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tomgoddard(at)ns.sympatic
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 Posted: Fri Nov 28, 2008 8:33 am Post subject: comm tx causing avionics fluctuations |
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Hi Noel,
We have worked most of this out. I know it has been a few months. We are going to externally mount the antennas. We did a trial test with a dummy load and then with the antenna hooked to a long coax and tried different external mounts and all the RFI went away. We are fairly certain then that all our trouble is radiated emission. I have some questions:
<![if !supportLists]>1. <![endif]>How far apart should the external antennas be… what’s the minimum distance
<![if !supportLists]>2. <![endif]>Can they share the same ground plane as long as they both make ground with it
<![if !supportLists]>3. <![endif]>Can the TXP antenna share the same ground plane
<![if !supportLists]>4. <![endif]>Do you have an antenna recommendation?
<![if !supportLists]>5. <![endif]>Should one be top mounted and one bottom mounted on the fuselage tail cone?
Tom
From: owner-avionics-list-server(at)matronics.com [mailto:owner-avionics-list-server(at)matronics.com] On Behalf Of Noel Loveys
Sent: Tuesday, October 09, 2007 10:53 AM
To: avionics-list(at)matronics.com
Subject: RE: 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.
[img]cid:image001.jpg(at)01C95155.2DA974C0[/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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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Fri Nov 28, 2008 10:44 am Post subject: comm tx causing avionics fluctuations |
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I believe the recommended distance antennae should be is greater than three feet. (for COM.Nav antennae) Some planes have two phased antennae for com. The installation instructions will specify the distance between the antennae. That is done on some installations to give a directional quality to the com radio. VOR antennae are usually horizontal and therefore are usually mounted on the tail of the plane.
XPNDR antenna are usually placed on the bottom of the airplane because their primary purpose is to communicate with ground stations. They should also be limited to about 200 mi in range so that they don’t cause distance interference.
The ground plane is actually a form of reflector so the short answer is yes several antennae, even on several different frequencies, can share a ground plane. In fact on metal planes the whole of the skin is considered to be ground plane. Tube an rag planes the whole frame is considered to be ground plane.
Transponders use different frequencies than con/nav antennae. Therefore their antennae are shorter. Use the antenna recommended with your transponder.
Com radios are usually vertically polarized antennae mounted on the top of the plane where they can reach other planes and distant ground based towers. So the long and short of it is Com on top.. XPNDR on the bottom and VOR on the tail. GPS if you have it will have the antenna on top where it can see the sky, RADALT will be on the belly of the plane. ADF goniometer (loop) antenna can be either on top or bottom of the plane the same is true of the ADF sense (long wire) antenna.
Without the aid of an avionics shop I wouldn’t try to install an HF rig in a plane... but if you do 80 meters is a blast at even 1000’.
Hope this helps
Noel
From: owner-avionics-list-server(at)matronics.com [mailto:owner-avionics-list-server(at)matronics.com] On Behalf Of Tom Goddard
Sent: Friday, November 28, 2008 1:01 PM
To: avionics-list(at)matronics.com
Subject: RE: comm tx causing avionics fluctuations
Hi Noel,
We have worked most of this out. I know it has been a few months. We are going to externally mount the antennas. We did a trial test with a dummy load and then with the antenna hooked to a long coax and tried different external mounts and all the RFI went away. We are fairly certain then that all our trouble is radiated emission. I have some questions:
<![if !supportLists]>1. <![endif]>How far apart should the external antennas be… what’s the minimum distance
<![if !supportLists]>2. <![endif]>Can they share the same ground plane as long as they both make ground with it
<![if !supportLists]>3. <![endif]>Can the TXP antenna share the same ground plane
<![if !supportLists]>4. <![endif]>Do you have an antenna recommendation?
<![if !supportLists]>5. <![endif]>Should one be top mounted and one bottom mounted on the fuselage tail cone?
Tom
From: owner-avionics-list-server(at)matronics.com [mailto:owner-avionics-list-server(at)matronics.com] On Behalf Of Noel Loveys
Sent: Tuesday, October 09, 2007 10:53 AM
To: avionics-list(at)matronics.com
Subject: RE: 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.
[img]cid:image001.jpg(at)01C95169.FA77C920[/img]
Noel Loveys, RPP, AME intern
Campbellton, Newfoundland,
Canada
Tel: 709-261-2705
noelloveys(at)yahoo.ca (noelloveys(at)yahoo.ca)
-----Original Message-----
From: owner-avionics-list-server(at)matronics.com [mailto:owner-avionics-list-server(at)matronics.com] On Behalf Of Tom Goddard
Sent: Monday, October 08, 2007 10:09 PM
To: avionics-list(at)matronics.com
Subject: comm tx causing avionics fluctuations
| 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-Listhref="http://forums.matronics.com">http://forums.matronics.com |
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09/10/2007 8:44 AM
Checked by AVG.
11/28/2008 8:17 AM
| | - The Matronics Avionics-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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| Filesize: |
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| Viewed: |
237 Time(s) |
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