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RG6 coax vx. RG400 coax

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mattreeves(at)yahoo.com
Guest

Posted: Sun Nov 19, 2006 3:10 pm Post subject: RG6 coax vx. RG400 coax

I built a whole lancair with RG6 cable cause the guy at Radio Shack said it was better than RG58U - then, when it came time to install a Garmin 430, I noticed the RG6 is 75ohm cable and it calls for 50ohm cable.

   

  I fiberglassed the comm antenna into the tail of the aircraft.  I could just add another antenna in the baggage area and run RG400 cable but am wondering if there is really a whole lot of difference and if it's worth the weight.

   

  I'm hoping for some opinions.  I also used RG6 on a Nav antenna in the roof of the airplane all glassed in.  Maybe the RG6 is okay on a receive signal rather than a send?

   

  Hoping some of you guys smarter than me can respond.

   

  Thanks.

   

  Matt



   [quote][b]

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jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Sun Nov 19, 2006 4:38 pm Post subject: Re: RG6 coax vx. RG400 coax

you can probably still use the installation by trimming the com cable to a tuned length, and connecting to the 430 at this point.

 to find the correct length requires use of a reflectometer - also known as VSWR meter - such as Bird model 43 or equivalent, or a modern ramp test set. A radio ham or VHF tech may be more experienced at doing this.



start with about 36" more cable than you need, temporarily connecting the end to the output terminal on the reflectometer (an adaptor and rewirable F connector for the RG6 cable would simplify this).



the input of the reflectometer is connected to the com Tx and the Tx is keyed on highest, mid, lowest frequencies of the band. The reflected power readings need to be noted (ratio of forward to reflected power if the forward power is not always same). then cut off around 3" cable and repeat. At some point you will find the reflected power to have reached its lowest reading on the low frequency, and cutting off more cable will bring the minimum reflected power to occur around the mid frequency. When this happens put the connector on the cable and record all 3 readings. I think VSWR spec for a com antenna is for 3:1 which equates to 25% reflected power.



If the cable is too short make up the balance with 50 ohm to the com Tx.



an alternate method would be to cut the RG6 at 32" from the antenna (roughly a half wavelength for foam type cable) and fit a BNC connector and run new RG58 back to the com Tx.



I would not be concerned for the nav cable, so long as its not required to be certified. it is likely that both cables will meet the spec, just as they are. Some com antennas have a 12 ohm resistor in the base to give a better match, and you loose a third of your signal this way. similarly, a long run of lossy cable helps. The most important issue is not damaging the transmitter and immunity from overload in receivers, the actual coverage range when airborne is seldom affected.

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_________________
Ralph - CH701 / 2200a

bobf(at)feldtman.com
Guest

Posted: Sun Nov 19, 2006 6:58 pm Post subject: RG6 coax vx. RG400 coax

Before you tear it up - get a ham to show you how to check "SWR" I bet

it will be less than 2:1 - I bet it will work okay, just not "perfectly"

So what if your comm output is 4 watts instead of 5 watts..



The problem is the dipole is 50 ohms so the mismatch is there, but

really not all that bad.



try it before you tear it up.



Bobf

W%RF - amateur



Matt Reeves wrote:

 	  Quote: 	
		   I built a whole lancair with RG6 cable cause the guy at Radio Shack said

 it was better than RG58U - then, when it came time to install a Garmin

 430, I noticed the RG6 is 75ohm cable and it calls for 50ohm cable.

  

 I fiberglassed the comm antenna into the tail of the aircraft.  I could

 just add another antenna in the baggage area and run RG400 cable but am

 wondering if there is really a whole lot of difference and if it's worth

 the weight.

  

 I'm hoping for some opinions.  I also used RG6 on a Nav antenna in the

 roof of the airplane all glassed in.  Maybe the RG6 is okay on a receive

 signal rather than a send?

  

 Hoping some of you guys smarter than me can respond.

  

 Thanks.

  

 Matt

 

 *

 

 

 *

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nuckollsr(at)cox.net
Guest

Posted: Mon Nov 20, 2006 6:47 am Post subject: RG6 coax vx. RG400 coax

At 08:57 PM 11/19/2006 -0500, you wrote:



 	  Quote: 	
		  



Before you tear it up - get a ham to show you how to check "SWR" I bet

it will be less than 2:1 - I bet it will work okay, just not "perfectly"

So what if your comm output is 4 watts instead of 5 watts..



The problem is the dipole is 50 ohms so the mismatch is there, but

really not all that bad.



try it before you tear it up.



Bobf

W%RF - amateur


	


   Good suggestion Bob. I agree. While it's not the

   best we know how to do, the consequences of having

   the 'foreign' coax in place are probably not all

   that bad.



   Getting a good sweep of antenna SWR as-installed over

   the full frequency range of interest would be a good thing

   to do. If it stays below 3:1, it's probably not worth

   the $time$ to rip out and/or work around the compromised

   system.



   Bob . . .

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