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h&jeuropa

Joined: 07 Nov 2006
Posts: 645

Posted: Fri May 02, 2008 3:52 pm Post subject: Comm Antenna

I'm having a problem with my comm antenna installed in my Europa. It is a common copper tape antenna with toroids over the coax. Materials and plans came from RST Engineering. Installed in the vertical rudder fin per Europa directions and tuned using a SWR bridge. 



When I transmit, certain frequencies cause my ACK ELT to transmit. It is located on the bottom of the fuselage touching the rear bulkhead (under the elevator torque tube). This is about 20 inches forward of the antenna and near the tip of the copper tape.  If I disconnect the cable going to the remote actuator, there is no problem. (I had this problem previously and added toroids to the four wires but no change). 



Also, when I transmit, certain frequencies cause the magnetometer in my GRT EFIS to swing heading 20 degrees. The magnetometer is mounted on the port side of the fuselage essentially on the fuselage parting line and just at the rear of the D panel bulkhead. This is about 60 inches forward of the antenna and about aligned with the center of the copper tape.



Have others had difficulity with the dipole in the rudder causing problems? Found a solution? How about using other antennas, like Bob Archer? 



Thanks



Jim Butcher

Europa N241BW

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nuckolls.bob(at)cox.net
Guest

Posted: Sat May 03, 2008 8:28 am Post subject: Comm Antenna

At 04:52 PM 5/2/2008 -0700, you wrote:



 	  Quote: 	
		  



I'm having a problem with my comm antenna installed in my Europa. It is a 

common copper tape antenna with toroids over the coax. Materials and plans 

came from RST Engineering. Installed in the vertical rudder fin per Europa 

directions and tuned using a SWR bridge.



When I transmit, certain frequencies cause my ACK ELT to transmit. It is 

located on the bottom of the fuselage touching the rear bulkhead (under 

the elevator torque tube). This is about 20 inches forward of the antenna 

and near the tip of the copper tape.  If I disconnect the cable going to 

the remote actuator, there is no problem. (I had this problem previously 

and added toroids to the four wires but no change).



Also, when I transmit, certain frequencies cause the magnetometer in my 

GRT EFIS to swing heading 20 degrees. The magnetometer is mounted on the 

port side of the fuselage essentially on the fuselage parting line and 

just at the rear of the D panel bulkhead. This is about 60 inches forward 

of the antenna and about aligned with the center of the copper tape.



Have others had difficulity with the dipole in the rudder causing 

problems? Found a solution? How about using other antennas, like Bob Archer?


	


   It's probably not the antenna . . . in fact, the BETTER the antenna

   works, the worse the problem(s) get.



   When crafting products to live in the real worlds aboard

   aircraft, the the thoughtful and through designer tries to

   anticipate and accommodate every expected stress that might

   attack the product's performance.



   This process generally includes an assessment of the

   design against guidelines cited in DO-160. See:



http://aeroelectric.com/articles/do160.html



   Under the topic of Radiated Susceptibility, DO_160

   cites a variety of test levels which a product

   must stand of without degradation of performance.

   Things that are buried in the guts of an all metal

   airplane and have no direct sight of external antennas

   (either on airplane or off) have the lowest test

   levels. Things that have clear sight of and perhaps

   close proximity to antennas have the highest test

   levels. There is a HUGE difference between the highest

   and lowest levels.



   In a small plastic airplane, we're faced with the

   worst case scenario for friendly system integration.

   The exposure probabilities and magnitudes for the

   small plastic airplane are as bad or worse than

   any of those encountered by the heavy iron birds.



   The variables for electro-whizzies responding badly

   to comm (or even transponder) signals fall into

   three categories.



   (1) strength and signature of the interferring

   source. Is it radiating stresses that are normally

   expected to be under control? In the case of your

   transceiver and antenna installation, the strength

   is obviously strong . . . and it's unlikely that

   were seeing the effects of spurious outputs.



   (2) coupling mode. In this case you have close

   proximity, un obstructed spaces between an

   antenna that's radiating normally and a device

   that operates normally . . . except while you're

   transmitting.



   Can this feature be mitigated? Probably not in

   that size airplane constructed of radio energy

   transparent materials.



   (3) inability of the offended device to operate

   in this environment. Unfortunately, may devices

   that have served us well in metal airplanes for

   decades are seeing new and un-anticipated problems

   in plastic airplanes. They may have been

   artfully crafted with the metal airplane in mind.

   A further consideration is that devices intended

   for the OBAM aircraft market are not being produced

   by individuals with knowledge of (or perhaps an

   interest in) what's needed to work properly

   in this relatively unique environment.



   I have one such case on my bench right now. The

   Van's aircraft ammeter would be hard pressed to

   run the gauntlets necessary for qualification

   onto type certificated aircraft . . . much less

   an OBAM aircraft.



   The task before you is daunting without the

   support of skills and experience of individuals

   who solve these problems for a living. Your

   best bet is to get with the manufacturers of

   the victim devices and see if any other customers

   have experienced difficulty and what remedies

   were useful.



   Bob . . .

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khorton01(at)rogers.com
Guest

Posted: Sat May 03, 2008 9:07 am Post subject: Comm Antenna

On 3-May-08, at 12:24 , Robert L. Nuckolls, III wrote:



 	  Quote: 	
		   

 >



> 

> >

>

> I'm having a problem with my comm antenna installed in my Europa.  

> It is a common copper tape antenna with toroids over the coax.  

> Materials and plans came from RST Engineering. Installed in the  

> vertical rudder fin per Europa directions and tuned using a SWR  

> bridge.

>

> When I transmit, certain frequencies cause my ACK ELT to transmit.  

> It is located on the bottom of the fuselage touching the rear  

> bulkhead (under the elevator torque tube). This is about 20 inches  

> forward of the antenna and near the tip of the copper tape.  If I  

> disconnect the cable going to the remote actuator, there is no  

> problem. (I had this problem previously and added toroids to the  

> four wires but no change).

>

  It's probably not the antenna . . . in fact, the BETTER the antenna

  works, the worse the problem(s) get.




	


It seems that the cable between the ELT and the cockpit control is  

acting as an antenna, picking up COM transmissions, and feeding it  

into the ELT.  This signal coming in on the cable is somehow  

interpreted as if the cockpit control had been activated.  So, I  

wonder if there is a way to shield the cable between the ELT and the  

cockpit control, to reduce the amount of energy that it is picking up  

from COM transmissions.  Also, rerouting that cable further away from  

the COM antenna would reduce the energy level it sees.



Bob - the ACK ELT uses a length of apparently conventional telephone  

cable to connect the ELT to the cockpit control.  Is it worth trying  

to replace that telephone cable with some sort of shielded cable,  

splicing it into a piece of telephone cable at each end so it has the  

correct connectors?  If that was tried, where should the shield be  

grounded (keeping in mind this is a composite aircraft, and that the  

ELT enclosure and cockpit control are both made of plastic)?



--

Kevin Horton         RV-8 (final assemby)

Ottawa, Canada

http://www.kilohotel.com/rv8

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nuckolls.bob(at)cox.net
Guest

Posted: Sun May 04, 2008 5:19 am Post subject: Comm Antenna

 	  Quote: 	
		  It seems that the cable between the ELT and the cockpit control is

acting as an antenna, picking up COM transmissions, and feeding it

into the ELT.  This signal coming in on the cable is somehow

interpreted as if the cockpit control had been activated.  So, I

wonder if there is a way to shield the cable between the ELT and the

cockpit control, to reduce the amount of energy that it is picking up

from COM transmissions.  Also, rerouting that cable further away from

the COM antenna would reduce the energy level it sees.


	


    I've heard this about the ACK ELTs in the past. I

    'presume' they've done their homework and meet all

    the test requirements levied on them for qualification

    in metal airplanes.



 	  Quote: 	
		  Bob - the ACK ELT uses a length of apparently conventional telephone

cable to connect the ELT to the cockpit control.  Is it worth trying

to replace that telephone cable with some sort of shielded cable,

splicing it into a piece of telephone cable at each end so it has the

correct connectors?  If that was tried, where should the shield be

grounded (keeping in mind this is a composite aircraft, and that the

ELT enclosure and cockpit control are both made of plastic)?


	


    I presume the problem goes away when the cable is unplugged?



    Hmmm . . . shielding a bundle is almost never the best way

    to go after an RFI problem. A filter assembly at the ELT's

    remote switch connector is the best shot at it. Do you know

    which wires in the conductor are common or ground? A few

    chokes and capacitors from Digikey could be crafted into

    a compact filter that would have high attenuation at

    VHF Comm frequencies.



    Bob . . .

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frazitl

Joined: 04 Feb 2008
Posts: 9

Posted: Mon May 05, 2008 8:14 am Post subject: Comm Antenna

FWIW, I looked up the installation instructions on the artex ME406 ELT

http://www.artex.net/products/oneproduct.php?categoryid=15&productid=87 .

They call for a shielded cable between the ELT and the control panel.  I

decided to go with the C91a ELT, but ran the shielded cable from the ELT

location to the panel for future upgrade ease.  I'm no expert, but I'd try

the shielded control cable first.



Terry

7A Panel

SNIP

Time: 09:12:01 AM PST US

From: "Dennis Jones" <djones(at)northboone.net>

Subject: Re: switch for smoke pump



Time: 10:07:23 AM PST US

From: Kevin Horton <khorton01(at)rogers.com>

Subject: Re: Comm Antenna

On 3-May-08, at 12:24 , Robert L. Nuckolls, III wrote:



 	  Quote: 	
		   >



> >

>

> I'm having a problem with my comm antenna installed in my Europa.

> It is a common copper tape antenna with toroids over the coax.

> Materials and plans came from RST Engineering. Installed in the

> vertical rudder fin per Europa directions and tuned using a SWR

> bridge.

>


	


 	  Quote: 	
		  


	


It seems that the cable between the ELT and the cockpit control is

acting as an antenna, picking up COM transmissions, and feeding it

into the ELT.  This signal coming in on the cable is somehow

interpreted as if the cockpit control had been activated.  So, I

wonder if there is a way to shield the cable between the ELT and the

cockpit control, to reduce the amount of energy that it is picking up

from COM transmissions.  Also, rerouting that cable further away from

the COM antenna would reduce the energy level it sees.



Bob - the ACK ELT uses a length of apparently conventional telephone

cable to connect the ELT to the cockpit control.  Is it worth trying

to replace that telephone cable with some sort of shielded cable,

splicing it into a piece of telephone cable at each end so it has the

correct connectors?  If that was tried, where should the shield be

grounded (keeping in mind this is a composite aircraft, and that the

ELT enclosure and cockpit control are both made of plastic)?



--

Kevin Horton         RV-8 (final assemby)

Ottawa, Canada

http://www.kilohotel.com/rv8



SNIP

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_________________
Terry Frazier
RV7A
Finishing

rampil

Joined: 04 May 2007
Posts: 870

Posted: Mon May 05, 2008 9:01 am Post subject: Re: Comm Antenna

Hi Jim,



I agree that the ELT remote head cable is picking up the com signal.

Most Europas I have dived into (including mine) use standard

unshielded telephone quad cable. My King 450 ELT does not pick up

the RFI as a test button press however.



The right sized toroids at the ELT end of the cable might help,

Changing the length of the cable (to move it from a resonance, might help,

finally, shield the cable.



Unless of course you think its time to get a new UHF ELT

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_________________
Ira N224XS

Alfio

Joined: 05 May 2008
Posts: 6

Posted: Mon May 05, 2008 9:06 am Post subject: Re: Comm Antenna

This may not be your problem or situation, however ..... 



One thing I noticed about the ACK installation, is that their antenna cannot be substituted for another one. It is part and parcel of the EMC filtering of the unit. According to ACK, other RF signals (e.g. comm) can be received via the ACK antenna port and falsely trigger the unit. 



If you did not substitute the antenna, then this would not apply.



Alfio

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mprather(at)spro.net
Guest

Posted: Mon May 05, 2008 9:42 am Post subject: Comm Antenna

 	  Quote: 	
		   


	


snip



 	  Quote: 	
		   Unless of course you think its time to get a new UHF ELT 


	


A UHF ELT might not have remote circuitry that is any more immune to VHF

RF energy than one of the VHF ULTs...



Regards,



Matt-

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h&jeuropa

Joined: 07 Nov 2006
Posts: 645

Posted: Tue Jun 10, 2008 4:38 am Post subject: Re: Comm Antenna

I seem to have these problems pretty well solved.  Initially tried shielded cables (shield grounded to A/C ground point near inst panel - unconnected at other end) with lots of toroids to the ELT and Magnetometer. Also made the interconnect between the Primary Flight Display and the MFD a shielded cable.  Had little or no effect.  Then tried just shielded cable (no toroids) to ELT and wrapped the entire ELT in aluminum foil.  Foil is connected to the cable shield and ground is as before.  That solved the ELT problem.  From other forums I learned that there are D Sub filters which have capacitors built in bypassing each connection to the shell of the connector.  Acquired Spectrum Control Series 100 filters, 5600 pf (from Newark Electronics)  for the Magnetometer and the PFD (main display).   At the Magnetometer I connected the cable shield to the D Sub shell since the shell is floating inside the Mag and I needed a return for the filter.  Found the PFD filter didn't have any effect but the Mag helped some.  Finally wrapped the Mag in aluminum foil which is connected to the shield and that seemed to solve the problems.  I think  there is so much RF from the com antenna and no shielding because of the composite, that it overwhelms the ELT and Magnetometer.



I also found a local ham with an antenna analyzer (MFJ Model 269) and had a look at my com antenna.  Found it had very low SWR but the resistance of the antenna changed if I walked near the antenna and the resistance was not constant or nearly so across the 118 to 136 Mhz range.  We both suspected the toroid balun just didn't perform properly.  So we decided to make a balun from coax as the ham radio books have shown for years.  It is very much like the one on the Aero Electric website except the length of the balun section is an electrical half wavelength, so you must take into account the velocity factor of the cable.  Using RG400 (vf=.695), the balun section is 16 1/4" long.  Took off the toroids and installed the new balun.  Had to trim the antenna to get the SWR low near the center of the band, but now the SWR is less than 2:1 across the entire band, the resistance stays constant and walking near the antenna makes no difference.  



Hope this helps someone else.



Jim Butcher

Europa N241BW

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