Homebrew PWR/SWR Meter: Part 2 - Directional Coupler Idea

Video looking at an initial idea for a directional coupler. Please note that this is not suppose to be a competition or commercial grade device. My aim is to keep the costs down, but still have a functional power/SWR meter at the end. The aim now is to condition the DC before feeding it into the Arduino.
I did not mention in the video that I placed the directional coupler between the antenna tuner and the dummy load. I did not see any increase in the tuner's SWR meter as a result of the directional coupler.
Details at: zl2ctm.blogspo...

[@genetomblin2883]

The 1954 ARRL handbook shows construction of a coax directional coupler like this one.

[@davelapp4740]

Charlie, Thanks for the interesting video, as usual. I had been reading about using op amps to make a log amp as well. Glad to hear you might experiment and show how well they work ... or don't. One thing I'm sure you've seen is mention of using a BJT instead of a diode in the feedback path. I'd be really interested in a comparison of the 2 approaches.

[@CharlieMorrisZL2CTM]

Cheers Dave. I will endeavour to look at both. The BJT is suppose to provide greater range, so that would be useful.

[@radicalradiotech4944]

Hi Charlie! Why Why Why does someone always do what i thinking was of doing?? HAHA no problem! as i told (you if you recall) i have used your software to provide a wide VFO for my Druzhba russian Kit. To that end your software is very very different now, its 10 bands, usb/cw/lsb. the bottom line i have programmed to be options for S meter , dBm and Batt volts , and on Tx Fwd power, Rev Power and Batt Volts. Soo I've been researching various dir couplers for my project. I looked at math with in the arduino .. hmm on my calculator ( HP42s) i couldnt get good results from the resolution. I thought ok an Array. Compactness i think for me is a important.. for the coupler, anyway i have only seen part one of your adventure i will watch the rest with trepidation.. TNX Charlie.

[@CharlieMorrisZL2CTM]

Hi Ian. Sounds like you are making great progress on your project. I m about to embark on an analogue VFO for something different. Hopefully, I can make it nice and stable.

[@ashave9100]

Hi Charlie-thanks for the new project Do you think that a NE3916-N bargraph ic could be used instead of meters ? They are supplied in cheap kits from E-bay for u/v meters, but maybe could be adapted to form a millivolt meter the leds could be changed to different colours (10) to show relative power out.

[@CharlieMorrisZL2CTM]

That's certainly an idea. I'll jot it down, but will stick with the Arduino for now. I like the idea of an optional audio out too.

[@adrian_sp6def]

I do not get one thing. Why this two almost identical pickup circuits mesure forward and reflect power. How this works? At first glance it should output same voltage but with inverse sign. How it sense forward and reflect part of RF? Additionally is it that You introduce na 25 ohm impedance on the path from in to out by connecting two 50 ohm coaxes in parallel? This is so interesting that I can not wait to try it myself. But to be honest, having knowledge how this works is much more important to me, than having working hardware. Thanks a lot. 73

[@bundylovess]

Looking good Charlie big thumbs up 👍

[@CharlieMorrisZL2CTM]

Cheers Alan. The diode based op amp log amp works really well (well at least so far).

[@moustaphamuhammad7013]

Wow really Wow.t that's the spirit

[@robertbobbitt8448]

Just one problem Two pieces of 50 ohm in parallel is 25 ohm

[@raafatalheety826]

Good morning

[@CharlieMorrisZL2CTM]

Hi Raafat. I hope things are going well for you.

[@berndb.5097]

Hi Charlie, In many of my refurbished SWR Meters I found 100 or 150 ohm resistors in place of the 50 ohm resistors. Is it absolutely necessary to use 50 ohms? If the resistors are high-impedance you also get a higher measurement voltage. The two resistors only have to be equal. Or am I wrong? 73 Bernd

[@CharlieMorrisZL2CTM]

I have also seen 150ohm Bernd. I tried a range and didn't see any great difference. In the end I kept the 50ohm resistor as I got the impression from the tests that the directional coupler was around 50ohm and was not introducing an impedance mismatch into the transmission line. I'll repeat it again tonight with 150ohm and update this comment.

[@CharlieMorrisZL2CTM]

Bernd. I have just repeated my tests and you get a good 1V improvement with a 150 ohm terminating resistor. I also couldn't detect any adverse SWR when i cranked the TX up to full CW power (should be approx. 100W). That being the case, I'll keep it at 150 ohm, which is in keeping with similar commercial meters. Thanks again for the prompt. Charlie

[@tinkerman9525]

Hi. About a year ago i came across a article with the maths about the resistanc of the resistors. For 50 ohm measurements you need 150 ohms and for 75 ohm the resistance must be 100 ohm

[@vicenteponsrequena6427]

Is valid for U-VHF swr ??

[@Frisky0563]

Hi Charlie do you have any schematics for a 6m amplifier radio output is about 1watt. I would like to amplify power to 20watts this will be used in FM mode 12volts dc supply. Thank you Don

[@CharlieMorrisZL2CTM]

Hi Don. No I don't sorry. I tend to stick to the lower HF bands. If that changes I'll let you know. Charlie

[@Frisky0563]

Ok thanks I just bought a uBitx kit going to build it into a manpack radio.

[@CharlieMorrisZL2CTM]

@@Frisky0563 Nice. Sounds like a great project.

[@gogorowokan1056]

goodd

[@CharlieMorrisZL2CTM]

Thanks Reno.

[@gogorowokan1056]

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[@kcnicely]

I suspect the logarithmic amplifier is going to be needed, especially for the reflected power since that is typically some fraction of the forward power. I had never really researched using an op amp as a logarithmic amplifier, but a quick google search turned up this article which I found helpful to understand how it works. www.electronicshub.org/operational-amplifier-applications/ I am looking forward to hearing your progress.

[@CharlieMorrisZL2CTM]

Thanks Ken. I have quite a good reference document from Analog Devices too. Its MT-077 Log Amp Basics.