16 - The Dark Art of the Band Pass Filter

[@acestudioscouk-Ace-G0ACE]

Great video, well explained as usual and I'm beginning to think I could actually manage some homebrew. Glad to see you settled in your new home and hope you have many happy years there.

[@M0NTVHomebrewing]

Thank you very much.

[@boxingday11]

Good to see you back 👍🇬🇧

[@M0NTVHomebrewing]

Thanks very much.

[@tdumnxy]

Very good video. I find all your videos helpful and clearly explained. Thanks.

[@M0NTVHomebrewing]

Thank you very much Stephen. Glad you enjoyed it. 73 :)

[@w1rcp]

I've built two filters using Elsie, and holy crap my insertion loss is terrible. I keep fussing with them, and it gets better all the time, but I'm shooting for a 3-pole design that I haven't been able to achieve sub -3dB insertion loss yet. It's a dark art for sure! -W1RCP

[@M0NTVHomebrewing]

Keep going! 73, Nick

[@TheTemporalAnomaly]

I`m getting to be a regular here, I do like your work. Very well done. I have however spotted a mistake on the diagram (time 6:49), that will have newcomers chasing their tails if not corrected. The hot ends of L2 and L5 should not be connected as transformer action here is needed to match 50 ohms. Likewise L1 and L2 hot ends should not be connected either. Also I recommend low loss variable caps making up some of the capacitance in the tuned circuits to allow for fine adjustments. I have also been known to `cheat` with C2 using twisted teflon wire as a `gimmic capacitor.` Complements, Chris. UK.

[@M0NTVHomebrewing]

Thanks Chris, I realised this quite early on so there is a bit marked 'IMPORTANT' in the video description that highlights it. Thanks for watching. 73 Nick

[@radiotec76]

Nice presentation! I’m looking for bandpass filter circuits that can handle 100 watts full duty cycle. Should just a matter of using toroids, capacitors and wire sized to handle the increased voltage and current.

[@M0NTVHomebrewing]

Hi! Thanks for watching. It depends where in the signal chain your bandpass filters are going. Usually they won't need to take a lot of power - unlike the transmit low pass filters that will need, as you rightfully say, components that can handle the larger power. In my latest rig (which puts out just over 80W at full power) in the transmit low pass filter I used T68 toroids and mica capacitors (good for 500V). You'll also need to be mindful of shielding (I put mine in metal tins) and your coaxial connections - use good rf connectors and coax that will take the power. Thanks again. All the best.

[@BShocked]

Very useful! I found similar schematics online for band pass filters. I'm always confused about what rating components to use though. If I want to operate at 100w how do I know a component, part, or wire can handle that sort of power?

[@M0NTVHomebrewing]

Thanks. Check out my reply to radiotech76 below. Basically for 100W I would use a T68 toroid and high voltage caps (e.g. rated 400-500V) + the thickest wire you can get away with. You could probably get by with less but I'd rather build in a bit of headroom. Hope this helps. 73 Nick.

[@alanmacdonald65]

Great video as always Nick! M0VLT

[@M0NTVHomebrewing]

Thanks mate :)

[@batica81]

Great video! I want to build one of those and use it with my cheap sdr receiver that has no frontend filtering. Idea is that it will keep very strong local interference away and give me much better reception on a specific band. Do you think that may work?

[@M0NTVHomebrewing]

Hi Vojislav, thanks very much. Yes it should work very well for that. As long as your SDR input is expecting a 50 Ohm impedance - which is what my filter was designed for. You can of course design a filter to match different impedances if necessary. Happy homebrewing! 73 Nick (M0NTV)

[@renuabel]

Have you got the ferrite core part number, cannot find any...

[@M0NTVHomebrewing]

Hi Abel, the toroids are T37-6. (Note they are iron dust and not ferrite cores). Hope this helps. 73 Nick

[@davidbrodie5044]

I've been making filters recently, both using Hans kits and freestyle from his published data which have worked well - I do need to get mine boxed up like yours to stop stuff bypassing the filter by radiation/coupling. Thanks for the vid, very useful.

[@M0NTVHomebrewing]

Thanks very much David and well done on your own filter construction. 73

[@paulchapman603]

Why does the schematic appear to show primary and secondary of both transformers in parallel? Is it a mistake, or some standard I'm unfamiliar with?

[@M0NTVHomebrewing]

Hi Paul, yes that is a mistake on the schematic I'm afraid. It is documented in the video description. The transformers should be wired the conventional way and not with their signal lines shorted together! 73 Nick

[@twobob]

as a musician. such filers are obviously second nature. Might I ask is it possible to create a (or maybe a stack? of) low and high pass filter(s) and just have no losses on your wanted band?? Maybe I'm being stupid - in music I would do that???

[@M0NTVHomebrewing]

Hi! You can stack high and low pass filters but unless your wanted pass band is very wide indeed I don't know why you'd bother - that's precisely what a band pass filter is for. Also there is no such thing as a lossless filter in the real world. Every filter you insert will have some attenuation upon your signal. However, if you build your filter carefully then these losses will be acceptably low and you can compensate for this by adding gain (i.e. amplification) in other places. Hope this helps.

[@twobob]

@@M0NTVHomebrewing it makes perfect sense. Many thanks :)

[@donepearce]

How is that supposed to work? It isn't an Altoids tin.

[@M0NTVHomebrewing]

I know! I had to buy those tins without even getting any mints inside. It's never right!

[@kp4md]

A curiously strong filter. Well done! de KP4MD

[@M0NTVHomebrewing]

Thank you!

[@frequencywatchers]

Flor Gluing These One Sided Copper Pcb Material I Use Soda And Super Glue, WORKS WONDERS And You Will NEVER Get Them Appart

[@M0NTVHomebrewing]

Thanks for the tip! 73 Nick

[@JimSky]

The schematic is wrong. The transformers are drawn as parallel inductors. Thanks

[@M0NTVHomebrewing]

Hi Jim, the second sentence in the comments (marked 'IMPORTANT') makes this clear. Re-recording with a corrected schematic was not an option at the time. Well done for spotting it though! 73 Nick

[@JimSky]

@@M0NTVHomebrewing Thanks OM, I didn't see the correction. Enjoying your projects. 73

[@M0NTVHomebrewing]

No worries Jim. Thanks for watching. 73 Nick