I may need to measure reflected power from some PA at 868 MHz with a maximum power of 5W and there are readily available SMD directional couplers to buy. However, I found only 90 deg. couplers on immediate stock and I wonder if can I measure vswr (maybe combine two?) with them. Actually I intend to use in some protective stage that suppose to keep the PA safe.
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1Frequency of use, bandwidth required, Power levels on channel? We need more detail to help. – Kortuk May 06 '13 at 18:18
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869MHz, few MHz, 5W maximum – user2274696 May 27 '13 at 16:00
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http://www.minicircuits.com/pdfs/BDCN-20-13+.pdf SMT, 360 - 1000 MHz, handles up to 10 W, in stock, $3 each. – The Photon May 28 '13 at 14:44
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To determine the VSWR you only need to know the magnitude of the reflection coefficient.
If you know the input power and can measure the returned power, that is also sufficient to determine the VSWR.
Therefore there's no reason a 90 degree directional coupler shouldn't be used in a VSWR measurement.
Edit, to your comment, "I'm asking about 3dB couplers used usually for splitting or combining amplifiers.",
No, these are not appropriate for measuring VSWR because they don't separate the forward and reverse travelling wave effectively. A directional coupler can provide 20 dB or more directivity, meaning less than -20 dB of the forward wave interferes with your measurement of the reverse wave. Since you hopefully have a much less power in the reverse wave than the forward wave, you need this isolation to get a good measurement of the reflection.
The Photon
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I'm asking about 3dB couplers used usually for splitting or combining amplifiers. One input and two 3dB outputs each one 90 deg. apart. There are a lot on market, this one for example http://www.farnell.com/datasheets/78984.pdf – user2274696 May 28 '13 at 09:03