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tomgoddard(at)ns.sympatic
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 Posted: Mon Oct 08, 2007 4:40 pm Post subject: comm tx causing avionics fluctuations |
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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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reichec
Joined: 23 Apr 2007
Posts: 26
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| Posted: Mon Oct 08, 2007 6:51 pm Post subject: comm tx causing avionics fluctuations |
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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
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rjquillin
Joined: 13 May 2007
Posts: 123
Location: KSEE
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| Posted: Mon Oct 08, 2007 7:01 pm Post subject: comm tx causing avionics fluctuations |
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At 17:38 10/8/2007, you wrote:
| Quote: | 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. [quote][b]
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j.rippengal(at)cytanet.co
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| Posted: Mon Oct 08, 2007 9:07 pm Post subject: comm tx causing avionics fluctuations |
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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
[quote]
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
[b]
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jetboy
Joined: 22 Jul 2006
Posts: 233
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| Posted: Tue Oct 09, 2007 12:54 am Post subject: Re: comm tx causing avionics fluctuations |
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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
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Tue Oct 09, 2007 5:54 am Post subject: comm tx causing avionics fluctuations |
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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.
[img]cid:820484612(at)09102007-0518[/img]
Noel Loveys, RPP, AME intern
Campbellton, Newfoundland,
Canada
Tel: 709-261-2705
noelloveys(at)yahoo.ca (noelloveys(at)yahoo.ca)
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j.rippengal(at)cytanet.co
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| Posted: Tue Oct 09, 2007 7:48 am Post subject: comm tx causing avionics fluctuations |
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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
[quote]
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: |
href="http://www.matronics.com/Navigator?Avionics-List">http://www.matronics.com/Navigator?Avionics-List
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Savannah174(at)msn.com
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| Posted: Wed Oct 10, 2007 9:58 am Post subject: comm tx causing avionics fluctuations |
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<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]--> 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
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