Part of our Getting On The Air: Modes, Operations, and Making Contacts guide.
Every modern transceiver lets you narrow or widen the receive filter, and a lot of operators leave it on the factory default forever. That is a shame, because filter width is one of the simplest ways to pull a usable signal out of a crowded, noisy band.
What the filter is actually doing
Your receiver’s IF filter sets how much of the radio spectrum around your tuned frequency actually reaches your ears or your decoding software. A wider filter lets more of the band through, which is good when you want to hear a broad, natural sounding voice signal, or when you want to see many digital signals across the waterfall at once. A narrower filter blocks more of what is on either side of your signal, which cuts down on adjacent channel interference and noise, at the cost of making voice sound thinner and boxier, or clipping the edges off a digital signal if you go too narrow.
The tradeoff is always the same: wider means more natural sound and more signal energy captured, narrower means better rejection of everything else nearby. Where you land on that spectrum should depend on band conditions and the mode you are running, not just whatever the radio defaults to out of the box.
SSB starting points
- 2.4 kHz is a solid general purpose SSB width, wide enough for natural sounding voice without wasting receiver bandwidth on frequencies your ears do not really need.
- 2.7 to 2.9 kHz gives fuller, more broadcast quality audio, good for casual ragchewing on a clean band with no one nearby.
- 1.8 to 2.0 kHz is worth switching to when a strong station is parked close to your frequency, trading some audio fullness for a real improvement in readability.
CW starting points
- 500 Hz is a comfortable general purpose CW width for casual operating, letting you copy a bit of the surrounding activity while still narrowing well past SSB.
- 250 Hz or narrower is the move in a crowded pileup or contest, isolating a single CW signal from everything stacked up around it.
Digital modes: it depends which kind
Digital modes split into two very different categories when it comes to filtering, and mixing them up is where a lot of operators get confused.
Modes like FT8, FT4, and JS8Call are normally run through a computer sound card while the radio sits in a standard SSB filter setting, typically 2.4 to 3 kHz wide. You actually want that width, because dozens of individual FT8 signals, each only about 50 Hz wide, are stacked up across that passband at once, and the decoding software needs to see all of them on the waterfall to work multiple stations. Narrowing the radio’s IF filter here does not help. It just throws away signals you could have decoded.
Modes like RTTY and PSK31 behave more like CW. RTTY typically wants a filter around 250 to 500 Hz, since a single RTTY signal only occupies a few hundred hertz. PSK31 signals are even narrower, often just 31 Hz wide, but in practice most operators still run a filter in the 200 to 500 Hz range so they can see and tune between several PSK31 signals near each other rather than isolating down to one at a time.
The general rule that ties all of this together: match your filter width to the actual bandwidth of what you are trying to copy, plus a little room. Too wide and you invite in noise and adjacent signals you do not want. Too narrow and you start clipping the very signal you are trying to hear.
For more on the modes mentioned here, see What Is WSJT-X? for the FT8/FT4 side, What Is JS8Call?, and The Fldigi Family for RTTY and PSK31.