Why Standard Band-Pass Filters Fail in Multi-Operator Stations

Why Standard Band-Pass Filters Fail in Multi-Operator Stations

Running more than one operator in the same station brings out the limits of conventional band-pass filters (BPFs). What works fine for single-op contesting often falls short when you add multiple transmitters and antennas, even if they’re on different bands.

The Example: FT8 on 17 m and CW on 20 m

On paper, this looks easy: two separate HF bands with plenty of frequency spacing. A standard 20 m BPF seems like it should keep the 17 m station quiet, right?

In practice, it doesn’t. Here’s why:

  • Harmonics and IMD products — A 20 m rig at 14 MHz throws off harmonics at 28 MHz and 42 MHz, which can land uncomfortably close to 15 m and 10 m operators.
  • Out-of-band skirt rejection — A typical contest-grade 20 m BPF may offer 30–40 dB rejection at 17 m. In a single-op station that’s enough. In a multi-transmitter site running 100 W per band, it’s not.
  • Near-field coupling — With antennas close together, even “filtered” transmitters leak significant energy into each other’s receivers.

What Level of Isolation Do You Need?

For true multi-two or multi-multi contesting, 70–80 dB isolation between bands is the practical target. Below that, you’ll start to see front-end overload, AGC pumping, and birdies across your digital waterfall.

Isolation at a Glance

Filter Setup Typical Rejection Use Case
Basic single BPF ~35–40 dB Casual operating, single-op
Contest-grade BPF bank ~55–60 dB SO2R, small multi-op
Cascaded filters (BPF + BPF) 65–75 dB Multi-two, light contest use
BPF + Notch/Trap combination 70–80 dB Serious multi-multi stations

Stacking Filters and Hybrid Combinations

A normal 20 m BPF won’t be sufficient against a nearby 17 m transmitter. The usual approach in serious multi-multi setups is:

  • Cascaded BPFs — two high-Q filters in series, tuned for sharp skirt selectivity.
  • Band-stop or notch filters — dedicated rejection of the neighbor’s band.
  • Triplexers/Duplexers — when sharing antennas, these networks combine BPF and notch behavior for very high isolation.
  • Physical antenna separation — distance is still the cheapest filter.

Don’t Forget Common-Mode Filtering

Band-pass filters handle the differential signal path — the intended RF traveling inside the coax. But the real killer in multi-op setups is the common-mode current riding on the shield. That path bypasses your filter network completely and couples straight into your neighbor’s receiver.

  • Why it matters: With two transmitters on different bands, each running 100 W, even microvolts of shield leakage can push the other rig’s front end into overload.
  • Why normal chokes fail: A handful of mix-31 beads may give 10–15 dB of suppression. That’s fine for single-op use, but nowhere near enough isolation for contest-class multi-multi stations.
  • What you need: At least 40–45 dB of wideband common-mode rejection per feedline, across 1.5–30 MHz.

That’s why a dedicated high-power isolator is essential. A unit like our 2025-F-010-45 9 kW Wide-Band HF Line Isolator delivers up to 45 dB of suppression from 1.5–30 MHz, ensuring shield-borne energy doesn’t reach other receivers.

Without strong common-mode filtering, your BPF skirts won’t matter — the interference sneaks around them via the coax shield.

Real-World Takeaway

If you’re planning to run two stations side-by-side — like FT8 on 17 m and CW on 20 m — don’t assume that a single commercial BPF will protect you. For 100 W class rigs, plan for at least 70 dB of cross-band isolation. This often means either cascaded contest-grade filters or a hybrid of band-pass and band-stop filtering, plus smart antenna placement and high-grade line isolators per feedline.

Mini-FAQ

  • Can one BPF per rig solve the problem? — Not for multi-op. You’ll need higher rejection, often via cascaded filters or BPF + notch combinations.
  • Is antenna spacing enough? — Helpful, but not sufficient. Coupling happens through both space and cabling.
  • What about QRP multi-op? — Lower power reduces stress, but isolation requirements are the same if you want clean reception.
  • Do I need a line isolator per antenna? — Yes. Each feedline must have serious common-mode suppression to avoid shield leakage ruining other receivers.

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Written by Joeri Van Dooren, ON6URE – RF engineer, antenna designer, and founder of RF.Guru, specializing in high-performance HF/VHF antennas and RF components.