Ferrite Toroids For RF Filtering ? Novice Level Quest's, Please

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Dec 15, 2004
Boston, MA

Not sure what is the best Forum for this question.
If not here, please just tell me where, and I'll re-post it.

Do listening only, <30 MHz.
Have a SDRPlay RSPdx.
Use a 70 foot horizontal. long wire ant outside, a Balun, and then a coax run into the house.
Using SDRConsole.

Works pretty well, but am convinced i could do a lot better.

The RSPdx is in my Study which also has two other desktop PC's which are on all the time,
as well as my Comcast Router and Modem.
Lots of Ethernet wiring.

Probably picking up all sorts of noise, but I'm not smart enough to figure out from where
exactly. Also, bit too old now to crawl around and to trace all my wiring anymore, etc.

I do have those cylindrical ferrite clip-ons on most everything i could
get to. All the AC power lines, the coax from the Router to the pc's, etc.
Probably helps, but hard to quantify how much.

Was wondering about those ferrite toroids where you run and loop the coax's, etc. once or twice thru them for rf filtering.
Any opinion on these ? Particularly relative to the cylindrical ones i now have all over.

Worth a try ? Why ?

if so, would appreciate any very specific recommendations.
Amazed at the prices on them.
No way i can spend $40 or so for just one !

Cheaper options if you feel worth doing ? Brand, ferrite material, etc. ?

Any thoughts on this, and suggestions, would be most appreciated.



Jun 30, 2006
So Cal - Richardson, TX - Tewksbury, MA
The #31 mix ferrite is good for most HF jobs. For Toroid rings the FT-240-31 is good and about 8 to 10 turns of coax or AC/DC power lines makes an effective choke. You can place one at the antenna end of the coax near the balun and sometimes another near the radio can help reduce noise from getting induced onto your coax shield. Try to put it after your coax runs past any computer cables or other wiring that might have noise.

The really big snap on ferrites with 1" inner dia hole are great as they can snap over existing cables with out having to thread them through the hole. It usually takes 4 to 6 loops through the core to be effective at HF. These cores can be expensive but this guy has about the best pricing I've seen at $6.50 for an FT-240-31 and $12.80 for the 1" internal hole big snap on cores. I've seen other places charge upwards of $10 for the FT-240 size core and $24 for the 1" snap on. Fair-Rite Mix 31 Ferrites - Pro Audio Engineering

When I used to go to the Dayton Hamvention I would seek out Pro Audio Engineering and buy up about 50lbs of ferrite, I have the stuff everywhere here and my noise floor is impressively low with them and not so much without them.


Aug 9, 2019
I agree. I excellent results in the low HF bands in trems of noise reduction by clipping a 13mm ferrite onto the usb and coax line near my rspdx with as many turns as i can fit, 4+ min


Active Member
Aug 4, 2020
Fort Worth
Palomar Engineers has a good selection of ferrite toroids.

Where I’ve spent hundreds. The site is a little bit of a tail-chase, but persevere. Tech articles and catalog descriptions are an education.

In my assigned Peterbilt, I likely have (20) pieces or more at work. I haven’t A-B tested in every instance.

More like, “when in doubt, add some”.

Definitive success in some instances. A big truck is a noise factory all on its own.

See their advice about treating victim and victimizer. I treat every separate power or coax piece AT BOTH ENDS.

Antenna system has preference.

Bold is thread subject devices:

Palomar Engineers CMNF-500 Filter
Morgan Systems 411CB Bandpass Filter
PE CMNF-1500 Filter
FT-240-31 toroids with (8) 3” dia. turns

Then there is the powered speaker pair and the mic cord.

Then the 12V power which has several loops thru a BIG Mix 31 snap. And (5) powered devices with yet more snaps on each end of each cord.

An unrelated device charger thru an FT240-61 toroid and Mix 31 snap.

A refrigerator/freezer on a GIGANTIC toroid.

A genset and an inverter which could each use some help.


Anything I don’t use here can go in one of three family vehicles or two house base stations.

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