I need info about special PI networks, like one with all reactances 50 ohms - Page 1
Forum section: electronics / rf-microwave

author: LM21
author_rank: Regular Contributor
timestamp: June 10, 2025, 06:23:33 pm
content: I am searching for documents about PI networks, like one where every reactance is 50 ohms. I think there are others almost similar.I have read PDFs about PI network values, but I can't find those pdfs now. It is of course easy to use what ever values I like, but I want to use something less random values.I am putting together a board with some simple circuits for demonstrating NanoVNA, and PI LP filters/networks are good for that. 50 to 50 ohms, 50 ohms to 100ohms, and 50 ohms to 25, should be nice to measure. Google is of no help, it offers filters for old style mains power supplies. (Those with a bad power factor)

author: TimFox
author_rank: Super Contributor
timestamp: June 10, 2025, 09:53:46 pm
content: See (among others)https://www.allaboutcircuits.com/tools/pi-match-impedance-matching-calculator/Note that any calculation of a pi-network requires specification of aQvalue for the network, as well as the impedance values at each end (which can be equal or unequal).For many ham-radio applications, 10 or so was considered reasonable.Depending on the ratio between impedances, the requiredQmay not be reasonable.

author: radiolistener
author_rank: Super Contributor
timestamp: June 11, 2025, 02:28:54 am
content: Q-factor depends on working bandwidth:Q = 1/BWwhere BW is normalized bandwidth relative to center frequency:BW = bandwidth / FcenterFor example if carrier is 1000 kHz and required bandwidth is 10 kHz:BW = 10 kHz / 1000 kHz = 0.01Q = 1/0.01 = 100You can also use Q = Fcenter / bandwidth:Q = 1000 kHz / 10 kHz = 100

author: Andy Chee
author_rank: Super Contributor
timestamp: June 11, 2025, 07:02:12 am
content: I use this online calculator. It allows you to specify input impedance and output impedance.https://markimicrowave.com/technical-resources/tools/lc-filter-design-tool/Ooops, you meant PI-network. In which case, I use Owen Duffy's calculatorhttps://web.archive.org/web/20211206154224/https://owenduffy.net/calc/pi.htm(he took his main website offline, hence the archive version)The nomograms from page 34 onwards may also be of relevance, despite being a vacuum tube specific application note.https://w5rkl.com/wp-content/uploads/2014/04/Eimac-care-and-feeding-of-power-grid-tubes.pdf

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: June 12, 2025, 06:49:04 pm
content: The classic: Motorola AN267.https://www.nxp.com/docs/en/application-note/AN267.pdfGreetings,Rainer DG1SMD

author: LM21
author_rank: Regular Contributor
timestamp: June 15, 2025, 11:33:01 am
content: My thanks to all of you, but formulas and net calculators are everywhere. Today, I am a bit lazy and use SimSmith or SimNEC. But what I would like to have, is some analysis of the values.

author: LM21
author_rank: Regular Contributor
timestamp: June 15, 2025, 12:44:54 pm
content: I can't really explain well what I want, not in my own language, even less in english. But I think I have seen studies of PI networks, its limits and border cases. Google finds only rubbish like virtual money and such.I have Motorola RF databook from the year 1978, Its application notes are a treasure. And I have a lot of PDFs, but I could not find anything about this. Of course there are a lot matching networks and their formulas. So perhaps deeper info has been in a magazine, QEX maybe.

author: antenna
author_rank: Frequent Contributor
timestamp: July 02, 2025, 10:21:02 pm
content: you said 50ohm, are you looking for a half wave filter like those used in CB radio output networks?https://kitsandparts.com/PCBs/LPF/filter0.pdf