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1.5DB???

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billhuntersemail(at)gmail
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Posted: Sun Apr 23, 2017 1:28 pm Post subject: 1.5DB???

Hi, 



I am installing a GTN-650 in my Velocity and the antenna location is only 2.0 feet from the unit itself. The Garmin instruction manual states that the cable loss needs to be between 1.5 and 6.5 DB. The manual also states that the calculated loss of a 6.5 foot cable with two BNC ends is 1.5 DB. 

I assume that this means that I need to run a 6.5 foot cable for my short run? 

If so, what is the minimum radius of the bends I would need to make in the cable to coil it up? 



Thanks for your kind help!!!  



Bill Hunter

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cbirdsall6(at)cox.net
Guest

Posted: Sun Apr 23, 2017 2:05 pm Post subject: 1.5DB???

Bill,



Yes, you need a minimum of 6.5 feet  of cable length. The GPS antenna is an active antenna, having a pre-amplifier built into it that's powered by a bias voltage applied on the antenna center conductor (power comes from the GTN). The 1.5 dB of loss is to keep the receiver in the GPS unit from being over-driven and thus degrading the signal.



The minimum 1-time bend radius from the RG400 spec is 1" - and the FAA spec from AC 43.13-1 is 10 times the diameter of the cable (which would make it just under a 2" radius).  Both of these are a bit tight in my mind, and I would suggest a 3" radius (6" dia) or larger loop.  That'll give about 19" of length per loop, which means three loops would get you what you need (or there about).



Chuck 



---- William Hunter <billhuntersemail(at)gmail.com> wrote: 

 	  Quote: 	
		   Hi,

 

 I am installing a GTN-650 in my Velocity and the antenna location is only

 2.0 feet from the unit itself. The Garmin instruction manual states that

 the cable loss needs to be between 1.5 and 6.5 DB. The manual also states

 that the calculated loss of a 6.5 foot cable with two BNC ends is 1.5 DB.

 

 I assume that this means that I need to run a 6.5 foot cable for my short

 run?

 

 If so, what is the minimum radius of the bends I would need to make in the

 cable to coil it up?

 

 

 Thanks for your kind help!!!

 

 Bill Hunter

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user9253

Joined: 28 Mar 2008
Posts: 1935
Location: Riley TWP Michigan

Posted: Sun Apr 23, 2017 3:52 pm Post subject: Re: 1.5DB???

In order to meet the minimum cable length requirement, I purchased a 10 foot RG400 cable with TNC connections from eBay for $12 with free shipping. Item number: 131817000902

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Joe Gores

billhuntersemail(at)gmail
Guest

Posted: Mon Apr 24, 2017 7:32 am Post subject: 1.5DB???

Well I came to the right place for advice. Thanks for the item number as I will visit eBay Aircraft Supply this afternoon in order one



Thanks again, 



Bill Hunter 

On Apr 23, 2017 16:59, "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)> wrote: 	  Quote: 	
		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>

 

 In order to meet the minimum cable length requirement, I purchased a 10 foot RG400 cable with TNC connections from eBay for $12 with free shipping. Item number: 131817000902

 

 --------

 Joe Gores

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=468711#468711

 

 

 

 

 

 

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donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Mon Apr 24, 2017 5:26 pm Post subject: Re: 1.5DB???

 	  cbirdsall6(at)cox.net wrote: 	
		  Bill,



Yes, you need a minimum of 6.5 feet  of cable length. The GPS antenna is an active antenna, having a pre-amplifier built into it that's powered by a bias voltage applied on the antenna center conductor (power comes from the GTN). The 1.5 dB of loss is to keep the receiver in the GPS unit from being over-driven and thus degrading the signal. 	




What's the long term impact from an antenna lead that's too short? 



I have a GTN625 which has been operating for about 18 months with a 1.5' antenna lead.

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user9253

Joined: 28 Mar 2008
Posts: 1935
Location: Riley TWP Michigan

Posted: Mon Apr 24, 2017 6:29 pm Post subject: Re: 1.5DB???

Read Stein's post is this thread:

http://forum.matronics.com/viewtopic.php?p=467885&sid=0551c7cee028f1697ba54f038595af00

I think that if it works, do not worry about it.

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Joe Gores

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