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Transponder Radio Noise?

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darinh

Joined: 20 Apr 2006
Posts: 327
Location: Utah

Posted: Sat May 09, 2009 9:06 pm Post subject: Transponder Radio Noise?

I've got a question for any of you that know more about avionics than I do...which is everyone.  I have noticed that whenever my transponder is interrogated I get a little short buzz in my headset.  It is not major but enough to really bug me.  I know it is from the transponder because I hear the noise at the exact second the little "R" for receive lights up on the readout.  I can also set it to "Standby" and the noise stops.  I have checked all my grounds including to the antenna and they seem to be good (checked with ohm meter for continuity).  Anyone have any ideas what could be the problem?



Also, I have read a couple of times about the snap together ferrite rings you can buy to reduce noise.  Anyone every used any of these and are there and rules of thumb about how to use them (i.e. on the power wire of a device, signal wire, etc.)?

_________________
Darin Hawkes
Series 7
914 Turbo
Kaysville, Utah

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sun May 10, 2009 5:37 am Post subject: Re: Transponder Radio Noise?

Hi Darin,



I have the same annoying little buzz from the transponder being interrogated. It goes off if I turn off the radio or the transponder or  the intercom. The intercom Mfg. thinks it is coming through one of the antenna wires in the plane, but I didn't have it for almost two years then it just showed up. I have checked the wires and have not been able to make it go away yet. I'm still looking. I'll let you know if I find out what is causing it.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

darinh

Joined: 20 Apr 2006
Posts: 327
Location: Utah

Posted: Sun May 10, 2009 5:51 am Post subject: Re: Transponder Radio Noise?

Roger, I will do the same if I find the problem.  I suspect I either have a ground loop problem, or that it is coming from one of the cables.  I am going to check the connectors again for good ground on both ends.  So far I have only checked ground on the antenna side.  Tracing RF noise seem a bit akin to catching ghosts...actually catching a ghost might be a bit easier!  good luck

_________________
Darin Hawkes
Series 7
914 Turbo
Kaysville, Utah

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Sun May 10, 2009 6:20 am Post subject: Transponder Radio Noise?

What you are hearing is your transponders reply to the interrogation...  not

the interrogation itself.  Assuming that at one time you did not get this

buzz then something has changed.  If this is the case ferrite beads and

rings may not help you.



What ai would look at first is where the antenna for the transponder

attaches to the plane.  This is a location outside the plane where a little

corrosion is more than possible.  Remove the antenna and make sure the base

of the antenna, the surface under it and the braid attachment are all nice

and shiny.  



I suspect you have a high SWR on that antenna.  This results in the

transmitter section of the Xpndr trying to pump more power into the antenna.

That in turn could with a little time cause you to lose your Xpndr RF power

transistors.  The extra power drain and load on the transmitting transistors

cause the spurious emissions you are hearing in your headset.



This may be a place where the best thing to do, if you can't track it down,

may be to get an avionics shop do an SWR test on your antenna system.



Noel



--

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

lcfitt(at)sbcglobal.net
Guest

Posted: Sun May 10, 2009 8:11 am Post subject: Transponder Radio Noise?

Darin,



Another thing you might try is to check all wire runs.  My understanding is 

that all antenna cables should be run independent of each other and other 

wires.  If the coax has to cross another wire, try to make the crossing 

angle as close to 90° as possible.



Lowell



---

taildragon(at)msn.com
Guest

Posted: Sun May 10, 2009 10:44 am Post subject: Transponder Radio Noise?

<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]-->  Hi,

  

 Yes, I have the same thing, didn't have it for five years and just started  showing up one day. I tried reseating the transponder in its rack and that  seemed to help for a few days. Now it just comes and goes with no particular  pattern that I have been able to correlate. Also, when it is making its buzz,  ATC has said that they are not seeing my transponder. When there is no buzz, ATC  is happy.

  

 Roger

 [quote]   ---

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Sun May 10, 2009 2:49 pm Post subject: Transponder Radio Noise?

Roger:  

   

In your case disconnect the antenna cable at both ends and first do a continuity check of the centre conductor of the coax.  Be sure to wiggle the wire while you are doing the test.  If it passes that test Gently tug on the end of the centre conductor from both ends.  Do this to make sure the centre conductor hasn’t broken a couple of centimeters ( inches) inside the cable.  You will not have to pull on the centre conductor very hard to have it come out of the cable if it is already broken.  While you are at it make sure the connection to the exterior of the 256 connector is good the easiest way of checking that is by doing a continuity check on the outside of the coax connectors.  

   

Noel  

       

From: owner-kitfox-list-server(at)matronics.com [mailto:owner-kitfox-list-server(at)matronics.com] On Behalf Of Roger Standley

 Sent: 10 May 2009 04:10 PM

 To: kitfox-list(at)matronics.com

 Subject: Re: Re: Transponder Radio Noise?  

  

  

       

Hi,  

    

   

    

Yes, I have the same thing, didn't have it for five years and just started showing up one day. I tried reseating the transponder in its rack and that seemed to help for a few days. Now it just comes and goes with no particular pattern that I have been able to correlate. Also, when it is making its buzz, ATC has said that they are not seeing my transponder. When there is no buzz, ATC is happy.  

    

   

    

Roger  

  [quote]    

---

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

darinh

Joined: 20 Apr 2006
Posts: 327
Location: Utah

Posted: Sun May 10, 2009 7:33 pm Post subject: Re: Transponder Radio Noise?

Noel, What is SWR?  I told you I am not an avionics guru so some of these acronyms are not familiar to me.  By the way, the plane is virtually brand new and there is not corrosion.  I did remove the antenna and checked anyway and even sanded and brightened up the metal again.  There is good ground between the airframe and the antenna and I have had this problem since day one.  I have pretty much ignored it to this point as the buzz is not really noticeable while flying due to cabin noise and ATC has never said anything about it so I don't think they hear it.  I think I will try the ferrite option as it is inexpensive and easy.  



Here is the question though...Where should I attach the ferrite cores?  On the antenna side or the Tx side?  Or should I put them on both the radio and the Tx?



Lowell, I ran all my power, grounds and other wires on the Right side of the fuse...the Tx cable is in the center and the radio cable is on the left side.

_________________
Darin Hawkes
Series 7
914 Turbo
Kaysville, Utah

Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Sun May 10, 2009 10:40 pm Post subject: Transponder Radio Noise?

 	  Quote: 	
		   From: darinh [gerns25(at)netscape.net] 

 Noel, What is SWR?


	


Allow me, please.

A Standing Wave Ratio meter is a simple intrument with two dials that you connect between your transceiver and your antenna via the coaxial cable. It has two dials. The first one shows the energy going out to the antenna and the second, the energy coming back.



The aim is to have as much as possible energy out and as little as possible coming back bacause, in the worst-case-out it may even grill your output amplification circuit.



An antenna is supposed to shoot out electromagnetic energy and it does that best when in harmony, i.e. when one end is at the peak positive, the other end of the antenna should be at the peak negative of the alternative current. This is why antenna are always as long as the wave length or a fraction of it.



Antenna are weird things and you can never be sure that they work to the best until you have seen their efficiency with a SWR meter. Perhaps your antenna is very close to your tail and the tube of your tail works as a reflector. They are many things that may affect it. At sea, I used to have my isolated backstay as an antenna that tuned with ... a tuner, which is a set of condensers and coils that "balance" the antenna. But still, when I wanted to have good reception from the south (sailing in Norway means that ... the rest of the world is at the south!  I turned the boat heading north. Something to do with my aluminium mast that was in the way, I guess.



Cheers,

Michel Verheughe

Norway

Kitfox 3 - Jabiru 2200 .... flying as PAX



Do not archive



<pre><b><font size  color="#000000" face="courier new,courier">

_________________
Kitfox 3 - Jabiru 2200

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Mon May 11, 2009 6:15 am Post subject: Transponder Radio Noise?

Standing Wave Reflection.  It is caused if there is a problem with the

transmitting antenna and some of the power being pumped into that system is

reflected back to the output transistors.  Most of what you have to know is

antenna systems are designed and tuned to have as low an SWR as is possible.

(less is better!)  If there is a break in the center conductor of the coax

or corrosion under the antenna etc the SWR will climb...and climb.  This

puts a lot more pressure on your output transistors and they can even burn

out.  



Electronic devices are not too bright.  Give them a job to do and they will

do it.  Make the job harder and the just try harder and harder to do the job

until they eventually burn themselves up.  Electric motors are that way and

so are antenna systems.  If you are getting a buzz when the XPNDR

(transponder) is transmitting then the problem is probably either in the

Xpndr itself or in the antenna system.  Being that most modern electronic

devices are pretty much fool proof the only place to look for problems is in

the connection to the plane in this case that is the antenna.  Once you have

your antenna, the coax and the connectors completely checked out then you

may have to spend a few dollars to have the Xpndr checked out.  I did say

they were pretty much fool proof this is true but there is always the 10% of

the time that grounds are not the problems.



BTW do you notice any lowering of voltage or increase in current used when

the Xpndr is transmitting?



Several times in the past I've seen RMI (Radio Magnetic Interference)

problems that were fixed by the installation of additional grounds.  No one

can quite figure out why but it did work.  One of those things was on a

coast guard rescue boat.  They have depth sounders and VHF (Very High

Frequency) radios similar to the ones on planes.  Every time the sounder

made a ping a loud click went through the radio.  Attaching a heavy duty

grounding strap to the depth sounder cured the interference.  Note: the unit

was grounded to spec before but the extra ground caused the clicking to

stop.



I think I would try a ferrite ring with a few of turns of the hot wire to

your radio on it.  Be sure to wrap it with heat shrink so it can't ground

and cause a little static of its own.  I'd be surprised if that will work

because the power supplies of the radios are pretty well filtered inside the

boxes.



Clear as mud??

Noel



--

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

lcfitt(at)sbcglobal.net
Guest

Posted: Mon May 11, 2009 7:42 am Post subject: Transponder Radio Noise?

Darin,



It sounds like you have done everything right.  You may get another thought 

on this, but it is my understanding that the ferrite cores are placed over 

the power leads rather than the antennas to eliminate any noise passing into 

the unit from those feeds.



SWR stands for Standing Wave Ratio.  Personally, and this is my opinion 

only, I don't think it would be a  problem in your case as you are likely 

using off the shelf antennas and mounted to the factory mounting pads.  If 

there was a problem in either of these areas, your noise situation would be 

more common.  SWR becomes a problem when you use antennas of the wrong 

length.  The desired length has to do with the frequency you are 

transmitting.



Lowell



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