Snap-on ferrite chokes helpful?

[SpaceForceCmdr]

Just wondering if these would be of any help for my SDR? The USB cable that powers it already has two, but the USB that powers my bias-t (from an MLA antenna) does not. Any other cables where a choke might be helpful, like on the feed line from my antenna, or the short line that feeds from the bias-t to the SDR itself?

 

[vagrant]

I use an OCFD (off center fed dipole) antenna and after putting a MyAntennas CMC (common mode choke) at the feed point where the coax connects to the antenna, it reduced my noise considerably around two S units. Some people also use a choke indoors before the coax connects to the radio. The indoor choke did not make a difference for me, but everyone's setup is different.

If you're going to go with snap-on ferrite beads, at least use five, if not six of the right mix. MyAntennas even sell a kit if your coax will wind through the bead enough times you could use just one large one.

RFI-Kit-2 – MyAntennas.com

 

[bharvey2]

I have a big box of snap on ferrites. Most are about 1/2" I.D. I've found that they work quite well in reducing noise when placed on power cords and many control cables. They can also be effective on the antenna feedline itself. As vagrant mentioned, use five or six. If the cable is small and flexible enough, multiple wraps through the ferrite will likely increase the ferrite's efficacy in reducing unwanted noise.

 

[Ubbe]

Frequencies consist of electrical and magnetic waves. The idea are to have the ferrites cover the coax where the amplitude, and the impedance of any interference riding on the outside of the coax, are the highest to let the ferrite work most efficient. If you monitor different frequency bands the place on the coax that have the signal at max will shift. You will need to cover a half wavelength of the frequency to be sure to hit the interference, or to decouple a coax from the antenna.

It's easier to do if the coax can be wrapped several turns around the ferrite. At 800MHz a half wavelength are 18 centimeter 7 inches. At 450MHz its 33 cm 13 inches. At 150Mhz its 100cm 39 inches.

/Ubbe

 

[prcguy]

I've never heard of covering a specific fraction of a wavelength with ferrite and the ferrite will absorb RF on the outside of a cable no matter where its placed. The different mixes of ferrite will have varying amounts of resistive choking impedance for one turn or one pass through the ferrite and usually one ferrite with one pass of the cable through it will not do much of anything due to the minimal amount of choking impedance.

For VHF through UHF a #43 mix ferrite is common as is a Laird 26 mix and possibly a #61 mix. Its nice to have at least 1,000 ohms of choking impedance and one #43 mix snap on with a 1/2" hole might have 200-300 ohms across VHF/UHF so a minimum of three in series would be a good start. You can also wrap two or three turns through the ferrite and two turns would multiply the inductance 4X and also increase the choking impedance. For lower frequencies like HF its more practical to wrap many turns around a ferrite toroid rather than a bunch of snap on types in series. You can get upwards of 30dB isolation by wrapping the correct amount of turns around a toroid and for snap on beads in series about 20dB isolation is the most you can get without covering many feet of coax in ferrite.

There could be a slight advantage where you place the ferrite along the cable but you usually place them right at the output of the device that might be generating RFI and/or at the input to the device that would be sensitive to the RFI.

Frequencies consist of electrical and magnetic waves. The idea are to have the ferrites cover the coax where the amplitude, and the impedance of any interference riding on the outside of the coax, are the highest to let the ferrite work most efficient. If you monitor different frequency bands the place on the coax that have the signal at max will shift. You will need to cover a half wavelength of the frequency to be sure to hit the interference, or to decouple a coax from the antenna.

It's easier to do if the coax can be wrapped several turns around the ferrite. At 800MHz a half wavelength are 18 centimeter 7 inches. At 450MHz its 33 cm 13 inches. At 150Mhz its 100cm 39 inches.

/Ubbe

 

[ratboy]

Snap on chokes can be almost a miracle, in some circumstances. Back years ago, I had bought my first Info-Tech/Universal Radio RTTY convertor and the old monitors (You want green or amber?) were huge RFI generators, especially the ones that Universal sold for use with those convertors!
I bought snap on chokes at every hamfest in the area, and had hundreds of them in all sizes by the time it all was over with. I had them on every input cable, antenna cable, video cable, oscilloscope cable, etc. I found by trial and error that making cables as short as possible after winding them around the chokes in a cluster (1 to about 6 or so) as many times as I could helped sometimes as much as the chokes did. Sometimes, putting the chokes on the cable spaced a couple of inches apart with multiple turns on each one was helpful. The biggest improvement was when I found, just by pure luck, a metal cased green 17" monitor for sale in an industrial supply catalog at work. That monitor alone got rid of I would say 50% of all my RFI on HF, and I tried several monitor cables and found one that had double shielding on it and with the chokes, I got rid of almost all the remaining buzzes and whines. Some otherwise great monitors are just trash, RFI wise. I had a fantastic looking Sony PC LCD monitor that I had to get rid of, it trashed all my HF stuff in my listening room. A friend still uses it on his one PC.

Before I moved to my present RFI hell, where nothing seems to really work, the chokes came in handy to quiet down my RSP1 SDR, which was super touchy about where it was located, and it's orientation to my laptop. The USB cable needed to be as long as possible to keep the buzzing to a usable limit. A metal case from EBAY helped too. Every SDR since then has been metal cased. As of this point, the only HF listening I can do is in my car, with a magnetic mount active antenna on the roof, to keep the SDR's from picking up their own display noises or my car's RFI. I use one of those Mahalit V3 Clones for it. It works fine, as long as you keep the receiver from picking up it's own display noise. If someone could get a quiet display and make an all in one, I would have my credit card out in seconds.

 

[SpaceForceCmdr]

I got a few of these snap-ons, and put them on my cables. Adding them to the USB power cable didn't seem to do anything, but when I popped one on the feed line that comes from my antenna, my SNR did seem to increase a little. I was on one of the time stations, and the SNR was varying, so I'd have to do more testing. For a couple of bucks, it's worth it to experiment with them.