Coax Noise Filters
Coax Common-Mode Noise Filters
Kill braid-borne shack noise by up to 4–6 S-units without touching your antenna. Install at the radio end to stop common-mode currents on the coax shield.
Part of the Palomar Noise Filter Series – Eliminate RFI at Every Path
Why it works: The outside of the coax acts like a second receiving antenna. Our filters add high choking impedance on the braid, while passing desired signals inside the coax. With multiple antennas, put a filter on every feed line—switches usually only switch the center pin, but braids remain paralleled.HF 50 Ω (500 W or less)
CMNF-500-50 — the go-to station filter for most transceivers and SDRs. Place at the radio before any switch/splitter.
HF 50 Ω (Amplifier)
CMNF-1500HF / CMNF-5000HF — same suppression with higher PEP ratings. Install at station entry; one per feedline.
LF (0.2–7 MHz) & 160 m
CMNF-1500LF / CMNF-5000LF — optimized below 10 MHz. Excellent for 160 m, AM BCB, and low-band SDR.
75 Ω TV/CATV
CMNF-500-75 (CMNF-TV) — reduce RFI on TV/cable/satellite runs and cable-modem coax.
VHF / UHF
CMNF-500-50VHF / -50UHF — targeted suppression on higher bands for clean VHF/UHF receive.
HF 50 Ω (500 W or less)
CMNF-500-50LF — the go-to station filter for most transceivers and SDRs at LF. Very effective on AM broadcast band.
Compare Models
| Model | Range (MHz) | PEP | Connectors | Notes |
|---|---|---|---|---|
| CMNF-500-50 | 1.8–65 | 500 W | SO-239 | General HF station filter |
| CMNF-1500HF / 5000HF | 1.8–65 | 1.5 kW / 5 kW | SO-239 | Amplifier duty |
| CMNF-1500LF / 5000LF | 0.2–7 | 1.5 kW / 5 kW | SO-239 | Low-band optimized |
| CMNF-500-75 (TV) | 1.8–65 | 500 W | F-type | 75 Ω consumer/video |
| CMNF-500-50VHF / -50UHF | 70–170 / 225–500 | 500 W | SO-239 | Higher-band suppression |
| CMNF-50-50LF | .1-30 | 500 W | SO-239 | Low Frequency/AM Broadcast DX |
60-second: do I have common-mode noise?
- Unplug the coax; note baseline noise.
- Touch the **center pin only** to the antenna jack; note noise level.
- Touch the **shell only** to the jack; if noise rises, your braid is carrying noise → a CMNF will help.
Where should I place the filter?
At the **radio/station end**, before any switch/amp/splitter. For multi-antenna stations, put a filter on **every feed line** that enters the shack. Most switches only switch the center conductor; the braids remain tied together and share noise.
Will this affect transmit power or SWR?
No—the filter chokes noise on the **outside** of the shield. Desired signals on the center conductor/inside shield pass normally. Choose a model with the correct **PEP rating** for your mode/duty cycle.
