-C Cable with Ferrites and although short (38 cm) and pricey ($13), it has totally eliminated the noise I hear from the PC when I'm set up as I was in the field I'm happy to report.

I've not tested this yet in the field, but just did an experiment using the same equipment, no ground connection, with everything close together as I would be in the field. My original USB cable, very bad noise; this new cable; noise gone. I'm not sure when I'll get to try this out in the field since snow will be upon the Rockies again later this week. I enjoy warmth much better than the cold for some reason.

I wanted to share this solution for others that may (and will??) encounter this issue. Not all USB-A to USB-C cables are the same. A good one is totally worth the price for this application.

If you've found a resolution to this same problem, please go ahead and reply so we can share the knowledge with the group.

73!

Roy - KI0ER Littleton, Colorado, USA

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73, Oleh, UY2ZA

The cable was sold as shielded. In fact, the screen was not connected to anything. After connecting the screen to the connector body, the cable became protected.
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73, Oleh, UY2ZA

I don't usually do digital modes on HF, but I've seen improper shield handling in USB cables. The shield of a shielded twisted pair cables (e.g. USB, Ethernet) must ground the shield at one side only (usually on the host side) but some cables/devices ground both sides, which partially defeat the purpose of shielding. I think it is generally a good idea to check whether the peripheral device leaves the shield pin of the USB connector unconnected, especially with noise-sensitive equipment like radio and audio.

A large clamp-on #31 ferrite core like Fair-Rite 2631181381 is handy for troubleshooting like this (though not lightweight for hiking operation). It's big enough that it can make a few turns of a USB cable as well as feedline coax on the core to see where the conduction path may be. #31 core inserts a greater choke impedance than #43 or #61 but like with any ferrite core it often requires multiple turns to see clear effects.

Another good tool to have is a USB isolator. There are a few chips that are used in many USB isolators, that electrically isolate the port for safety, and those are very lightweight.

On Tue, Oct 24, 2023 at 02:48 PM, xyroto wrote:

It opens other questions. Whether USB sockets these cables plug into are connected? If they are shielded, why did the connection cause less noise without it? Because it seems like there could be intentional shielding breaks between computers and devices?

A shield functions as a (Faraday) shield if the entire shield maintains the same potential (against a distant reference potential). That assumption is immediately violated if both ends of the shield is connected to something that can pour some current. So, the correct way to use a shield as a shield against noise emission/reception/conduction is to connect only one side. (Coax shield is a different story, it's a transmission line. Consumer audio equipment's line cables are an example of how to do a bad job, though adequate for that purpose.)

The actual signals in USB are transmitted through twisted pairs, so the signal can be detected without using the shield as the reference potential.

Another issue is, like anything RF, common mode current. The computer and the TRX both have some electrostatic coupling with the ground, and the noise can conduct through the entire bundle of the cable against the ground. If it's transmitted RF power causing RFI, the best way is usually to create an additional short direct connections between the transmitter ground and the grounds of all connected devices so that RF current get bypassed and not flow with the data cables. But noise like this might be better dealt with by putting a high impedance common mode choke on the connecting wire.

I will describe it in more detail. The cable shown in the photo is only 30 cm long. There were problems with it even though it is shielded. Also I have the cable is 120 cm long. Also in the screen. And it worked without problems. Of course, I was interested in why such a long cable was more protected than a short one. It turned out that the screen in it was grounded to the connector body at both (!) ends. I made the same connections as in the photo from both ends of the cable and forgot about the problem.
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73, Oleh, UY2ZA

So other's don't waste time trying this device to isolate the usb connection between the QMX and PC to prevent computer generated RFI ...

Don't bother trying this: -- DSD TECH SH-G01B USB Isolator with High-Speed ADI ADUM3165 Chip - 480Mbps

All I got was the dreaded "USB device not recognized" warning from MS Windows based computers (I tried it on 2 PCs) after plugging the QMX into it and then booting up the QMX (an already running QMX being plugged into a Windows PC seems to always generate the "USB device not recognized" warning; the QMX needs to be booted up AFTER it's been plugged into the PC for it to be recognized by Windows ... at least from my experience.)

Following one of Ryuji's recommendations for using a small usb isolator to solve USB related rfi issues between one's PC and QMX, I thought I'd give the above a try, which did not work at all for me.

To be clear, Ryuji did not recommend this device, he just suggested that something like this may solve the issue, and well, it should. Note that there are far less expensive but slower USB 2 type isolator devices available, but I've not tried them (yet). With this particular device I tried around 6 or so different USB A to USB C cables I had lying about, one of which was a USB 3 rated cable (with blue tab inside the USB A connector). Most of the cables, but not all, would allow a properly recognized USB connection between the QMX and PC without the above adapter, but none of them worked with the adapter in place, well, except for one cable that did work.

The only USB A to USB C cable that worked with the above isolator was the short Digirig USB cable, Digirig Shielded Short USB-A to USB-C Cable with Ferrites, for some reason (?). But when using that Digirig cable there's no RFI problem in the first place, so there's no need to use the isolator.

Has anyone used any of the slower-speed and cheaper USB isolators with their QMX that actually worked?

73!

Roy - KI0ER Colorado, USA

I see in Rob VK5ZIK's photo here: /g/QRPLabs/message/113034 that he's got an add-on ferrite on his USB cable. I wonder which ferrite he's using? I'm assuming that has solved the RFI issue for him.

Thanks for sharing that photo Rob!

73!

Roy - KI0ER Colorado, USA