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1.57542 GHz Filter

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speedy11

Joined: 29 Jun 2015
Posts: 62
Location: Port Orange, FL

Posted: Sat Jul 22, 2017 10:10 am Post subject: 1.57542 GHz Filter

In the Garmin installation manual, it says the following:

 

 "The GPS antenna is lass susceptible to harmonic interference if a 1.57542 GHz filter is installed on the COM transceiver antenna output."

 

 Does anyone know of anyone who had to do this?

 

 Getting ready to install a GTN650 in my RV-8 so trying to anticipate potential problems.

 

 Thanks,

 

 Stan Sutterfield

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Jul 22, 2017 11:44 am Post subject: 1.57542 GHz Filter

At 01:10 PM 7/22/2017, you wrote:

  	  Quote: 	
		   In the Garmin installation manual, it says the following:



 "The GPS antenna is lass susceptible to harmonic interference if a 1.57542 GHz filter is installed on the COM transceiver antenna output."



 Does anyone know of anyone who had to do this?



 Getting ready to install a GTN650 in my RV-8 so trying to anticipate potential problems.



 Thanks,



 Stan Sutterfield 	


   These were found useful on some makes of comm transceivers that

   had unhappy amounts of harmonic radiation on or near

   GPS frequencies.  I don't now recall the brands/models of

   radio involved but as I do recall, it was just a select

   few.



   Try it without the filter. It's easy to add later if

   indeed you do need it. It's simply inserted into

   the transceiver coax feedline



   

   I doubt you will find it necessary.



 



 

 

   Bob . . .

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alec(at)alecmyers.com
Guest

Posted: Sat Jul 22, 2017 6:29 pm Post subject: 1.57542 GHz Filter

I've tried the filters and they didn't make any difference.



On Jul 22, 2017, at 14:10, speedy11(at)aol.com (speedy11(at)aol.com) wrote:

In the Garmin installation manual, it says the following:

 

 "The GPS antenna is lass susceptible to harmonic interference if a 1.57542 GHz filter is installed on the COM transceiver antenna output."

 

 Does anyone know of anyone who had to do this?

 

 Getting ready to install a GTN650 in my RV-8 so trying to anticipate potential problems.

 

 Thanks,

 

 Stan Sutterfield

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Jul 23, 2017 7:54 am Post subject: 1.57542 GHz Filter

At 09:28 PM 7/22/2017, you wrote:

  	  Quote: 	
		  I've tried the filters and they didn't make any difference. 	


   What symptoms suggested that the filter

   was, perhaps, necessary?



 

   Bob . . .

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alec(at)alecmyers.com
Guest

Posted: Sun Jul 23, 2017 8:37 am Post subject: 1.57542 GHz Filter

I'm sorry - I should have given more detail. 

The GPS reports zero satellites in view and takes itself offline during, and for a few seconds after, any transmission on either VHF COM radio on one of a select few frequencies: 121.175MHz I think is one of them. It’s an old GPS and not a new problem. I’ve put a fair amount of resource into trying to fix it some while ago, but not found a resolution. 



The inline filters didn’t make any difference.  	  Quote: 	
		  On Jul 23, 2017, at 11:53 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

  At 09:28 PM 7/22/2017, you wrote:  	  Quote: 	
		  I've tried the filters and they didn't make any difference. 	
   What symptoms suggested that the filter   was, perhaps, necessary? 

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Jul 23, 2017 12:23 pm Post subject: 1.57542 GHz Filter

At 11:35 AM 7/23/2017, you wrote:

  	  Quote: 	
		  I'm sorry - I should have given more detail. 



 The GPS reports zero satellites in view and takes itself offline during, and for a few seconds after, any transmission on either VHF COM radio on one of a select few frequencies: 121.175MHz I think is one of them. It’s an old GPS and not a new problem. I’ve put a fair amount of resource into trying to fix it some while ago, but not found a resolution. 	


   Understand.



   Some background on cross-system interference issues

   may be helpful . . .



   When one runs the DO160/MIL-STD 461/462 gauntlets, 

   it is incumbent on the manufacturer of a new product

   to measure and document extraneous emissions to

   be sure they are below tolerable limits for other

   systems.  We know that its difficult if not impossible

   to drive all potential interference emission to zero

   but the design goals call for reducing them to insignificance.

   I.e. the victim system's performance is not degraded.



   

 [img]cid:.0[/img] 



   A fairly recent edition of DO160 offers these testing

   limits for radiated emissions. Note those 'notches' in

   the allowable emissions plot. They are centered over specific

   ranges of spectrum where RECEIVERS of itty-bitty signals

   would like to have a clear shot at detecting the information

   on those frequencies.



   Note the notches cover vhf nav/comm, uhf nav/comm, transponder/

   tcas and gps bands.  Not sure what service is being protected

   up round 5 ghz . . . 



   The energy levels allowed are the sum total of energies

   radiated from all components of a device and associated

   cables when laid out on a copper-top table like so.



 [img]cid:.1[/img]

    All this is fine and dandy for potential antagonists

    that are NOT designed to radiate buckets of energy on purpose

    . . . i.e. transmitters.  Tests for bkackbox/harness radiation are

    generally no big deal . . . I've never failed one.



    Then there are transmitters. By nature, they're intended to

    radiate signal levels on the order of 130 dBuv/m at 3 meters

    from an antenna. Given that 'extraneous' emission limits

    at GPS frequencies are on the order of 85 dBuv/m LOWER, this

    means that the 13th harmonic (121 mHz x13 = GPS) power has to

    be on the order of 1 times 10^-8 lower than the fundamental.



    Modern designs have no problem achieving this goal but

    there's a goodly number of legacy comm equipment that

    was designed and qualified before GPS became the backbone

    of aerial navigation.



    GPS signals are exceedingly weak . . . generally at or below

    atmospheric noise. The predictable nature of the incoming

    signal makes if possible to digitally sift useful data

    out of what would otherwise be noise.



    If adding the notch filter didn't fix the symptoms, then

    perhaps the interference isn't being radiated from the

    antenna . . . there have been cases of incompatible

    appliances were the interference was getting out of the

    enclosure (i.e. wouldn't pass the bench test as described above).



    Will your gps interference symptom repeat on the ground?



    You might try dummy loading the transceiver and see

    if the interference goes away. Oops, just noticed that it

    repeats on two separate transmitters (same brand/model?)

    How far apart are your comm and GPS antennas?    



 



      



    



 

 

   Bob . . .

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Eric Page

Joined: 15 Feb 2017
Posts: 260

Posted: Sun Jul 23, 2017 2:04 pm Post subject: 1.57542 GHz Filter

On Jul 23, 2017, at 1:22 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com> wrote:

 	  Quote: 	
		   Not sure what service is being protected up round 5 ghz . . .


	


Probably 802.11a WiFi systems.  In the U.S., four channels between 5.180 and 5.240 GHz, and five channels between 5.745 and 5.825 GHz.  Might be other stuff as well; that's in an Industrial, Scientific and Medical (ISM) band.



Eric

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alec(at)alecmyers.com
Guest

Posted: Sat Aug 05, 2017 4:34 pm Post subject: 1.57542 GHz Filter

Hi Bob

I’m *very* happy to have your expert attention consider this, even though it’s not a homebuilt.



VHF comm equipment is (are?) a King KX165 (com1) and a KX155 (com2), each connected to its own roof-mount antenna.



The GPS is an Apollo GX50, with the “patch” antenna on the tailcone, so there’s about 6 feet between the antenna systems. The coaxes are routed diversely, the GPS under the floor and the COM  via the pillars to the cabin roof. 



The interference is generated from both com1 and com2.



A

 	  Quote: 	
		  On Jul 23, 2017, at 4:22 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

  At 11:35 AM 7/23/2017, you wrote:  	  Quote: 	
		  I'm sorry - I should have given more detail.  The GPS reports zero satellites in view and takes itself offline during, and for a few seconds after, any transmission on either VHF COM radio on one of a select few frequencies: 121.175MHz I think is one of them. Itâs an old GPS and not a new problem. Iâve put a fair amount of resource into trying to fix it some while ago, but not found a resolution. 	
   Understand.   Some background on cross-system interference issues   may be helpful . . .   When one runs the DO160/MIL-STD 461/462 gauntlets,    it is incumbent on the manufacturer of a new product   to measure and document extraneous emissions to   be sure they are below tolerable limits for other   systems.  We know that its difficult if not impossible   to drive all potential interference emission to zero   but the design goals call for reducing them to insignificance.   I.e. the victim system's performance is not degraded.    <b12a4d0.jpg>    A fairly recent edition of DO160 offers these testing   limits for radiated emissions. Note those 'notches' in   the allowable emissions plot. They are centered over specific   ranges of spectrum where RECEIVERS of itty-bitty signals   would like to have a clear shot at detecting the information   on those frequencies.   Note the notches cover vhf nav/comm, uhf nav/comm, transponder/   tcas and gps bands.  Not sure what service is being protected   up round 5 ghz . . .    The energy levels allowed are the sum total of energies   radiated from all components of a device and associated   cables when laid out on a copper-top table like so. <b12a51e.jpg>    All this is fine and dandy for potential antagonists    that are NOT designed to radiate buckets of energy on purpose    . . . i.e. transmitters.  Tests for bkackbox/harness radiation are    generally no big deal . . . I've never failed one.    Then there are transmitters. By nature, they're intended to    radiate signal levels on the order of 130 dBuv/m at 3 meters    from an antenna. Given that 'extraneous' emission limits    at GPS frequencies are on the order of 85 dBuv/m LOWER, this    means that the 13th harmonic (121 mHz x13 = GPS) power has to    be on the order of 1 times 10^-8 lower than the fundamental.    Modern designs have no problem achieving this goal but    there's a goodly number of legacy comm equipment that    was designed and qualified before GPS became the backbone    of aerial navigation.    GPS signals are exceedingly weak . . . generally at or below    atmospheric noise. The predictable nature of the incoming    signal makes if possible to digitally sift useful data    out of what would otherwise be noise.    If adding the notch filter didn't fix the symptoms, then    perhaps the interference isn't being radiated from the    antenna . . . there have been cases of incompatible    appliances were the interference was getting out of the    enclosure (i.e. wouldn't pass the bench test as described above).    Will your gps interference symptom repeat on the ground?    You might try dummy loading the transceiver and see    if the interference goes away. Oops, just noticed that it    repeats on two separate transmitters (same brand/model?)    How far apart are your comm and GPS antennas?                 

   Bob . . .

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