Uncommanded 26 dB attenuation, SMA connector tightening

There is a software bug (v1.665 and earlier) that has been observed by a number of people now. The attenuator becomes set to about 26 dB of attenuation even though the slider on the user RF tab and admin page config tab all say zero. Worse, the problem persists across restarts.

I have reviewed the code and can't see an obvious issue. So the next release, v1.666, has a bunch of debugging added to log the problem so I can fix it. You can clear this problem by simply changing to attenuator to a non-zero value and then back to zero on the user RF tab slider (changing the admin slider won't fix the problem). If your Kiwi is public you might want to consider making the RF attenuator adjustable by "everyone" for this reason. Setting on the admin page, config tab. My apologies for this very annoying bug.


About SMA connector torque: Many of you will know more about this than me. But I'll tell you what I know.

You have to be careful with SMA connectors. Because the fittings are brass they have to be torqued tight enough to make a good ground connection on the outer threads. But you don't want to use a wrench that supplies too much torque because that will deform the soft brass and then the connector will be ruined.

So if you think signal levels from your antenna are down try finger tightening the threads a little bit more. But don't use a tool unless you're extremely careful. You can use the self-test function of the signal generator extension with the SMA-to-SMA cable that was supplied with the Kiwi to verify the Kiwi's RF front end is working properly. Procedure described here: http://kiwisdr.com/quickstart/#id-self-test

Hewlett-Packard used to ship a special SMA/SMB connector torquing tool as part of the service kits for their test equipment just for this reason. But it was very expensive if purchased separately. Hundreds of dollars! Anyone remember that?


  • Can you trust that thing not to over-torque though?

    An eBay search of "sma connector torque wrench" is kind of amusing. Triple digit prices down to double digits from China. Not clear if the China ones deliver the spec'd torque (0.3-0.6 N.m says https://en.wikipedia.org/wiki/SMA_connector)

  • I wouldn't trust a Chinese made torque wrench.

    It's unlikely that the KiWi will be repeatedly connected and disconnected as much as some other equipment.

    Slightly past hand tight is fine, and don't use a big adjustable wrench.



  • jksjks
    edited April 12

    When running v1.666 (or later) on your Kiwi-2, and you experience the "26 dB" problem, before using the workaround to fix the problem please email serial number and admin password to support@kiwisdr.com. Then I can look at the new log messages and try to find this bug. Thank you.

  • I worked in an HP microwave lab and as I remember the production line spec for torque was surprisingly low, only 7 inch-pounds. That was with quality connectors on the highest quality vector network analyzers as well as on swept signal sources. As time went on other connector types good to above 18 GHz came to the fore but I don't think they were tightened more.

    As Martin suggests, it's very easy to over-tighten these beyond that specification thinking it will make things better. It probably will only make things worse.

    Glenn n6gn

  • Has anyone seen the uncommanded 26 dB signal drop problem in recent software releases where I have additional debugging prints enabled? I'd sure like to find this problem if it still exists.

    Thank you

  • no problems on the signal drop anymore,

    but the antenna switch extension stopped working :-o

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