Bandpass Filter Calculator

You might need one if:

• Your radio gets "overloaded" by strong local AM/FM stations (audio sounds distorted or you hear stations where there shouldn't be any)
• You're using an SDR (software defined radio) and see tons of signals even on "empty" frequencies
• You live near broadcast towers, airports, or other strong RF sources
• You want cleaner transmitted signals with fewer harmonics

You probably don't need one if: Your radio works fine and you're not experiencing interference problems. Don't fix what isn't broken!

Nope! This filter works the same in both directions. You can connect your antenna to either side and your radio to the other - it doesn't matter.

That's because it's a "passive" filter made of simple coils and capacitors. There's no "right way" to hook it up. The schematic shows "INPUT" and "OUTPUT" just to make it easier to read, but you could swap them and it would work identically.

Bonus: This also means you can use the same filter for both receiving AND transmitting without switching anything!

It works for both! But there's a catch with power levels:

• Receiving: Works great as-is. Just build it with the parts shown.
• Transmitting low power (under 10 watts): Works fine with standard parts.
• Transmitting higher power (10-100 watts): Use thicker wire (14-16 AWG) and capacitors rated for higher voltage (500V or more).
• Transmitting 100+ watts: This simple design isn't ideal. Look into commercial filters or beefier homebrew designs.

For the capacitors:

• Look for "NP0" or "C0G" ceramic capacitors - these are stable and work well for radio frequencies
• Mouser.com and DigiKey.com have huge selections
• Amazon has assortment kits that work for experimenting
• Hamfests often have bins of surplus capacitors cheap

For the wire (inductors):

• Enameled "magnet wire" from Amazon or hardware stores
• 18 AWG is a good all-around size

Pro tip: Get an assortment of capacitor values - you'll likely need to combine values to hit the exact number.

No problem! You can combine capacitors:

• Parallel (side by side): Values ADD together. Need 100pF? Use 68pF + 33pF = 101pF. Close enough!
• Series (in a row): Gets more complicated, but useful for fine-tuning

Even easier: Use a "trimmer capacitor" (adjustable) in parallel with a fixed cap. This lets you tune to the exact value you need. A 5-30pF trimmer plus a fixed cap slightly below your target value works great.

Best way: Use a NanoVNA (about $50 on Amazon). It shows you exactly what frequencies pass through and what gets blocked.

Budget way: Connect your antenna to the filter input and your radio to the output. Tune around:

• Signals on your target band should be strong
• Signals on other bands should be weaker than before
• If everything is weak, something's wrong - check your connections

The "it's working" signs: Less noise, cleaner signals, nearby broadcast stations don't blast through anymore.

Nope - that's the whole point! A bandpass filter only lets ONE band through. That's what makes it useful.

If you work multiple bands, you have options:

• Build several filters and swap cables (cheapest)
• Build a "filter bank" with a switch to select which filter is active (convenient)
• Just build one for your most-used or most-problematic band

Many hams start with one filter for 40m or 20m since those are popular bands with lots of activity.

Yes! The filter passes the entire band (like all of 40m from 7.0 to 7.3 MHz). All the different modes live within that range, so they all get through.

Your radio handles picking out individual signals within the band - the filter's job is just keeping signals from OTHER bands out.

A little bit weaker is normal. Every filter loses a small amount of signal (called "insertion loss"). This design typically loses 1-2 dB, which is barely noticeable - maybe half an S-unit on your meter.

If signals are WAY weaker:

• Check all your solder joints
• Make sure coils aren't touching the metal box
• Verify capacitor values are correct
• The filter might be tuned to the wrong frequency - try adjusting the trimmer caps

The metal box is important! It does two things:

• Shielding: Keeps outside signals from sneaking around your filter
• Grounding: Provides a solid reference for the circuit

Can you skip it? For testing, sure. But for real use, the filter won't work nearly as well without shielding. Unwanted signals will just "go around" the filter instead of through it.

Cheap box options: Altoids tins, aluminum project boxes from Amazon, or even a tin cookie box with holes drilled for connectors.

It depends on the design goals. Both can handle high power when properly designed, but they have different trade-offs:

Toroid inductors (iron powder cores):

• Self-shielding - magnetic field stays contained, no metal enclosure interaction
• More compact - smaller size for equivalent inductance
• Heat dissipation is the limit - not saturation (a common misconception)
• Typical power ratings per core size (iron powder, in resonant circuits):
  • T-50 (12.7mm): ~25W
  • T-68 (17.5mm): ~75W
  • T-106 (26.9mm): ~150W
  • T-130 (33mm): ~200W+

Air-core inductors:

• Highest Q - lowest losses in the passband
• No saturation limit - power limited only by wire gauge and capacitor ratings
• Requires shielding - magnetic field radiates, needs metal enclosure
• Larger size - takes more space for equivalent inductance
• Tunable - can adjust inductance by spreading/compressing turns

The bottom line:

• Receive-only: Either works; toroids are more compact
• QRP (under 10W): T-50 or T-68 toroids are fine
• 100W: Use T-106 or larger toroids, OR well-built air-core
• 200W+: Use T-130 toroids (like W3NQN designs) or air-core with heavy wire

Important: Use iron powder cores (T-XX-2, T-XX-6) for RF power, NOT ferrite. Iron powder resists saturation much better than ferrite at RF frequencies.

Sources: QST Dec 1988 "Toroids - Some Practical Considerations", ARRL/W3NQN "Clean Up Your Signals with Band-Pass Filters", Micrometals "Iron Powder Core Selection for RF Power Applications"