SWR protection schematic that compares FWD to REV levels?? - Page 1
Forum section: electronics / rf-microwave

author: Chris Wilson
author_rank: Super Contributor
timestamp: March 11, 2017, 12:57:39 pm
content: I am looking for schematics for a standalone SWR protection circuit. Most seem to just compare REV power to a fixed input voltage to a comparator. I want a circuit that works at all power levels, so it has to compare the relative levels of FWD to REV voltages. As by illustration say I am running 1 Watt QRP. A high reflected power may be a tiny voltage out of the bridge compared to a safe REV level when running 1kW. I am sure someone has addressed this before and built SWR protection that looks at the relativity between FWD and REF voltage levels, but I can't find much, and the one I have found is hand written and illegible, unfortunately. Please bear in mind I am a beginner so I need real schematics and not general ideas to create my ownIt may be easy peasy to you, but I need to build to a known working schematic at my level, although I do try and understand what's occurring! I have the bridge built and working.Thanks for reading.

author: rheb1026
author_rank: Regular Contributor
timestamp: March 11, 2017, 04:10:15 pm
content: I've had a similar project on my todo list. I came across the site for W6PQL which has a lot of good, well documented projects. This isn't quite what you're asking for, but it's at least a good starting point:http://www.w6pql.com/1_8_to_54_mhz_dual_coupler.htmThis should be able to give you a forward and reverse voltage that you can use in your own circuit. Hope this helps

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 11, 2017, 04:40:49 pm
content: Mind that your circuit is okay with large SWR, as long as it is inductive.Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: March 11, 2017, 09:05:17 pm
content: Thanks, but just run that by me again please. If the antenna plug is left out, what would the amp make of that? And the opposite extreme, the plug shorts out?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 11, 2017, 10:02:04 pm
content: Also, how close is the antenna to the amplifier?Open: probably not good, because of the energy stored in the choke. Same is true if gate drive suddenly falls. If the impedance changes slowly instead, then open is okay.Short: fine, if the supply is current limited (you need a constant current source to drive it, and no, a fuse isn't enough current limiting). If not, then |Z| must stay above the design minimum.Between those, the phase angle can be resistive to inductive without much consequence.Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: March 21, 2017, 01:18:42 pm
content: Sorry, missed these replies, I have been working away. Thanks for that, the aerial is within 20 feet of it, fed with a toroidal impedance matching transformer outside at the end of the co-ax, and then a huge loading coil inside a wheelie bin, that is variometer tuned, hopefully soon remotely as I am a bit fed up with running up and downstairs and then outside dozens of times peaking the damned brute upI am working on a current measuring device using an ACS712 Hall effect sensor and comparator to trip the driver IC abaove a certain level, but turn on current surge may be an issue to resolve? Thanks for the reply!For SWR I was thinking of bread boarding and adapting this circuit to try, as it compares the balance of FWD and REV power rather than just measures REV.http://www.n5dux.com/ham/files/pdf/Pocket-Size%20VHF%20SWR%20Meter.pdfNot sure whether it's worth it over the W6PQL one.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 09:36:12 pm
content: Hi,you could use a Bruene bridge with recitified outputs and compare them, like here:https://electronicprojectsforfun.wordpress.com/rf-module-gallery/swr-bridges/

author: Unknown
author_rank: Unknown
timestamp: August 16, 2018, 09:55:15 am
content: Hi, if you want to protect your finals then there is no need to calculate SWR, just prevent conditions that will blow them. Current limit the supply & measure the output voltage peak. Depending on the power levels involved the limiting can be as simple as a zener across the output device or an active ALC reducing drive levels. You can measure the temperature too for belt & braces.