Related to this question: Is it legal to broadcast an FM radio signal with a Raspberry Pi Zero?

Is it possible to build a super-low-tech low-pass filter for VHF harmonic suppression without a PCB or any other non-trivial-to-find components? (e.g. nothing surface-mount or tiny) Would a bunch of coax-stub (or twisted pair) reflective notch filters work? Hookup wire wound around a pencil inductors? etc.

  • $\begingroup$ A PCB counts as "non-trivial-to-find"? Tuning a filter is certainly a league above making a PCB, so I'd say you are SOL with those requirements. Not that making a filter with commodity parts is difficult: it's just inductors (which you can make) and capacitors which you can buy or make (one way is to cut them from PCB stock, ironically). You can make an impedance bridge easily enough, and an oscillator, and a detector, but you've set the bar pretty low. $\endgroup$ Feb 26 '17 at 0:24
  • $\begingroup$ Well I suppose blank clad FR4 is available fairly cheaply for next day delivery from amazon. But how is a high school kid at a local Raspberry Pi hackathon (or similar event) going to cut it? The idea might be to convince them to at least try to be legal before hooking up an antenna to some digital output. $\endgroup$
    – hotpaw2
    Feb 26 '17 at 4:27
  • 1
    $\begingroup$ @hotpaw2 I think you should add some of that to the question — not so much the "convince them" part but to clarify what kind of "trivial-to-find" you mean and what sort of equipment might be already available. $\endgroup$
    – Kevin Reid AG6YO
    Feb 26 '17 at 4:44
  • $\begingroup$ "trivial-to-find" will vary significantly depending on the reader, so I'll leave that off my question. There's no longer a radio shack next to every mcdonalds around my area. Maybe next-day delivery from a popular and well known web site and on a high school student's fast food budget qualifies for some percentage of readers. $\endgroup$
    – hotpaw2
    Feb 26 '17 at 18:23

If you've got an RTL-SDR or similar broadband VHF SDR, you could construct a wideband noise generator and use it as a sort of poor man's VNA to tune the notch on a set of coaxial stubs. A dozen or so meters of high quality coax, RG-213, 9913F, or even better, hardline, could be used to build a set of stubs that would suppress harmonics fairly well. A pair of them properly connected should be able to suppress 2nd order harmonics by something close to 30dB. 3rd order is trickier, and requires carefully designed "T" type stubs that have reactance compensation included to avoid attenuating the primary frequency as well. It is not a trivial process, but the required materials are the same.

The same SDR/Noise Generator combination could potentially be used to tune a typical multipole LC filter, but that will not be at all trivial. You'll need a good L/C meter, and ideally an oscilloscope and a good clean variable signal generator to get the same attenuation as the coaxial stubs, and the components cannot be scrounged typically. You'll need low loss NP0 capacitors in very precise values and inductors wound carefully to avoid stray capacitance and thermal variation as well.

  • $\begingroup$ +1: Good idea. An RTL-SDR can be cheaper than a Pi or Arduino. I wonder if the Pi itself can be used to generate a wideband noise source or frequency sweep? So. Where's a good description of building T-type notching stubs? You might want to add the latter to your answer, if possible. $\endgroup$
    – hotpaw2
    Feb 26 '17 at 18:24
  • $\begingroup$ If you have the poor man's VNA you don't need an L/C meter, and I think winding the inductors and sourcing the capacitors is a lot easier than you make it sound. For capacitors under 220 pF (which is probably all you'll need at VHF) NP0 is probably the only dielectric you'll find. I doubt temperature stability is so critical with the low-order filter that would be necessary for harmonic suppression. $\endgroup$ Feb 27 '17 at 0:49

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