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Old May 26, 2011, 05:25 PM
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5.8 Ghz SWR meter

I've started building a 5.8Ghz SWR meter with the build log here:

http://www.radialmind.org/projects/58-ghz-swr-meter

Schematics will be added soon, but it's a simple 50 Ohm termination on both ends with 2x schottky diodes inspired by the Marconi LNB diode setup as in the link in the article. It seems no one has vested any interest in 5.8 Ghz SWR's, as there are no well-explained articles available on the internet.


The problem is that it is not working.... yet!


I opted for a coax-based directional coupling and merged the coupling lines, merged the transmission lines and then soldered SMD components on a couple of manually cut tracks on a PCB.

I measured everything through and there are no shortcircuits. The DC resistance values make sense too. When measuring resistance on a multimeter wtih the potmeter at 0, the current meters immediately go to full whack. Changing the locations of the probes a bit and measuring again and getting the same result tells me that the circuit should be functional.

Anyone see any obvious failures in this design? Schematics to be posted later.
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Old May 26, 2011, 06:57 PM
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Radial,

Shouldn't your bi-directional coupler be split into two, one in each direction with separate dummy loads on two pieces of coax like in the diagram (Dual directional couplers)...

http://www.microwaves101.com/encyclo...alcouplers.cfm

The article flippantly suggests this is best but gives no explanation why, this is the way I would do it,with two separate couplers and loads.

A.
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Old May 27, 2011, 01:24 AM
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Hmm... right. I guess mine is more like the bi-directional coupler. The problem is then probably that I did terminate mine on both ends, which, from the looks of the picture, shouldn't be done in that case.

Even without the terminators it wasn't yet working, but I'll try again. This probably leads to the recommended design eventually of separated coupled coax's and proper termination on either one. Another thing I'm suspecting is the distance between the cores, which now has a bit of glue there. Possibly a better way is to use some fishing line or similar to pull them together more.

To be continued...
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Old May 27, 2011, 04:01 AM
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I've looked at the pictures you took and I'm not sure why there needs to be anything other than a resistor at the load end of the coax. I've only used waveguide couplers but my understanding is that there's a section of loosely coupled transmission line to the main through-line and one end is connected to a resistive load to match the characteristic impedance of the cable etc. The other end is taken away to a detector of some sort which needs to have a diode capable of rectifying at the frequency in question.

A.
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Old May 27, 2011, 10:16 AM
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Radial,

I think this is what you were trying to make, the article only covers frequencies to 150MHz but I think the principles are the same, one interesting difference is the secondary transmission line is grounded at its centre...

http://members.aceweb.com/n6mm/tech/matchbox.htm

A.
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Old May 27, 2011, 05:15 PM
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I actually based the coupling on this one here:

http://www.frars.org.uk/cgi-bin/render.pl?pageid=1085

It may be a bit of a cheapo solution, but I was getting concerned about getting different readings, since the cores may have different distances to the center. My intention was to make this a relative SWR meter. So the forward A-meter is just turned up to full whack and then I just read the other meter to get the SWR reading.

Today I resorted to the use of the non bidi coupling, as it is recommended. So two coupled ports with double HF schottky diodes and a current meter between there with 10K resistor and 1nF capacitor. That means two coax's with 50 Ohm terminations at either end. Good news is that one end is apparently working ok, the other still doesn't, because its termination resistor died.

SMD resistors don't like being relocated on PCB's. You get the body off, but then notice the little conductive side on them was left behind in the solder bath where it used to be....

To be continued...
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Old May 27, 2011, 05:45 PM
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Do you know the diode type you are using?

A.
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Old May 28, 2011, 01:42 AM
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Schottky BAT62 - SOT23:

http://shop.conrad.nl/componenten/co...on/154053.html

http://www.produktinfo.conrad.com/da...iode_BAT62.pdf

I know the forward voltage threshold and the current it lets through then is also critical. I tried to find a diode with a low forward voltage, reasonable forward current. I'm not so much concerned with reverse voltage.

If you know of a diode that performs better then I'm all ears of course.

This diode is not the best choice because it has 4 pins. In the soldering phase though, I took care not to connect the other 2 pins. (I actually ordered the SOD323, but they gave me these and I'm not going to send this back on a $1 envelope .
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Old May 28, 2011, 05:26 AM
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The BAT62 claims it works into the GHz region but doesn't specify how far it goes. You might better sensitivity using something like this...

http://www.farnell.com/datasheets/461166.pdf

A.
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Old Jun 08, 2011, 02:46 PM
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I checked in other locations. The BAT62-03W does seem to make it up to 6Ghz.
Someone else helped me out in the analysis. I'm getting 4mV at the pins of the ammeter with the potmeter to 0. I measured the ammeter through and its internal resistance (100uA) is 1kOhm. With that kind of voltage, that's a bit high. Ideally, this would be 40 Ohm or lower. I need at least 0.1V now to get 100% deflection. At the moment, with 0 Ohm at the potmeter and 200mW transmitter, I'm only getting 4uA deflection.

Any tips here?
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Old Jun 08, 2011, 05:02 PM
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Is that 4mV the forward or the reflected voltage? Either way you are probably going to need an amp of some kind. The way your diodes are shown, you will need a +/- supply. If I were doing it my first inclination would be to power it with 2 AAA cells and use an amp similar to this. http://focus.ti.com/lit/ds/symlink/opa2369.pdf
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Old Jul 15, 2011, 05:28 AM
um.. yeah, I can make that fly
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Any progress guys? I'd love one of these to tune my 5.8ghz cloverleaf and helical antennas.

CJ
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Old Jul 16, 2011, 04:26 AM
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This setup is currently on hold. The coupler is the critical component in this setup. If I can get my hands on a decent 5.8 GHz directional coupler (bidi even), then I can continue. Unfortunately, they seem to be really expensive. I'm also considering making yet another coupler, but then stripping away just a bit more of the internal isolation. There is currently about 1mm or slightly more of distance between the wires. Another idea would be to make this coupler on a piece of PCB by etching it out very carefully.

As for the antennas, I just bent them in their shape and just hope for the best. If the angles are right, they seem to perform pretty good. I have not yet done a range test on them.
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