Measuring 2.4 GHz SWR using a directional coupler

Oct 28, 2015 4 Replies
J

Measuring 2.4 GHz SWR using a directional coupler.



SWR meters for those frequencies are normally expensive. There is a way to do good measurements using a directional coupler. I got myself this one for 9 $:

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A simple BAT diode detector is easiy made:



1/2 BAT15-099

---|>|-------------------- | | |/ [ ] === O Volt meter | 1k | 1n |



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This 2.4 GHz linear:

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and a normal WiFi access point for test.



Good SWR (not much reflected):

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And with a metal object disturbing the antenna:

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By reversing in and output from the directional coupler you can get the forward voltage, and then you can calculate the SWR.



Here the method is described in more detail:

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Most expensive and most complicated part is the connectors, seems everybody uses something else...



So, some giggle Hertz tronix for the day.


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J

Oooooh. I want one or two. Thanks.

Nicely done. I prefer a bridge arrangement as in: I've built a few of these that work well. I suggest that you build yours so that the diode is easily replaceable. I keep blowing up mine with static electricity. PCB construction is nice, but just soldering the parts together in the air is good enough (if you have a good eye and steady hand).

I also use a return loss bridge: and an RF sweep generator.

Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
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S

AD has a nice 2.7 Ghz Chip for this, so you can see gain and phase...

Ham use:

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Steve

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J

On a sunny day (Wed, 28 Oct 2015 07:51:39 -0700) it happened Jeff Liebermann wrote in :

Do not open it, was full of what could be Beryllium dust, beryllium resistor inside? maybe it was just ceramic, but I could imagine a Chinese workshop.. Construction is nice tough, thick metal strips hold apart by some plastic type spacers.

But, but, that was also in the picture:

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Circuit diagram:
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PIC code in asm:
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That one needs some more work.

This is what it looks like after several diode replacements:

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The 50 Ohm SMDs are 2 x 100 Ohm on top of each other.

I have blown up about 3 BAT diodes now, using my Baofeng at 70 cm 2 W / 4 W. It is really a low power device, that is why I got the directional coupler, now I can live monitor SWR (reflected).

One interesting thing I found, when using the Raspberry RF out (can go to several hundred MHz)

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I consistently got strange readings. Finally realized that is a _square_ wave of several hundred MHz with harmonics into the GHz range, and reflections happen even in very short connection cables.

Interesting, had not seen that before.

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R

Use a complete directional coupler; a meter on both ends - not terminator on one end and meter on the other (like you have now).

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