SWR protection schematic that compares FWD to REV levels??

[Chris Wilson]

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 own It 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.

[rheb1026]

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.htm

This should be able to give you a forward and reverse voltage that you can use in your own circuit. Hope this helps

[T3sl4co1l]

Mind that your circuit is okay with large SWR, as long as it is inductive.

Tim

[Chris Wilson]

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?

[T3sl4co1l]

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

[Chris Wilson]

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 up I 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.pdf

Not sure whether it's worth it over the W6PQL one.

[Wolfgang]

Hi,

you could use a Bruene bridge with recitified outputs and compare them, like here:

https://electronicprojectsforfun.wordpress.com/rf-module-gallery/swr-bridges/

[Z80]

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.