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Scalar Network Analyzer question: Cable requirement

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noobler

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My measurement is in Ka band. Should I use a Ka band cable from SNA RF out to RF in of the directional bridge? I know its a bit expensive but is it a requirement?

Thanks in advance. :wink:
 

tony_lth

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I don't think so, because you are using scalar network. Just try to find some good cable, not so good as the VNA used. It's about 1,000 RMB in China. It's some plastic cable, can't be operated in the high or low temperature.
 

biff44

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I wouldn't go crazy with the cable. I would pick one with fairly good VSWR, so there is not too much ripple vs. frequency. A scalar can not readily correct the fast ripples, as they can change depending on what you are trying to measure, and if they chage your scalar calibration is no longer going to correct for the ripples.

But there is no need for the lowest loss cable....a couple of extra db of insertion is tolerable, as long as it is not rolling off very quickly with frequency.
 

noobler

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Thanks for your inputs guys. Not much concern on the loss, but on frequecy; so if I use a low freq cable, say less than 5 GHz from SNA RF out to bridge's RF in, is not an issue if my DUT is in 30GHz+ range?
 
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tony_lth

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You can try. And keep the cable fixed, i.e. keep cable shape fixed between calibration and measurement. Otherwise you can try to change cable position when you measure, you will see the result changed a lot.
 
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biff44

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In my lab I have some expensive cables that work great at Ka band, I have some cheap cables that would have 15 dB of loss at Ka band, and I some cheap cables that work ok up there (maybe 5 dB loss). What you should do is go thru your cheap cables, measure insertion loss, and use the one you find that works the best!

At some point in frequency the SMA connectors themselves will stop working, because the outer diameter will be too big and they start to multimode. If you see sudden spikes in insertion loss, that is probably the cause, and you will have to switch to cables with smaller connectors like 2.4mm, etc
 
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