[Radio Wrangler]

You want to pass 1MHz, and reject 2, 3, 4, 5, etc. MHz so you could use a lowpass filter with its 3dB point chosen to be a bit above 1MHz, but a high enough number of meshes to get decent attenuation in place before 2MHz. Getting artful, you can design an 'elliptic' type (aka complete Cauer) filter and plant its nulls on some of your harmonics.

Alternatively you could use a bandpass centred on 1MHz. You can trade off between having a narrow filter of fairly low order, or having a wider filter of higher order. Narrow filters create more insertion loss, and get fiddly to tune. Higher order, wider filters are less touchy, but use more parts.

On-line filter design programmes are a lot crude. They'll design you an elliptic if you ask, but won't let you play games with placing nulls. They're worse at bandpass designs. There are two approaches to designing bandpass filters. One is to design a lowpass (think of it as a bandpass centred on zero hertz, extending to -3dB point at a positive frequency, and one at a negative frequency) and then to transform it into a bandpass filter. These filters are OK only in fairly mild forms. They have two problems... they soon run into trouble requiring extreme component values and they are very intolerant of strays. Many of the on-line thingies only know about transformed lowpass filters.

The other bandpass structure is the coupled resonator style. These are the only approach once they get narrow. They can absorb strays by twiddling component values and are generally more house-trained. Relatively fewer on-line calculators even know this type exist. These programmes seem to have been written by people more from the programming end of the spectrum than from the filter expert end.

Maybe a 2 resonator design set for 1MHz +/- 50kHz pasband would be a good first try.

David