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[2E0ILY]

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.

[VK3BL]

Take a look at the circuits for open source antenna analyzers.  They all use very low power, and should give you some ideas.

I understand the desire, but you might find it hard finding a standalone circuit as QRP level reflected power isn't going to damage any of the commercial tramsceivers.  The Yaesu FT-817ND has a notoriously inaccurate "SWR Meter" for that very reason.

Perhaps ask in the homebrew forum - they tend to be QRP Guys.

Still, I doubt reflected power would damage any "PA" at those power levels, ao unless its for measurement purposes rather than protection I encourage you to question your need.

Reflected power kills PA devices due to voltage breakdown not dissipation, so if you chose an appropriately specified final (which should be very easy and cheap) you won't need any protection full stop.

[2E0ILY]

Hi Jarrad, the amp it will be used with is my home brew 1kW + at 136kHz, so not QRP unless run on a lower than 50V FET voltage. But I still would like Reflected trip to be a percentage of Forward power, not just an arbitrary fixed figure. Thanks for the reply

[VK3BL]

My mistake - on my iPhone atm.

A trivial modification of W6PQL's directional coupler should suffice then.

http://www.w6pql.com/hf/bandswitchsensor.htm

I'd email him if you can't figure it out, but all the details you'd need to accomplish your goal with that circuit shouls be on that page.

Cheers,

Jarrad

[W1VT]

http://www.n5dux.com/ham/files/pdf/Pocket-Size%20VHF%20SWR%20Meter.pdf

Clever hams have used IC chips like the LM3914 that compare the ratio of two voltages.

Zack W1VT