Re: uBitx v5 issue

Evan Hand

Thank you for the feedback.  It is a good summary of the data that has been presented in this forum as far as I know.  

No one has yet published any data on either the susceptibility to high RF (what is the actual threshold before failure), on the effect of the diode on Q90 to transmission, or back to back diodes on reception. 

There was concern about the reverse diode on the base of Q90 having a negative impact on the SSB transmission.  Has anyone tried that?  If so, it would be good to know if it works with no adverse effects.  

I have done the back to back diodes on the input to my v4 board and have not noticed any negative effects on the receive capability.  I have NOT done any significant testing to verify the before and after results of the diodes, so this is not conclusive of this being a good solution other than one that works at my QTH where the noise floor is very high.  The high noise could mask any of the potential problems that Farhan points out.  NOTE: the noise issue is on all of my rigs that include an Icom 7300, Hermes HL2, QCX+, and an RSP1a as well as 2 v4 and one v5 uBITX.  I have only modified one of my v4 boards to test the effect of the diodes.

I have not had the Q90 failure problem, mainly because I very rigorously only connect one HF rig to an antenna at a time (I do have three HF antennas up).  I do have a 50 watt Kenwood TM-D710G 2m/70cm that I do use at the same time, and so far it has not destroyed any of my uBITX Q90s.  I have also tried to measure the signal strength of the VHF/UHF antenna to the closest HF antenna, and could not get an effective measurement.  I tried that with my oscilloscope.  The sensitivity is only 2mv/division (1x probe setting).  That should have been low enough to measure any signal large enough to overcome the 0.6volt diode threshold.  The measurements would be specific to my QTH and antenna placement.

To sum up my opinion at this point; without more data, I would do one of the diode protection schemes only if you will be using the rig near other high RF transmitters when connected to an antenna, or if you do experience a Q90 failure.  I am still leaning towards the back to back diodes as that is switched out when the uBITX goes into transmit.  This could be modified to a single diode connected as to protect the base to emitter junction, just installed at the switched point like the back to back diodes.

As a side note, the back to back diodes was cited on this reflector (sorry, I do not have the original post), and lead me to further internet research on my part.  I found that the scheme (with variations) have been used to protect receivers quite often for the exact same reason: protecting sensitive components connected to the antenna from strong RF fields.  Here is one of the articles that I found:
and another
Both use variations on the back to back diodes.  These are only two of the references that I found.

Jerry, again thank you!  Also thanks to Iz and Farhan for sharing their thoughts.  This is how I learn.

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