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comm tx causeing avionics fluctuations

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tomgoddard(at)ns.sympatic
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Posted: Mon Oct 08, 2007 4:41 pm Post subject: comm tx causeing avionics fluctuations

Does any one have experience with comm tx causing transient changes in their engine analyzers egt/cht and voltages in glasairs or other fiberglass airframes? As we continue to debug our panel this problem is now up for a solution. I have heard that it may be "antenna standing wave reflections"? Our vert servo on the TruTrak is affected during Tx as well. We assumed that the other systems being affected were due to the close proximity of the comm antenna coax and the aircraft charging system as they all run through the center console of our Glasair III for a short distance in close proximity. Maybe I am wrong and it is more to do with the Standing Wave Reflection?  

   

Our other bugs caused by the comm. Tx are:  

      1. transient voltage rise on both buses from 14.1 to 14.6 (this is a real voltage change measured by and external meter)  

      2. egt and cht values decrease by 300 and 100 degrees F respectively (these are obviously not real temp changes)  

      3. Ray Allan trim and flap indicators flicker (the trim and flaps don't actually move)  

   

The following is something Tim Hedding of B&C alternators sent me:  

   

1. Check the transmitting antenna systems for SWR. Check that the antenna, transmission line and transmitter output are of the same impedance and that the connections and ground planes are good. Use an SWR bridge or antenna analyzer to check the antenna system matching / efficiency.  

   

There must be some other glass airplanes out there that have seen some of these quirks.  

   

Tom  

   

   

   

Tom Goddard  

100   Miner Lane  

Grand   Pre, N.S.  

B0P 1M0  

9026702511 Cell  

9025427315 Fax  

   

     

  08/10/2007 9:04 AM

     [quote][b]

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Craymondw(at)aol.com
Guest

Posted: Mon Oct 08, 2007 5:42 pm Post subject: comm tx causeing avionics fluctuations

I have the same problem and have lived with it for sixteen years. The TX  needs to have a diode that runs from the TX wire to the autopilot. Doing that  shuts off the auto pilot when transmitting and will solve that problem.   



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