Today's Message Index:
----------------------
1. 05:40 PM - comm tx causing avionics fluctuations (Tom Goddard)
2. 07:51 PM - Re: comm tx causing avionics fluctuations (Charles Reiche)
3. 08:01 PM - Re: comm tx causing avionics fluctuations (Ron Quillin)
4. 10:07 PM - Re: comm tx causing avionics fluctuations (John Rippengal)
Message 1
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| Subject: | comm tx causing 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
9:04 AM
Message 2
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| Subject: | Re: comm tx causing avionics fluctuations |
As impossible as it sounds I once installed a noise free wx-1000+ system
into a glassair that was already well thought out and had a single point
ground very close to the battery negative lead. Try and find someone
with a MJF or other make antenna analyzer and check that the ground
plane and antenna connections out of the back of the radio case look
like a good low SWR antenna. Also if this is transmitting into rg-58
antenna coaxes, you might think of upgrading the whole ship with rg-142
or rg-400 double shielded coax. This would help greatly if the real
problem is RF power leaking out of a poorly shielded coax. You could try
shielding the EGT box with MU metal if thats where it is being affected.
It might be coming into the box on the lead wires themselves. Check
your engine mount to grounding system resistance. Everyone will tell you
that idea numbers are 3 Milliohms but thats really hard to achieve in
the amateur built community, crap even the production stuff with
foberglass and mesh ground planes like cirrus or socata (the roof).
Good luck
Charles Reiche
----- Original Message -----
From: Tom Goddard
To: avionics-list@matronics.com
Sent: Monday, October 08, 2007 8:38 PM
Subject: Avionics-List: comm tx causing 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
Message 3
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| Subject: | Re: comm tx causing avionics fluctuations |
At 17:38 10/8/2007, you wrote:
>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)
This still may not be a real change. Meters, especially inexpensive
ones, in an RF field can rectify the RF energy and add it to actual
DC voltage as displayed.
One would not expect to see a rise in bus with additional load
applied, unless perhaps there was some disturbance to the regulator.
Same observed change with engine/alternator/generator off?
Do you have a 'scope available you could use to monitor the bus(s)
and see if you observe a change?
Ron Q.
Message 4
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| Subject: | Re: 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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