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1. 10:58 AM - Re: comm tx causing avionics fluctuations (Roger Roy)
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Subject: | Re: comm tx causing avionics fluctuations |
John I'll sing that praise forever. I was having a lot of problems same
as a lot of what's being talked about with a MicroAir M-760 I went on
Ebay and got a brand new one for $14.00 and was able to trouble shoot my
faulty BNC at the antenna in minutes, that saved me hours of
troubleshooting the system, so a SWR meter should be in every
homebuilder tool box.
RJ
----- Original Message -----
From: John Rippengal<mailto:j.rippengal@cytanet.com.cy>
To: avionics-list@matronics.com<mailto:avionics-list@matronics.com>
Sent: Tuesday, October 09, 2007 1:05 AM
Subject: Re: Avionics-List: comm tx causing avionics fluctuations
You have been given good advice by Tim Hedding; why don't you follow
it?
Anyone installing their own radios should have a cheap 2 metre band
VSWR meter and use it.
The other remote possibility is that you have your antenna in the
wrong place. The only place for a comm antenna in a plastic aeroplane is
in or near the tail or in the winglet in canards.
I take it also that all your negative power grounds are separate wires
from each instrument and they go back to a single ground buss near and
connected with a heavy lead to the battery/alternator ground. Similarly
the positive 12/14v leads are run (via a breaker) from one central
positive buss also near the battery/alternator. And finally that the
positive and ground leads to each instrument run together.
John
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
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