5-Pole Band-Pass Filters in a Multi-2 Station at 1.5 kW

Why “55–75 dB out-of-band rejection” changes what you can hear… and how hard you can push

In a Multi-2 contest station, two transmitters are active at the same time… often on adjacent contest bands such as 20/15/10 m or 40/20/15 m. In that environment, band-pass filters are no longer accessories. They determine whether you can copy weak multipliers while the other transmitter is running… or whether your receiver turns into hash every time the other station keys.

The core problem is inter-station interference: energy from one transmitter coupling into the other station’s antenna and receiver chain. The total RX/TX isolation you achieve is the sum of:

• Antenna-to-antenna isolation (spacing, height, polarization, pattern nulls)
• Filter rejection at the victim receiver’s band
• Receiver robustness, preselection, and any added attenuation

At 1.5 kW, a Multi-2 station is unforgiving. You need isolation margin where it matters most… on adjacent bands.

What 5-pole “XL-class” filters are built to do in Multi-2

High-end contest filters are designed to live behind the amplifier at full power, filtering both the fundamental and wideband PA noise before it ever reaches the antenna system. The key technical differentiator is topology: five resonant sections instead of the more common three.

More poles give steeper skirts. That directly translates into higher adjacent-band rejection without unacceptable in-band loss… exactly what Multi-2 stations need.

Typical figures for this class are 55–75 dB adjacent-band rejection with insertion loss around 0.5 dB… achieved with large enclosures and active cooling because real power is being dissipated.

What a “35–55 dB OOB” filter really means in practice

Many HF band-pass filters are rated for similar power levels but provide 35–55 dB attenuation on the nearest band. That difference looks modest on paper… until you translate it into what the receiver actually sees.

Every 10 dB is a factor of 10 in power. A 20 dB difference is a factor of 100.

A realistic Multi-2 example (with numbers that matter)

Assume:

• Transmit power: 1.5 kW (≈ 61.8 dBm)
• Antenna-to-antenna isolation: 60 dB (already quite good)

Interfering power at the receiver input is approximately:

P_RX ≈ P_TX − Isolation − Filter Rejection

• 45 dB rejection → −43 dBm (≈ S9 + 30 dB)
• 55 dB rejection → −53 dBm (≈ S9 + 20 dB)
• 70 dB rejection → −68 dBm (≈ S9 + 5 dB)

That last 15 dB step is often the difference between constantly riding RF attenuation… and being able to leave the receiver in a linear, sensitive operating state.

Does lowering power make fewer poles “good enough”?

Lowering power helps… but it does not cancel large rejection differences. Dropping from 1.5 kW to 400 W is only about 5.7 dB. That does not erase a 15–25 dB filtering advantage.

• 10 dB more rejection ≈ 10× less power
• 20 dB more rejection ≈ 100× less power
• 25 dB more rejection ≈ 316× less power

The station-reality “tipping point”

Most operators don’t need equal interference… they need no RX pain. A common practical target is keeping coupled interference below −50 dBm.

With ~60 dB antenna isolation:

• 45 dB rejection → workable up to ~300 W
• 55 dB rejection → workable well past legal limit
• 70 dB rejection → enormous margin

Insertion loss, heat, and why big filters exist

• 0.5 dB loss @ 1.5 kW ≈ 160 W heat
• 0.12 dB loss @ 1.5 kW ≈ 40 W heat

How to pick the right filter

• Identify your worst adjacent band pair (20/15, 40/20, 80/40)
• Measure or estimate antenna isolation (S21 if possible)
• Choose a comfort target (−50 dBm workable, −60 dBm comfortable)
• Rejection ≥ Ptx(dBm) − Isolation − Target

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