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GPS Antenna Length

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w_sweet(at)comcast.net
Guest

Posted: Wed Nov 28, 2007 2:52 am Post subject: GPS Antenna Length

For those who assert that there is a minimum WAAS  GPS coax length, please list references. Both NAVCOM is Visalia, Ca and Garmin  tech rep say no such CURRENT requirement exists. Older Garmin GPS's did require  such attenuation in the coax; not now.

 Wayne

    [quote][b]

schaefer(at)rts-services.
Guest

Posted: Wed Nov 28, 2007 4:05 am Post subject: GPS Antenna Length

I do not have the Install manual for the 430W or 530W. I do have the manual

for Garnin 900 or the WAAS GPS conponent which is the GIA63W.



The manual part number is 190-00719-00. The Install manuals are NOT on the

Garmin Web site so you need to talk to a certified Garmin Installer to get

the info.



 



The manual states:



 



Section 1.5 Wiring Considerations

Paul Smith

Joined: 28 Nov 2007
Posts: 1

Posted: Wed Nov 28, 2007 7:11 am Post subject: Re: GPS Antenna Length

My GNS480 Installation manual states: "The cable loss from the GPS antenna shall not be greater than 7 dB. If SatCom is installed on the aircraft, the cable loss shall be 3 dB minimum to ensure proper interference rejection from SatCom."

I understand this to mean that since I don't have SatCom I can make the cable as short as I want. It is preesently about 6 ft.

w_sweet(at)comcast.net
Guest

Posted: Wed Nov 28, 2007 7:58 am Post subject: GPS Antenna Length

Exactly. Those are the older GPS's that the Garmin  rep and NAVCOM referenced as needing the specific length. That GPS is no longer  supported by Garmin and THAT is the reason the manuals are not on Garmin web  site. The newer GPS's as I posted and as Garmin stated do not have those  limitations. The statements from the "person who is certified" is NOT in accord  with Eric at NAVCOM ( has installed WAAS's in certified aircraft) or the Garmin  tech rep. Seems there is some disconnect here; I'm betting on Garmin and Eric at  NAVCOM. 

 For those interested, call Garmin; 913-397-8200.  

 The "person who is certified" (by the way, that is  any certified avionics technician, AND........YIIPPEEE any builder of an  experimental aircraft) perhaps should call Garmin to get updated.

 Wayne

 [quote]   ---

mike(at)ferrer-aviation.c
Guest

Posted: Wed Nov 28, 2007 8:43 am Post subject: GPS Antenna Length

I have the Garmin GNS-430W installation manual, dated 11/2006, and this is

what is says...



"...Once the antenna mounting position has been prepared, route the coax

cable from the antenna to the

400W Series unit. Proper selection of coax cable and assembly of connectors

is critical to GPS signal

performance. The cable loss from the GPS antenna shall be between 3 dB and 7

dB in order to maintain

proper rejection to interference signals. The coaxial connectors and

adapters, such as TNC to BNC, add

additional loss to the cable and should be considered when computing the

cable loss. A typical loss of 0.2

dB can be used for each connection. The typical cable loss for 20 feet of

RG-142B or RG-400 coax with a

connector on each end is 4 dB. For very short runs, where the loss is less

than 3dB, additional cable

should be used to increase the loss to within 3 dB to 7 dB. This additional

cable may be coiled, taking

into account the minimum bend radius of the cable..."



I also have the Garmin STC Upgrade manual for upgrading existing 400/500

series to a 400W/500W series and it says the following...



"...The 400W Series Installation Manual and 500W Series Installation Manual

require that

the coaxial cable have a loss of 3 dB to 7 dB. If RG400 is used, 3 dB

equates to a length

of approximately 13 feet of RG400 cable with a connector on each end..."

w_sweet(at)comcast.net
Guest

Posted: Wed Nov 28, 2007 2:44 pm Post subject: GPS Antenna Length

OK, that is correct apparently as of the date of the publication of that 

manual. According to the Garmin rep (a different guy this time) I spoke with 

today, a minimum of 6.5 feet is required for proper attenuation; "it was 13 

feet" , again according to this rep. Did not ask when that change was made. 

He also did not mention any required maximum length; I should have asked. I 

was a bit shocked in his response in view of the first tech guy's response.

So the point is, most installations will meet this new requirement unless 

the antenna is mounted on the glare shield directly above the GPS panel 

mount.

I could have misunderstood the first rep in that my question was about a 13 

to 15 foot required coax length; his answer was not complete apparently.

Wayne



---

w_sweet(at)comcast.net
Guest

Posted: Wed Nov 28, 2007 2:46 pm Post subject: GPS Antenna Length

Just received this email from Garmin:

 Dear Wayne Sweet,



Thank you for contacting Garmin  International,



The WAAS install manual requires a loss of 1.5 to 6.5 DB.  1.5 DB equates

to about 6.5 feet of RG142B or RG400 with 2  connectors.



With Best Regards,



Walt Williams

Field Service  Engineer

Garmin Aviation Product Support

913-397-8200  voice

913-397-8282 fax



 [quote]   ---

Vince Palermo

Joined: 18 Aug 2007
Posts: 13

Posted: Wed Nov 28, 2007 4:04 pm Post subject: GPS Antenna Length

STC upgrade Installation Manual-400w/500/ SeriesManual Number 190-00357-06

Page 9

Rev. B



In paragraph 2.5.4 the following notes are inserted



the 400W series installation manual and 500W series installation series manual require that the coaxial cable have a loss of 3 db th 7 db. If RG400 cable is used, 3 db equates to a length of approximately 13 feet of RG400 cable with a connector on each end.



To maintain integrity of the WAAS signal, the GPS antenna coaxial cable must have a minimum of two shields(E.G. RG400 OR RG142B). IF THE EXISTING COAXIAL CABLE IS SINGLE SHIELDED(E.G. RG58 OR RG59), THE ENTIRE CABLE MUST BE REPLACED EVEN IF IT MEETS THE OVERALL CABLE ATTENUATION REQUIREMENT.



If the existing coaxial cable is terminated with a BNC connector and the GPS WAAS antenna being installed has a TNC connector, a TNC-to BNC adapter such as P?N 1-1478013-0 from AMP (Tyco) may be used. The adapter will add 0.5 db to the overall GPS antenna cable loss.



THERE IS A LOT MORE THAT THE MANUAL STATES, BUT MY TYPING IS SORT OF LIKE WATCHING PAINT DRY AND MY INDEX FINGERS ARE GETTING TIRED. The bottom line is that this is how I have installed dozens of WAAS upgrades since Garmin came out with the upgrades and this is how we have done them all.



 Vincent Palermo

vpalermo(at)tampabay.rr.com (vpalermo(at)tampabay.rr.com)

 



On Nov 28, 2007, at 5:50 AM, Wayne Sweet wrote:

 	  Quote: 	
		  For those who assert that there is a minimum WAAS GPS coax length, please list references. Both NAVCOM is Visalia, Ca and Garmin tech rep say no such CURRENT requirement exists. Older Garmin GPS's did require such attenuation in the coax; not now.

Wayne

 	  Quote: 	
		  http://www.matronics.com/contribution

href="http://www.matronics.com/Navigator?Avionics-List">http://www.matronics.com/Navigator?Avionics-List

href="http://forums.matronics.com">http://forums.matronics.com



	


	


 [quote][b]

_________________
Vincent J. Palermo

schaefer(at)rts-services.
Guest

Posted: Wed Nov 28, 2007 4:56 pm Post subject: GPS Antenna Length

You will be able to get away with just about anything for enroute GPS nav and VFR approaches.  

If you expect to use the GPS for the new precision approaches, and you do not want them flagged while you are still in the soup, you would be best served to follow all of the little details in the installation manual. Including those that talk about required DB loss in the GPS antenna path (needed for WAAS integrity), location of COM cabling and antennas in relation to GPS antennas, (Limit cross interference when communication while on approach), and potential low angle shadowing of airframe components between the GPS antenna from GPS satellites.  

         

  

From: owner-avionics-list-server(at)matronics.com [mailto:owner-avionics-list-server(at)matronics.com] On Behalf Of Vincent Palermo

 Sent: Wednesday, November 28, 2007 5:57 PM

 To: avionics-list(at)matronics.com

 Subject: Re: GPS Antenna Length  

  

   

STC upgrade Installation Manual-400w/500/ Series    

Manual Number 190-00357-06  

    

Page 9  

    

Rev. B  

    

   

    

In paragraph 2.5.4 the following notes are inserted  

    

   

    

the 400W series installation manual and 500W series installation series manual require that the coaxial cable have a loss of 3 db th 7 db. If RG400 cable is used, 3 db equates to a length of approximately 13 feet of RG400 cable with a connector on each end.  

    

   

    

To maintain integrity of the WAAS signal, the GPS antenna coaxial cable must have a minimum of two shields(E.G. RG400 OR RG142B). IF THE EXISTING COAXIAL CABLE IS SINGLE SHIELDED(E.G. RG58 OR RG59), THE ENTIRE CABLE MUST BE REPLACED EVEN IF IT MEETS THE OVERALL CABLE ATTENUATION REQUIREMENT.  

    

   

    

If the existing coaxial cable is terminated with a BNC connector and the GPS WAAS antenna being installed has a TNC connector, a TNC-to BNC adapter such as P?N 1-1478013-0 from AMP (Tyco) may be used. The adapter will add 0.5 db to the overall GPS antenna cable loss.  

    

   

    

THERE IS A LOT MORE THAT THE MANUAL  STATES, BUT MY TYPING IS SORT OF LIKE WATCHING PAINT DRY AND MY INDEX FINGERS ARE GETTING TIRED. The bottom line is that this is how I have installed dozens of WAAS upgrades since Garmin came out with the upgrades and this is how we have done them all.  

    

           

Vincent Palermo  

    

vpalermo(at)tampabay.rr.com (vpalermo(at)tampabay.rr.com)  

    

   

  

  



 

   

    

       

On Nov 28, 2007, at 5:50 AM, Wayne Sweet wrote:  

  



 

         

For those who assert that there is a minimum WAAS GPS coax length, please list references. Both NAVCOM is Visalia, Ca and Garmin tech rep say no such CURRENT requirement exists. Older Garmin GPS's did require such attenuation in the coax; not now.  

    

Wayne  

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       [quote][b]

GrummanDude

Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA

Posted: Wed Nov 28, 2007 10:38 pm Post subject: GPS Antenna Length

Quote "THERE IS A LOT MORE THAT THE MANUAL STATES, BUT MY TYPING IS SORT OF LIKE WATCHING PAINT DRY AND MY INDEX FINGERS ARE GETTING TIRED. The bottom line is that this is how I have installed dozens of WAAS upgrades since Garmin came out with the upgrades and this is how we have done them all." 

  Question:  When you say, "The bottom line is that this is how I have installed dozens of WAAS upgrades since Garmin came out with the upgrades and this is how we have done them all."

  

  ------ so, to what method do you refer?

  

   

     Vincent Palermo

  vpalermo(at)tampabay.rr.com (vpalermo(at)tampabay.rr.com)

  

 

  

 

   On Nov 28, 2007, at 5:50 AM, Wayne Sweet wrote:

  	  Quote: 	
		    For those who assert that there is a minimum WAAS GPS coax length, please list references. Both NAVCOM is Visalia, Ca and Garmin tech rep say no such CURRENT requirement exists. Older Garmin GPS's did require such attenuation in the coax; not now.

  Wayne

  	  Quote: 	
		  http://www.matronics.com/contribution href="http://www.matronics.com/Navigator?Avionics-List"http://www.matronics.com/Navigator?Avionics-List href="http://forums.matronics.com"http://forums.matronics.com  	


   _-========================http://wrget="_blank" href="http://forums.matronics.com">http://forums.matronics==============  	
  

      

 

   More new features than ever.  Check out the new AOL Mail! 

  [quote][b]

_________________
Gary
AuCountry Aviation
Home of Team Grumman

Vince Palermo

Joined: 18 Aug 2007
Posts: 13

Posted: Thu Nov 29, 2007 3:26 am Post subject: GPS Antenna Length

Not sure what the question is........

 Vincent Palermo

vpalermo(at)tampabay.rr.com (vpalermo(at)tampabay.rr.com)

 



On Nov 29, 2007, at 1:37 AM, teamgrumman(at)aol.com (teamgrumman(at)aol.com) wrote:

[quote]Quote "THERE IS A LOT MORE THAT THE MANUAL STATES, BUT MY TYPING IS SORT OF LIKE WATCHING PAINT DRY AND MY INDEX FINGERS ARE GETTING TIRED. The bottom line is that this is how I have installed dozens of WAAS upgrades since Garmin came out with the upgrades and this is how we have done them all." 

 Question:  When you say, "The bottom line is that this is how I have installed dozens of WAAS upgrades since Garmin came out with the upgrades and this is how we have done them all."

 

 ------ so, to what method do you refer?

 

  

    Vincent Palermo

 vpalermo(at)tampabay.rr.com (vpalermo(at)tampabay.rr.com)

 

 

  

 

  On Nov 28, 2007, at 5:50 AM, Wayne Sweet wrote:

  	  Quote: 	
		    For those who assert that there is a minimum WAAS GPS coax length, please list references. Both NAVCOM is Visalia, Ca and Garmin tech rep say no such CURRENT requirement exists. Older Garmin GPS's did require such attenuation in the coax; not now.

 Wayne

  	  Quote: 	
		  http://www.matronics.com/contribution href="http://www.matronics.com/Navigator?Avionics-List"http://www.matronics.com/Navigator?Avionics-List href="http://forums.matronics.com"http://forums.matronics.com  	


  http://wrget="_blank" href="http://forums.matronics.com">http://forums.matronics==============  [/url][url=http://www.matronics.com/contribution] 	
 

  

 

   More new features than ever.  Check out the new AOL Mail!

 

  	  Quote: 	
		  

href="http://www.matronics.com/contribution">http://www.matronics.com/contribution

href="http://www.matronics.com/Navigator?Avionics-List">http://www.matronics.com/Navigator?Avionics-List

href="http://forums.matronics.com">http://forums.matronics.com



	
[b]

_________________
Vincent J. Palermo

reichec

Joined: 23 Apr 2007
Posts: 26

Posted: Thu Nov 29, 2007 4:05 am Post subject: GPS Antenna Length

When Mr. FAA man comes and signs my paperwork, I am  certifying that I have installed the device PER THE MANUAL; and the manual  says I need to install a certain length of coax with a certain value of  attenuation between point A and point B, I legally have to follow what it says  in the manual...  Becuase when some moron crashes flying beyond his  capabilities below minimums, and his wife comes to sue our shop, I need to  assure that I have done my job 100%....   The only time I ever deviate  from what the installation manual says is when I have information from Walt  Williams or George Koelch or one of the other garmin tech guys,  AND when I  install it PER THE MANUAL and for some reason it doesnt work right for a certain  application in a certain airplane, then and only then do I modify what the  install manual says and I attach info that I recieve to my work order and It  lives on forever in my files at our repair station.   I dont  understand the mentality of "how cheap can I be" by using the wrong coax or coax  is too short... Is it reall that hard to install it per what the manufacturer  says in their publications?  99.99% of the time it works... so why are we  questioning what they are saying?

  

 [quote]   ---

GrummanDude

Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA

Posted: Fri Nov 30, 2007 12:14 am Post subject: GPS Antenna Length

What you say has some merit.  And, in the ideal world, the engineers idea of a generic installation fits all applications.  In the ideal world, the best place to mount the GPS antenna is always on the top of the plane.  In the real world, my experience with the FAA and certifying STCs has taught me that the only way to get something certified is to write installation instructions so that any idiot can install the STC and the installation will work.  In most cases, that is sufficient. 

  However.  This is not always the case.  For example:  Installation on Grumman-American aircraft

  

  (1) The turtle-deck on a Lynx/Traveler/Cheetah/Tiger isn't horizontal in flight.  

  (2) If mounted on the turtle-deck, about 180 degrees of forward horizon is blocked

  (3) The GPS antenna cannot be mounted on the canopy.  

  (4) The location of the comm antenna on the turtle-deck 

       and mounting the GPS antenna 2 feet away from the antenna, 

       puts the GPS antenna either right under the canopy when it's open or

       alongside the dorsal fin near the vertical stabilizer where it is blocked.  

  

  So, the solution: mount the GPS antennal on the glareshield.  This results in

  (1) The GPS antenna being horizontal in flight

  (2) The GPS antenna being visible to the horizon about 300 degrees horizontally

  (3) The GPS antenna being visible to the sky about 160 degrees vertically

  

  True, the GPS antenna doesn't see all of the horizon or all of the sky, but.  I've had really good performance with my GPS antennas mounted on the glareshield.

   

  Most people are happy to blindly follow any instruction given them and care nothing about why (or how) the instructions came to be.  After doing basic research in combustion instabilities for 22 years, I've become accustomed to not accepting 'that's-how-we've-always-done-it' as an answer.

  

  I asked the question, and called Garmin, because I wanted to know the answer.  I wanted to know if the GPS unit would really fail / burn-up / explode / decapitate / melt / make the wings fall off /  or whatever, if the antenna were mounted anywhere but where the instructions say.   I was told by Tim, at Garmin, that he'd rather see the antenna mounted on the glareshield (so that it could see more of the horizon) than on the turtle-deck at 7 to 10 degrees from the horizontal.  He suggested coiling up 6 feet of cable to get 1 1/2 db loss.  The next step is to investigate what the total loss in the cable actually is (at various lengths) with BNC/TNC connectors.  1 1/2 db is easy.

  

  

  

 

 --

_________________
Gary
AuCountry Aviation
Home of Team Grumman

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