Nick M0NTV designs, builds and tests a switching circuit for two RF band pass filters. This follows on from the last video about the construction of the filters themselves.
Nick, I think your videos have now become a gold-standard for beginner radio builders. For me you absolutely hit the mark for pace of delivery and level of detail. I can't help wishing that all this material were available in a book. I realise that would require a lot of work for a potentially quite small audience. I wonder if a GQRP member could collaborate. Best wishes and as always, thank you!
Thank you so much Stephen for your very kind appreciation. I'm pleased if some folk find this helpful. I suppose in a way I'm making the kind of videos that I wish I'd seen when I was starting out. I watched some brilliant stuff but at the time most of it was way beyond where I was at. The idea of the book is an interesting one. Perhaps that is something I can consider as a future project. Thanks again. 73, Nick
Great work. Sometimes leakage occurs when the individual relays are fed from the same supply. It's possibly noticeable with the high rejection in crystal filters, probably not with LC filters unless they have many poles and higher rejection. I have seen cross coupling between LC filters. The response had a little nurgle at the other frequency. It was coupling between toroids. Placing a shield between the filters cured it.
Great explanation Nick. I tried 5v relays but went to 12v relays, which consume the same power but draw less current. In a battery powered transceiver, its the milliamps you want to watch. It wasnt a big difference, 10 to 15mA per relay, but it all adds up. Theres a practice of DC whetting relay contacts that carry RF, the DC kind of burns off impurities and makes the contacts stay clean for longer. I decided that this was probably a problem limited to older open air relay contacts, and that modern sealed miniature relays would be much less prone to contact plaque. I've never had a relay go noisy. Maybe in 25 years time. But most homebrewers dont keep using their rigs anywhere near that long. 🤓 73 Paul VK3HN.
Thanks very much Paul. Yes, relays definitely have a shelf life because of the moving parts but they'll probably be fine for me. I'll have moved on to the next rig by then! Hope you're doing OK. 73, Nick
Nice! Surrounding the relay with copper tape may improve rejection and help with impedance control. Another interesting way of doing it, is having the relay laid on its side, with a copper strip around it, soldered to gnd at both sides.
Nice review of relay tech... But only two relays! I used to have a lot of fun building automatic test gear with relays, I think the record was 43 relays. You could even get it to play tunes with the contacts clattering in and out. For lots of relays there are some chips that you can use that provide the drive and interfacing, which saves a lot of messing about. But there is nothing wrong with a couple of transistors for simple circuits. Remember the 2N7000 mosfet as an alternative if you need it. Andy
Thanks, Nick, for another great video! I'm really glad you include a learner's section; I knew about relays but not much and I'm sure there are plenty like me. So, if the relay is drawing current, even minimal, I suppose you would put the band you use most on the Normally Closed option? There is probably minimal difference but if power consumption mattered, say SOTA or POTA, would this be a good way to go please?
Hi Ace, Thanks very much. Yes, I'll be putting 15m on the NC and 10m on the NO. If current draw is an issue then you can get latching relays which only draw power when they switch but not once they are switched. These are more expensive but very useful if you are building a portable rig. 73, Nick
Hi Nick. A random thought ... 10M & 15M are not harmonically related ... was there an option to parallel the two filters and not switch them?? Please don't think I'm criticising (I'm not), just thinking out loud about options and simplicity.
Hi Nev, no worries. No, in fact I tend towards the other extreme - filter, filter and then filter some more! I really don't want anything but the signals I want going into an amplifier, much less a mixer. The best rig I've ever built (The Optimiser) has at least 2 sets of band pass filters plus low pass filtering. The switching adds complexity and a little loss but means a much cleaner signal on RX and TX. Have a good weekend. 73, Nick
@@M0NTVHomebrewingHi Nick and Nev, I think you'd be OK to parallel 21 and 28MHz BPFs, as long as you could get the coupling in and out to match. There's not as much spectrum energy at these higher HF frequencies, and unless you have a neighbour who operates these bands, I think it might work. A resonant antenna also contributes to filtering unwanted RF energy. I probably wouldn't try it lower down, say 60 and 40m. That said, I've always done these things conservatively with a separate BPF for each band. 73 Paul VK3HN.
Hi Paul, I confess it is not something I've even considered. I prefer a more 'belt and braces' approach as you say with a BPF per band. Interesting thought though and thanks to Nev for raising it. 73, Nick
