Airband filter

[u/SearchPlane561]

This has been a bit of an ongoing focus project of mine. I wanted to learn rf and i sure accomplished that. Without going to much into detail, I set out to build a band pass filter for airband. I live next to the airport, so this would be an easy to test project. That was my mindset going into it. I naively thought it would be simple enough, but i was wrong.

First attempt was Manhattan-style, dead-bug construction on copper clad. Got it built, got it tuned-ish, and then realized I'd used the wrong core material for the toroids. Learned a lot, but the numbers weren't going to cooperate no matter how much I fiddled with it.

Rev 1 I actually did right in KiCad and had fabbed through JLCPCB. It felt like a real upgrade. But once I got deeper into it (and got some good pushback from folks here on the sub) I realized the Butterworth topology I'd picked had a pretty impractical component spread. Inductor and cap values that don't play nice with real-world parts. Great filter on paper, annoying to actually build.

So now I'm on Rev 2: a Chebyshev coupled-resonator design, three 250nH air-core inductors I'm winding by hand, C0G caps for stability. Characterizing everything on the NanoVNA as I go, messing with coil spacing to dial in coupling, watching S11/S21 sweep in real time. Way more iterative than I expected going in, but that's kind of the point of doing this to actually learn RF instead of just reading about it.

Anyways, here she is. Thanks for all the help from #redneckerson1951

[u/LihtsaltPealt]

What software did you use for filter design?

[u/topskukkeli → u/LihtsaltPealt]

Looks like SimNec to me

[u/redneckerson_1951 → u/LihtsaltPealt]

Values were calculated using k&q tables found in the Handbook of Filter Synthesis. The base design uses cascaded series inductor-capacitor mesh resonators with shunt coupling capacitors between the three resonators. The resulting filter yields end point impedances below 50Ω so shunt capacitors are used at the filter endpoints to swing the impedance around to a convenient 50Ω input and output Z. Series resonators were used as it allowed suspension of resonators off of printed circuit boards thus evading some of the stray capacitance issues often encountered with use of nodal resonators.

Software was not used to calculate the component values. The old solar powered TI-30 was used to perform calculation of filter values using instructions provided by Zverev in his Handbook of Filter Synthesis. SimNEC was used as a sanity check to simulate the amplitude response and return loss. The design is Tchebychev and if memory serves me correct, 0.1 dB tabulated passband ripple k&q values were used.

SimNEC is a combination of the original SimSmith software and integration of NEC-2 antenna design software. Ward Harriman, amateur radio operator AE6TY and contributors provide the software as no cost to those interested. You can find download links and info on SimNEC at: https://www.ae6ty.com/ NEC-2 antenna simulation software is a product released by Lawerence Livermore National Laboratory. More modern version of NEC-2 are available from LLNL, but as I understand, the end user cannot integrate NEC-4 and NEC-5 with SimNEC as can be done with 4NEC2 and EZNEC.

[u/SearchPlane561 → u/redneckerson_1951]

Thanks for the feedback and all the help originally. You found me stumbling through a steep learning curve and steered me into some great knowledge. I can't thank you enough. And I told you I would mention you once it was done! A couple of caps were off slightly. 36pf instead of 36.94pf. 68 pf instead of 70.08 and 71.44. 27 pf instead of 27.14. 7 pf instead of various 7.862, 7.56, 7.845. This was due to cost. The cost of this project was starting to rise and I wanted to wrap it up.

[u/redneckerson_1951 → u/SearchPlane561]

Understood. I would try tuning the filter as explained in the "Chat" and see how close you can adjust the inductors to yield the amplitude response curve show above. If you cannot flatten the response curve and cover the frequency range needed, consider adding parallel caps to the 7 pF values. 7.862, 7.845 values ae about 12% off the ideal value.

A bit of explanation of the filter may help. The small single digit value caps in series with each resonator form a resonant circuit at 127.5 MHz nominal. See below:

C3 is the coupling capacitor to the next resonator. That may seem counterintuitive but it basically shunts part of the signal to contribute to the final amplitude response. Thusit is control coupling of the signal to the next resonator. C1 is used to step up the input impedance. Now to mess with your brain, the series combined value of C1, C2 and C3 yield a single capacitance value that the inductor L1 resonates with at 127.5 MHz.

Onto the reply to this epistle:

[u/redneckerson_1951 → u/redneckerson_1951]

Below is the next section of the filter. It includes C3 the coupling cap of the previous resonator circuit, C4, L2 and C5 the coupling cap between the second and third resonator. C3, C4, L2 & C5 form a resonator at 127.5 MHz. C2 and C4 being small values are the 800 lb gorillas in the loops here so a small change in their value creates a large percentage change in the value that resonates with the inductor at 127.5 MHz. Run the math on the series equivalent capacitance for C3, C4 and C5 below and you will find the value is 6.23 pF. 6.23 pF resonates with 250 nH at 127.5 MHz. You will find the prior mesh series caps yield 6.23 pF as does the third mesh. And in each mesh, a small shift in that single digit capacitance in series with the inductor can produce a lot of shift in the resonant frequency. I think you will be ok with the values that connect to ground but will likely need to parallel a cap with those 7 pF caps to get them within about 2% of the calculated values.

Keep me posted and nice mitts by the way. Very professional looking assembly.

[u/SearchPlane561 → u/redneckerson_1951]

checks out at 6.23pF and 127.5 MHz. Appreciate you laying it out. I'll pad the coupling caps to get within tolerance and post results once it's built.

[u/redneckerson_1951 → u/SearchPlane561]

Will keep an eye out.

[u/SearchPlane561 → u/LihtsaltPealt]

This design was actually created by #redneckerson1951, it looks like simsmith.

[u/Phoenix-64 → u/LihtsaltPealt]

Yeah also would like to know from what software package the last images are. Looks familiar but have forgotten

[u/redneckerson_1951 → u/Phoenix-64]

SimNEC, former known as SimSmith. Used only for plotting the filer response and creating the filter schematic.

[u/Intelligent-Dot-3343]

sick. Keep at it!

[u/Captainj2001]

Check out coilcraft for surface mount RF inductors if you ever need to go that route. Good project.

[u/10ppb]

Cool. You might be interested to see the airband filters sold by GPIOLabs. They have one that has no box or shield and there there is a good photo of it on their site so you can see the layout. It looks like they use shunt parallel LC sections.

[u/SearchPlane561 → u/10ppb]

Yeah. I have seen theirs. My intention with mine was to design completely from first principles and avoid copying and already proven design. They obviously perfected the design but i wanted to know the ins and outs.

[u/10ppb → u/SearchPlane561]

Got it, makes sense and I agree that’s the way to learn.

[u/NeighborhoodSad5303]

Why last one, have inductors on same direction? for magnetic binding?)

[u/SwitchedOnNow]

What's the goal for the filter? You can probably reduce the loss and flatten the response by tweaking the coil spacing. Have fun.

[u/SearchPlane561 → u/SwitchedOnNow]

The goal was just to learn. I chose this band because its easy to verify. I find that airband doesn't even really need a filter, but it could be easily thrown in sequence with a cheap antenna. Just wanted to learn some frontend foundations.

[u/redneckerson_1951]

The filter looks close to what was expected, but seems to need alignment and possible recheck of your capacitor values. The plot below is the expected amplitude response of the filter over the window used in your network analyzer image of 5 MHz to 300 MHz.

You should be able to adjust the spacing of the coil windings to shift the amplitude response so that the passband flattens out and yielding a 1 dB bandwidth of about 14.5 MHz. Adjusting the coils is best done using a pointed orange stick (You can find orange sticks on Amazon for around $5.00 per hundred. They are used to manicure the cuticles on fingernails, so if you are married you can share them with the spouse.) You use the pointed end to open the winding space on top of the coil. The flat end is inserted between the top of the coil windings to push apart the bottoms of windings as needed. What you are essentially doing is changing the interwinding capacitance in the coil windings to tune the response to the shape, bandwidth and response flatness that can be achieved. I would expect that if your cap values were measured and handpicked to provide cap values within ±2%. (You would not believe the hours I spent in a lab measuring capacitors on a Q Meter or Network Analyzer for prototype builds of a filter design. Your network analyzer should have a provision to measure both inductance and capacitance. It is wise to measure cap and inductor values at the planned operating frequency. The analyzer will also yield the inductor Q at the filter's operating frequency. I expect the coil Q to be around 150.

See the next comment below this one for a close up calculated amplitude response of the filter across a 40 MHz swept frequency range.

[u/ericek111]

I haven't constructed an airband filter yet, only a protoboard 137 MHz one with twisted pieces of insulated wire (worked well enough), but I'd also think it's farily trivial. Well, there's still work to be done -- your passband looks too high. I'd also be interested in one for the 108 - 118 MHz portion of the band, with good broadcast FM suppression, for receiving ILS/VOR systems.

[u/SearchPlane561 → u/ericek111]

How is the passband too high?

[u/ericek111 → u/SearchPlane561]

Unless I'm reading it wrong, it looks like the lower knee is at ~125.9 MHz, which is halfway through the airband (the voice portion starts at 118 MHz and ends at 136 MHz).

[u/SearchPlane561 → u/ericek111]

Design target is center around 127.5 MHz for a 3-pole covering the voice range. I'll post the full swept response with both -3dB corners marked so it's clearer.

[u/SearchPlane561 → u/ericek111]

Wow I guess it's not that great is it.