The Ferrite Snap On Combo Pack is a convenient solution for troubleshooting RFI/EMI with various diameter cables ranging from 1/4″ to 3/4″ for use as either single turn or multiple turn ferrite chokes. The snap on chokes are useful for RF coax, AC/DC power cables, and device interconnection cables.
Always use a snap on with a larger inside diameter than the diameter of the cable and use multiple turns if possible (2 turns through the snap on is the same as 4 snap ons with 1 turns). Included in these combo packs are our most popular snap on sizes as follows:
CMNF-500-50 — Coax Common-Mode Noise Filter (HF, 500 W)
What it fixes (in plain English)
Common-mode current rides the outside of your coax braid like a second antenna. The CMNF-500-50 adds high choking impedance on the braid, so neighborhood hash and “mystery” RFI don’t get dragged straight into the receiver.
- Lower S-meter baseline • cleaner waterfall
- Less RF-in-shack (mic bite, router resets, audio buzz)
- Stabilizes multi-antenna stations where braids get paralleled by switches
Where to place it
- At the radio end of each coax run, before any switch/splitter/amp.
- Multi-antenna station? Use one filter per feed line. Most antenna switches only switch the center pin—the braids stay tied together and share noise.
- Portable: at the radio or station entry point works great.
Which model should I choose?
- Up to 100–500 W PEP: CMNF-500-50 (this page)
- Amplifiers (1.5/5 kW): CMNF-1500HF / CMNF-5000HF
- Low-band focus (0.2–7 MHz / 160 m): CMNF-1500LF / CMNF-5000LF
- 75 Ω TV/CATV: CMNF-500-75 (CMNF-TV)
60-second “Do I need this?” test
- Unplug the coax; note baseline noise.
- Touch the center pin only to the radio jack; note noise.
- Touch the shell only. If noise jumps, your braid is carrying noise → a CMNF will help (one per feed line).
- High choking impedance on braid; passes desired signals normally
- Use short jumpers; keep filter near radio/entry
- Best results with one filter per feed line in multi-antenna stations
- CMNF-500-50 filter (SO-239 both ends)
- Quick-start sheet + placement diagram
| Impedance | 50 Ω |
|---|---|
| Frequency range | 1.8–65 MHz (HF wideband) |
| Power | Up to 500 W PEP (SSB). Reduce for high-duty digital/CW. |
| Connectors | SO-239 (UHF female) both ends |
| Placement | At radio/entry; one per feed line in multi-antenna stations |
- 2× CMNF-500-50 + two 3-ft RG-8X jumpers
- Add a third unit if you use a separate receive antenna or second HF line
