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1. 01:56 AM - Re: comm tx causing avionics fluctuations (jetboy)
2. 06:54 AM - Re: comm tx causing avionics fluctuations (Noel Loveys)
3. 08:48 AM - Re: comm tx causing avionics fluctuations (John Rippengal)
Message 1
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| Subject: | Re: comm tx causing avionics fluctuations |
Apart from the previous advice: antenna far away and SWR checked,
you may need to fit ferrite beads to some of your instrument wiring. Try the clip
over style from electronics parts stores, particularly around temp. sensor
cable at instrument end and far end.
Also better to stay with normal grade coax, the greater loss actually helps the
VSWR match and lowers the power radiated into your instruments.
A similar method is used in many certified com antennas, a 12.5 ohm resistor is
built inside the base so you loose about 2 watts power ( not significant in radio
terms) but get a good 50 ohm match over the relatively wide com range.
Ralph
--------
Ralph - CH701 / 2200a
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=138927#138927
Message 2
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| Subject: | comm tx causing avionics fluctuations |
Tom:
Just a few Ideas.
Just as the radiating element of your antenna should be 1/4 wave length
the
ground plane should also be at least that size. If it isn't don't worry
you
can stick a ground plane inside your fuselage with contact cement using
aluminium foil for the ground plane itself. Make sure it is connected
to
the braid side of the coax at the antenna end. The ground plane should
also
be as close as reasonably possible to perpendicular to the radiating
element.
The advice you got to check out your antenna system sounds good to me.
Transmitters aren't too bright and when you feed their output to a
miss-matched antenna system the fool transmitter works harder to get the
signal out. This means it draws a lot more current and of course heats
up.
The end result can be a burned out final amplifier (expensive) or a
popped
circuit breaker. (Cheap if you don't reset it)
Most of the other things you mentioned can be easily attributed to the
same
antenna problem. High current draw by the transmitter will cause the
voltage regulator to cut in possibly to the point before an over voltage
indication.... BTW you generally get that OV indication after the
alternator has been shut down to protect it.
It just crossed my mind that I have seen high draw on a transmitter. It
occurred when there was a crack in the centre conductor inside the coax
to
the antenna. Do a continuity test on the centre conductor of the coax.
When you do that continuity test try wiggling the connectors while
hooked up
to the VOM. Your problem can also be caused by a cold solder joint or a
bad
crimp in a connector.
Finally my pet peeve is bad grounds. Aircraft electrical systems need
to
have as close to perfect ground as possible. You would be surprised the
effect of a bit of black corrosion under the grounding strap that
connects
your engine to the frame across the engine mount will have on your whole
system. Make sure any grounding straps are clean under the nuts/bolts
that
connect them to the various parts of the frame. Make sure there is good
continuity from the grounding posts behind your instrument panel to the
engine block. Also make sure the voltage regulator is mounted on a
clean
surface. Many newer solid state regulators require the case have a good
ground.
You may want to borrow another transmitter and temporarily connect it to
your antenna system and the AC power. If you get the same effect on
transmit then you will know for sure it is your installation and not the
transmitter itself.
The advice to get your antenna system checked out is capitol. Avionics
techs have equipment that can do amazing things. Ten minutes on a
Cushman
could end up saving many $$ chasing a problem.
To recap:
*Put your antenna system under the microscope. That's probably where
your
problem is located.
*Check the continuity of all grounding straps. a good idea to check on
an
annual basis.
*If possible try another transmitter (not a hand held) in your plane You
only need to connect the antenna and aircraft power and see if you get
the
same problem.
If this doesn't fix the problem then you are going to have to get some
local
expertise... Preferably with equipment. Good places to look for help
are
Telco, Amateur radio, Television stations. Only certified avionics
shops
should open avionics cases or make modifications to avionics systems.
You
should keep that in mind if you have a certified plane.
Where do you keep your plane?? if at YHZ you may want to contact Uli
Huber
at Atlantic AV in Springfield he has technicians who should be able to
get
you going in short order...... Bring money ;-)
Hope this helps.
Noel Loveys, RPP, AME intern
Campbellton, Newfoundland,
Canada
Tel: 709-261-2705
<mailto:noelloveys@yahoo.ca> noelloveys@yahoo.ca
-----Original Message-----
From: owner-avionics-list-server@matronics.com
[mailto:owner-avionics-list-server@matronics.com] On Behalf Of Tom
Goddard
Sent: Monday, October 08, 2007 10:09 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 |
Tom,
Not only did you not say where your antenna was mounted , you also did
not say what type of antenna you were using. I note some various advice
tweaking this and that but you have to realise before you do anything
that you have a gross error in your installation. This could be be the
antenna mounted right near the instrument panel or using completely the
wrong type of antenna, or trying to use a monopole antenna in a plastic
aeroplane without using a metal ground plane of at least 2 or 3 feet
diameter, or failing to connect the coax plugs correctly so there was no
proper connection etc etc. Your whole panel seems to be alive with RF
energy so don't try tweaking this and that until you have found out
where the really gross error is.
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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