Lowpass filter for 30m

2015-08-13 20.15.44

Flathagen made a 30m lowpass filter for his Softrock Ensemble RXTX tranceiver:

I just finished a 30m lowpass filter for the Softrock Ensemble RXTX tranceiver.  I used the schematic provided by WB5RVZ.  L200 and L201 is wound to about 0.9uH, whereas C200 and C202 is 100pF and C201 is 330pF. Everything is soldered to a PCB (ugly construction) and I added two switches and a coax to enable on and off switching of the filter.


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  1. When building filters, what is a decent [but cheap] test setup? The setup here used a function generator, and a scope in FFT mode.
    A Vector Network Analyzer [VNA] typically reaches the GHz range, and costs thousands of dollars. Massive overkill for HF – should I look at antenna analyzers instead?
    For a top frequency of about 300MHz?

  2. Try looking at the RF explorer spectrum analyzer and the matching RF Explorer tracking generator.

    I’m just a happy owner of these they give you coverage up to 2.7GHz

  3. Nice gear, good price, but…

    The explorer starts at 15MHz, and the generator starts at 24MHz. I’m willing to pay for GHz coverage I don’t use [yet] – but I need coverage down to 2MHz, preferably lower.

    Thanks for the feedback, but the quest for appropriate [and cheap] test equipment will continue.

  4. Alan,

    If you want to cover up to 24Mhz you can easily build your own kit for those frequencies.

    You can use a DDS board for a signal source and a rf detector based around the analogue devices logarithmic amplifiers, combined with a decent embedded processor board.

    Have a look at http://g4dcp.blogspot.co.uk/ Musings of an ancient radio ham in particularly the 26th June entry and you can see how I measure filters, crystals etc at low frequencies.


    1. Peter,
      That’s a nice blog. Just enough info to leave me thirsty for more…
      Time for an Ebay AD9851 board, to start the tinkering.

  5. That will get you up to about 50Mhz

    Sorry I forgot to mention that. The RF explorer is useful for above that.

    G4DCP – Peter

  6. I think that the important thing is to wind evenly around the core – so that for example, the 48’th turn is near to the 52’nd and so on. But the 75’th turn of a 100-turn coil should be 90-degrees away from the 50’th turn. The left photo is a 70-turn inductor for a 600m filter, and the right photo is a 105-turn inductor for a 2,200m filter.

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