Topband: 160m high power lowpass/bandpassfilter

Ford Peterson ford at
Sun Feb 26 19:43:33 EST 2006

> Anyone has info about this? (schematic, component values, capacitor voltages
> etc)
> Especially looking for info about what cores are necesarry when running 1kW.
> Is T200-2 large enough, or maybe I even can use T157-2 or T130-2???
> 73
> Dick, PA4VHF


I have designed and made competition grade bandpass filters for 80-40-20 that would handle 200W with good success.  At 200W the voltages on resonant L/C circuits are going to be on the order of multiple KV when exact resonances occur.  Even using T-140 parts and #12 wire, at 200W there was some warming--even with the bulky wire.  Wire heating will easily dominate over core heating as the circulating currents in the L/C at 200W was on the order of a dozens of amps.  

You are talking about a filter to handle 1KW.  My guess is you need to rethink your project as the components you are talking about are going to be massive, extremely awkward, and may likely be more costly than the radio used to generate the RF!  Most competition grade filters are placed between the exciter and the amp, making a worst case power requirement on the order of 50W to 100W, depending on the amp.  I am a low power competitor and needed the filters to handle at least 150W.  This design has been tested for very short periods at 200W but considerable periods at 150W.  Even at 100W, with extended period load mismatches, heating was noticed but not to the point of letting any smoke out.

The 160M version was far more complicated in that the Iron Powder cores could only handle a fixed number of turns.  The inductances needed for the resonant circuits required the use of either a) multiple cores in series, or b) very massive cores, or c) very tiny wire (wire heating was a problem).  I abandoned the 160M project.

In any event, you can investigate what you need by taking a look at this link I prepared in 2004 when I designed and constructed my filter set.  The paper discusses the design approach for 80-40-20, but the principles are identical but for the frequency used.  Your mileage may vary--good luck...

This paper was version 1.  By the time I had built 1 of them for each of the 3 bands, the procedure became pretty solid.  It was a lot of work and required painstakingly tuning of each filter element to get it right.  The construction required the use of my HP8690B sweep generator and HP-141T spectrum analyzer.  Other equipment suitably sized for looking at RF in the frequency domain is virtually essential to tuning.  In the end, it was worth it.  Insertion loss was about -0.5dB and the attenuation on the adjacent ham band was exceptional (in some cases >60dB).  I had a friend at the University of MN labs -- Ethan-K8GU, test these and a set of ICE and Dunestar filter in use by other local hams using calibrated equipment.  The comparison was that this approach proved superior to the available samples.  Once again, this is a very tweaky business--your mileage will vary (and possibly be even better than mine).  Lest I be accused of bad mouthing either of these fine companies making filters, had the same effort been placed on tuning the off-the-shelf and well-used (and no doubt abused) filters, the comparative results may have proved different.

Good luck.

ford at

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