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RG400 Splice

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n4zq(at)verizon.net
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Posted: Sun Aug 02, 2015 5:43 am Post subject: RG400 Splice

I may have a need to lengthen two of the four antenna cables to my Ryan TCAD9900B by approximately 6”.

Short of fabricating an extension with connectors on each end, would it be preferable to splice directly to the existing cables?



Thanks



N4ZQ

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billp(at)wwpc.com
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Posted: Sun Aug 02, 2015 7:51 am Post subject: RG400 Splice

If you can't find 6" of slack to pull from somewhere in the airframe 

you're stuck. You might be able to reroute the cables to get the extra 6".



Here's a whole lot of technical reasons why you should replace all your 

cables (not splice them) if that's what you need to do;



1) According to the installation manual all four cables MUST be the same 

length. Changing the length of just two of the cables is going to make 

target bearing inaccurate. The TCAD uses signal arrival time (phase) to 

determine the bearing to the target. changing a cables length will 

change the length of time it takes a signal to get through that cable 

retaliative to the time the same signal takes to get through the other 

cables.



The cables need to have 3.0 dB of loss +/-0.5 dB. The TCAD uses relative 

signal strength to determine the range to the target. Without using 

connectors it is nearly impossible to keep a constant impedance. A 

soldered splice just won't work well at these frequencies. In addition, 

the reflections caused by the impedance mismatch is going to add 

additional loss and may create ghost aircraft. I'm not sure about the 

ghosts but seems like a possibility given what the TCAD is trying to do. 

Splicing the additional cable will add 4 more connectors per cable. Good 

quality connectors rated for operation at these frequencies that are 

properly installed will add about 0.11 dB per connector for a total of 

0.44 dB. Getting real close to the 0.5 dB tolerance you have in the spec 

for the cables. Of course you don't know how much of that tolerance (or 

in which direction, + or -) has been used by the original installer. The 

installation manual doesn't talk about fabricating these cables but 

rather lists prefabricated cable sets that are compatible. My guess that 

they want the cables manufactured and tested as a set with a network 

analyzer so that they know that the cables are of equal length 

(electrical phase-wise) and equal loss at the 3 dB spec.



Piecing out the cable length with homemade extensions seems like a bad 

idea to me. You're taking a very expensive piece of avionics and 

sacrificing it's accuracy for the price of a new cable set.



Bill



On 8/2/15 06:42, Greenbacks, UnLtd. wrote:

 	  Quote: 	
		   



 I may have a need to lengthen two of the four antenna cables to my Ryan TCAD9900B by approximately 6”.

 Short of fabricating an extension with connectors on each end, would it be preferable to splice directly to the existing cables?



 Thanks



 N4ZQ

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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Mon Aug 03, 2015 5:44 am Post subject: Re: RG400 Splice

 	  Quote: 	
		  I may have a need to lengthen two of the four antenna cables to my Ryan TCAD9900B by approximately 6”.  Short of fabricating an extension with connectors on each end, would it be preferable to splice directly to the existing cables? N4ZQ 	




I'd follow Bill's suggestions, but if it absolutely has to be lengthened there are connectors to do it properly. There are good You Tube videos online regarding this. 



But whatever you do, remember that the RF signal is carried in the insulation, NOT the center conductor nor the shield! This makes all other joining schemes problematic. 



See my attached "Dabbling with Electricity" for this and other matters.

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Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Wed Aug 05, 2015 2:13 pm Post subject: Re: RG400 Splice

 	  Quote: 	
		  But whatever you do, remember that the RF signal is carried in the insulation, NOT the center conductor nor the shield! 	




This is the kind of stuff that makes my head explode.   



You're saying the signal is carried by the thing that is not supposed to conduct electricity?



And I'm not doubting you.  It's just that makes as much sense (to me) as my car battery being jammed full of AAA's.

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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Wed Aug 05, 2015 3:56 pm Post subject: Re: RG400 Splice

 	  Quote: 	
		  You're saying the signal is carried by the thing that is not supposed to conduct electricity? Don Johnson 	




The signal (which is obviously the important stuff) is DRIVEN by the "wave guide" which is the metal parts, but transmitted inside the wave guide...which is the insulation. It turns out that the wave velocity is exactly the same as the speed of light inside the insulating material-- Vacuum, Teflon or PE for example.



I point this out to make sure that builders understand the reason for proper termination (which includes joints) of the coaxial cable. I can't be "pig-tailed" at high frequencies without enormous loss of signal strength. At low frequencies (like audio) it just doesn't matter.



Electricity is strange. Please read my "Dabbling with Electricity".

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Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

edpav8r(at)yahoo.com
Guest

Posted: Wed Aug 05, 2015 4:19 pm Post subject: RG400 Splice

Not quite, Don.  It's actually full of AA's.  There are two of these...

http://www.youtube.com/watch?v=nzgDffMcsUU

...in every 12V car battery.

And every AA is actually a stack of coin cells in a plastic wrapper!  

Eric

do not archive





On Aug 5, 2015, at 6:13 PM, "donjohnston" <don(at)velocity-xl.com (don(at)velocity-xl.com)> wrote:



 	  Quote: 	
		  It's just that makes as much sense (to me) as my car battery being jammed full of AAA's.  [Laughing]

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donjohnston

Joined: 13 Dec 2009
Posts: 231

Posted: Thu Aug 06, 2015 3:12 am Post subject: Re: RG400 Splice

Those videos are great.



This is what prompted my statement.



https://www.youtube.com/watch?v=s_8n2Qgguto

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Aug 06, 2015 5:11 am Post subject: RG400 Splice

At 06:56 PM 8/5/2015, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "Eric M. Jones" <emjones(at)charter.net>



 

 > You're saying the signal is carried by the thing that is not supposed to conduct electricity? Don Johnson



 

 The signal (which is obviously the important stuff) is DRIVEN by the "wave guide" which is the metal parts, but transmitted inside the wave guide...which is the insulation. It turns out that the wave velocity is exactly the same as the speed of light inside the insulating material-- Vacuum, Teflon or PE for example. 	


   There is no parallel between coaxial transmission

   lines and waveguide.



   Transmission lines are metallic conductors with

   predictable surge impedance and velocity factor

   that is constant with frequency.



   Waveguides are as their name suggests, pipes

   for the conduct of electromagnetic energy,

   a constriction of free space that allows one

   to direct the energy from the antenna (a little

   radiator at one end of the 'pipe' over some

   distance and around corners before it is launched

   into free space . . . usually at the focal point of

   some reflective dish.



   A wave traversing the length of a guide literally

   reflects from wall-to-wall and in a manner that

   is influenced by the size and shape of the guide

   with respect to the operating frequency. Waveguide

   also exhibits a 'velocity factor' which is a function

   size and operating frequency. As one squirts energy

   down a wave guide in lower frequencies, the velocity

   factor increases until at some lower limit (cutoff

   frequency) the velocity factor is for all practical

   purposes infinite and no energy makes it down the

   pipe.



   Coax cable exhibits no such characteristic in

   that it's a conductor constrained within a die-electric

   and waveguide is a simple constriction of free space.

   Losses in waveguide and coax are separate operators.



 

 

   Bob . . .    [quote][b]

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