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com antenna problem

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theo.celis(at)skynet.be
Guest

Posted: Thu Dec 06, 2007 1:08 am Post subject: com antenna problem

Good morning Bob,



Our RV-7A is nearing completion here in Leuven (close to Brussels-Belgium)

On checking the radios, 2 SL30 with the GMA340 ASP, our com 1 has a problem

when transmitting: ammeter deflects full scale (towards the + side !?) when

pushing the PTT - same on both sticks. The com2 shows a normal neg ammeter

deflection when tx.

Switching boxes in the rack: same result. (My knowledge of electronics is 

very

limited; before becoming an airline pilot I was a chemical engeneer.)

We then switched antennas behind the instr panel and now com2 had the 

problem.

Maybe a bad crimp on the coax connector? So therefore we opened the baggage 

rear panel

and connected coax com1 to the belly Comant antenna 2 (com1 ant is on top of 

the fuselage)

and now the readings were normal: neg amm deflection. Bad Comant CI 121 

antenna #1?

Both ant are new ones, straight out of the box.

Maybe irrelevant , but using the Fluke meter, there is no short between the 

base and center

conductor and when testing capacity, the meter tests OL.

Very probably you have seen this before?



I have been reading the e-mails on the list since a few months now and I was 

very

interested in all yr comments.



Hope you can help us too. Thanks.



Théo Celis

AeroElectric-List

 	  Quote:

nuckolls.bob(at)cox.net
Guest

Posted: Thu Dec 06, 2007 5:43 am Post subject: com antenna problem

At 10:05 AM 12/6/2007 +0100, you wrote:



 	  Quote: 	
		  

<theo.celis(at)skynet.be>



Good morning Bob,



Our RV-7A is nearing completion here in Leuven (close to Brussels-Belgium)

On checking the radios, 2 SL30 with the GMA340 ASP, our com 1 has a problem

when transmitting: ammeter deflects full scale (towards the + side !?) when

pushing the PTT - same on both sticks. The com2 shows a normal neg ammeter

deflection when tx.

Switching boxes in the rack: same result. (My knowledge of electronics is very

limited; before becoming an airline pilot I was a chemical engeneer.)

We then switched antennas behind the instr panel and now com2 had the problem.

Maybe a bad crimp on the coax connector? So therefore we opened the 

baggage rear panel

and connected coax com1 to the belly Comant antenna 2 (com1 ant is on top 

of the fuselage)

and now the readings were normal: neg amm deflection. Bad Comant CI 121 

antenna #1?

Both ant are new ones, straight out of the box.

Maybe irrelevant , but using the Fluke meter, there is no short between 

the base and center

conductor and when testing capacity, the meter tests OL.

Very probably you have seen this before?


	


   You don't mention the type of aircraft. Composite?

   What kind of 'ammeter' . . . electronic? Digital

   display? The list of observations you've cited

   suggests an ammeter that is not designed to live

   in the high radio frequency fields that exist in

   and around the panels of composite aircraft.



   If you've checked out your antenna feedlines then

   this hypothesis is more likely. It's not uncommon for

   the interference condition to change markedly with

   changes of configuration. I've seen a 'problem' that

   went away when the mechanic was trying to demonstrate

   it for me . . . but came back when I left the cockpit.

   Seems that my body mass changed the radiation patterns

   in the cockpit enough to shift the condition!



   What kind of ammeter display do you have?



   Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

longg(at)pjm.com
Guest

Posted: Thu Dec 06, 2007 9:48 am Post subject: com antenna problem

Hey Bob, between the lines - it's an RV-7A, you know, one of those

slower metal jobs - sorry.



--

jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Thu Dec 06, 2007 12:47 pm Post subject: Re: com antenna problem

May need to put a clip-on ferrite over the ammeter sense wires,  at both ends or at cockpit bulkhead. Get the ferrites from a electronics or computer store. 13 to 20 mm size should do. Quite enough RF gets into the cabin of metal A/C, to upset the electronic gauges.



Ralph

_________________
Ralph - CH701 / 2200a

bobf(at)feldtman.com
Guest

Posted: Thu Dec 06, 2007 2:49 pm Post subject: com antenna problem

Palomar engineering sells ferrites for hams

  

 bobf



 

 On 12/6/07, jetboy <sanson.r(at)xtra.co.nz (sanson.r(at)xtra.co.nz)> wrote: [quote]--> AeroElectric-List message posted by: "jetboy" <sanson.r(at)xtra.co.nz  (sanson.r(at)xtra.co.nz)>



May need to put a clip-on ferrite over the ammeter sense wires,  at both ends or at cockpit bulkhead. Get the ferrites from a electronics or computer store. 13 to 20 mm size should do. Quite enough RF gets into the cabin of metal A/C, to upset the electronic gauges. 



Ralph



--------

Ralph - CH701 / 2200a





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=150847#150847  [quote][b]

theo.celis(at)skynet.be
Guest

Posted: Fri Dec 07, 2007 9:14 am Post subject: com antenna problem

Thank you Bob, Ralph & Bob F.,



A few tests confirmed exactly what you said, Bob.

Just moving my body while txmitting resulted in large

needle deflections.

As somebody mentioned, the RV is full metal.

The ammeter is one that Vans sells :  a 40A shunt model, -40/0/+40 scale.

The coil is housed in an aluminum cylinder, inside the plastic instrument.

There is a PCB with some resistors, transistors etc.

The instrument itself is located in a centre console under the main instr

panel.

The voltmeters we used to play with during elec lessons were far less

complicated...

We measured 7mV over the shunt with all avionics turned on and when

pressing the PTT it shot up to 12mV.

My friend brought a handheld RX and our tx tested fine.



Thanks a million for yr help.



Théo.



---

nuckolls.bob(at)cox.net
Guest

Posted: Sat Dec 08, 2007 2:31 pm Post subject: com antenna problem

At 06:11 PM 12/7/2007 +0100, you wrote:



 	  Quote: 	
		  

<theo.celis(at)skynet.be>



Thank you Bob, Ralph & Bob F.,



A few tests confirmed exactly what you said, Bob.

Just moving my body while txmitting resulted in large

needle deflections.

As somebody mentioned, the RV is full metal.

The ammeter is one that Vans sells :  a 40A shunt model, -40/0/+40 scale.

The coil is housed in an aluminum cylinder, inside the plastic instrument.

There is a PCB with some resistors, transistors etc.

The instrument itself is located in a centre console under the main instr

panel.

The voltmeters we used to play with during elec lessons were far less

complicated...

We measured 7mV over the shunt with all avionics turned on and when

pressing the PTT it shot up to 12mV.

My friend brought a handheld RX and our tx tested fine.



Thanks a million for yr help.


	


   Transistors are to radio frequency energy as

   mobile homes are to tornados. It is all too

   common in the OBAM aviation community that the

   designers of electro-whizzies have not considered

   the potential effects of radio frequency energy

   coming from a perfectly normal comm transmitter

   and antenna installation.



   I looked at Van's listing for the -40/0/+40

   instrument and it's not clear that this is

   an "electronically enhanced" instrument. Other

   than two wires from the shunt to the instrument,

   are there any other wires that need connection

   to say ship's power?



   By the way, the amount of RF found in the cockpit

   of an RV (due to proximity to a properly installed

   comm antenna system) is no greater than what is

   expected in light aircraft. It's an exceedingly

   rare situation where an observed interference

   between the comm transmitter and some piece of

   panel mounted equipment is the fault of the

   installer or serendipitous combination of conditions.

   It's nearly always a shortcoming of the victim

   system's design.



   Do the installation instructions for this insrument

   mention anything an installer should do to forestall

   such interference?



   Bob . . .

theo.celis(at)skynet.be
Guest

Posted: Tue Dec 11, 2007 9:12 am Post subject: com antenna problem

Connections on the ammeter: 4 studs (and 2 wires for the internal lt):

pwr, gnd, 2 from the shunt.

I reread the instructions (with diagram) and they refer to AC4313.13...

They mention  the use of 18awg for all wires - exc the light.

Nothing is said about meter sensitivity or the use of shielded

wire.

I should mention that the checks were done in the workshop where

ceiling and floor contain rebars and the door is aluminum.

Reception of nearby Brussels app is excellent.

The tip-up canopy was removed for easy access.

Best regards,



Théo.

---

nuckolls.bob(at)cox.net
Guest

Posted: Tue Dec 11, 2007 12:32 pm Post subject: com antenna problem

At 06:10 PM 12/11/2007 +0100, you wrote:



 	  Quote: 	
		  

<theo.celis(at)skynet.be>



Connections on the ammeter: 4 studs (and 2 wires for the internal lt):

pwr, gnd, 2 from the shunt.

I reread the instructions (with diagram) and they refer to AC4313.13...

They mention  the use of 18awg for all wires - exc the light.

Nothing is said about meter sensitivity or the use of shielded

wire.


	


   And it shouldn't. What you describe is the ordinary

   signal-powered instrument designed to be used with

   the conventional ammeter shunt. I can think of no

   reason for this device to be sensitive to radio

   frequency energy.



 	  Quote: 	
		  I should mention that the checks were done in the workshop where

ceiling and floor contain rebars and the door is aluminum.

Reception of nearby Brussels app is excellent.

The tip-up canopy was removed for easy access.


	


   None of this is relevant. Without launching a

   $time$ consuming research task to see why this

   instrument seems to be affected, we're probably

   not going to learn of the root cause and remedy.



   Know that there are a number of similarly affected

   displays on type certificated aircraft wherein the

   crew are instructed to "disregard the perturbations

   observed while transmitting."



   It would be interesting to know why and perhaps

   institute a fix . . . but it's not going to add

   value to your finished product in proportion to

   the cost of knowing what the fix is.



   Bob . . .

theo.celis(at)skynet.be
Guest

Posted: Wed Dec 12, 2007 6:33 am Post subject: com antenna problem

Thanks, Bob, we can now move on to the next problem: modifying the

RV-7A nose gear - a mandatory SB...



Do not archive.

---

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