What happens if I put two x RF DC-22MHz filters in parallel?

Short version: “Parallel” doesn’t help—and can make things worse.

Here are the three ways people mean “parallel,” and what actually happens:

1. Tie the two filters’ INPUTS together and OUTPUTS together (no splitter).
   • The source and load each see ~25 Ω instead of 50 Ω (two 50-Ω ports in parallel).
   • That’s a 2:1 VSWR (return loss ≈ 9.5 dB) before you even count the filters’ reactance near cutoff.
   • The mismatch distorts the designed response: worse passband ripple, unpredictable cutoff shift (typically not upward), and degraded stopband.
2. Use a proper 2-way splitter/combiner (e.g., Wilkinson) to “parallel” the paths.
   • Each filter still sees 50 Ω; the outputs are recombined in phase.
   • Net transfer ≈ the same as a single filter, aside from the splitter/combiner’s small insertion loss (~0.2–0.5 dB each).
   • No meaningful extension of the passband; main “benefit” is higher power handling / redundancy, not a higher cutoff.
3. Cascade (series) the filters.
   • This steepens the skirt and greatly improves stopband attenuation, but adds insertion loss and doesn’t raise the cutoff.

If your goal is to move the −3 dB point from 22 → ~24 MHz, paralleling won’t do it. The clean options are:

• Use a higher-cutoff LPF (e.g., 24–25 MHz model) and, if needed, add a small notch or a second stage for extra suppression above your band.
• Roll your own LC (Chebyshev/elliptic) at 24–25 MHz for 50 Ω terminations.