Antenna Switch Common-Mode Noise Filter Kits
Most antenna switches only switch the center pin; the coax braids remain tied together in parallel. That means noise from any non-selected line can ride the outside of its coax into your currently selected antenna port. The cure is simple: install one CMNF on each feed line before the switch so those braids are isolated.
What this fixes (in plain English)
Noise rides the outside of coax as common-mode current. Inside a switch, the braids are tied together, so non-selected lines can inject noise into the active port. Choking each input isolates the braids and typically lowers the noise floor by ~25–36 dB (≈4–6 S-units), station-dependent.
- Lower S-meter baseline & cleaner waterfall
- Less “RF in the shack” & fewer mystery RFI paths
- Stable multi-antenna operation with reduced interaction
Where to place the filters
- One CMNF on each feed line, located between the antenna coax and the switch input.
- Use short jumpers or barrels into the switch. Either port can be in/out; DC/bias-T passes.
- Optional but recommended: add a CMNF at the radio/amp input for belt-and-suspenders isolation.
60-second “Do I have it?” test
- Unplug all lines → note baseline noise.
- Touch the center pin only to the jack → note noise.
- Touch the shell only. If noise jumps, that line’s braid is noisy → put a CMNF on that line before the switch.
What’s included (by kit size)
- CMNF filters: One per selected port count (2–9)
- Short jumpers/barrels: One per filter for clean connection into the switch
- Quick-start: 60-sec test & placement diagram
Select your desired Port Count (2–9) and Power (500 W or 3 kW) from the options above.
- General HF (1.8–65 MHz): choose the ste CMNF in 500/1500/3000/5000 Watts power to match your station.
- Low-band focus (0.2–7 MHz / 160 m & AM BCB): ask about LF-optimized CMNF sets if your issue is primarily on the low bands.
- Best practice: CMNF on each switch input plus one at the radio/amp input for maximum isolation.
FAQ
Do I really need one per line? Yes. With braids paralleled inside the switch, a single noisy line pollutes the rest unless each input is choked.
Will filters change SWR or reduce power? No. They choke only the outside of the shield (common-mode). Desired RF rides the center pin & inner shield as usual—just select the power rating that fits your station.
Can I pass DC or bias-T? Yes—CMNFs pass DC on the center pin while blocking braid-borne common-mode.
Still noisy on 160 m? Add an extra CMNF at the station entry or use LF-optimized models for stronger low-band choking.
Antenna Switch Quick Start Manual
Bonus: Put a feed-point choke (MAXI-CHOKER™) at the antenna so the coax braid doesn’t become part of the radiator on TX; let these CMNFs handle the switch and the radio end.
