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Re: [TowerTalk] LMR-900

Subject: Re: [TowerTalk] LMR-900
Date: Sat, 17 Jul 2010 14:13:48 EDT
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Jim, if the braid and foil were in poor contact, they would constitute a  
very low impedance transmission line with the foil acting as center conductor 
 and braid as outer conductor.
After a very short distance any rf on the braid would be closely coupled to 
 the foil where it would then flow to the open end of the foil and around 
the end  into the inner surface of the foil.
With partial contact between the braid and foil, the rf path would be a  
combination of dc shorts between the two shields and the transmission line  
effect. The rf would make it's way to the inner surface pretty easily in 
either  case. There could be a small difference in electrical length when 
corrosion  causes the d.c. shorts to degrade. I have actually seen this happen 
432 MHz  phasing lines due to water ingress corrosion between the braid and 
foil. Water  is not our friend!
Gerald K5GW
In a message dated 7/17/2010 11:15:19 A.M. Central Daylight Time, writes:

On Sat,  17 Jul 2010 11:45:43 -0400, Joe Subich, W4TV wrote:

>Connector  installation techniques should always insure there is
>an adequate RF  connection between the connector body and the
>INSIDE of the inner  shield.

Interesting. Let's say I'm installing an Amphenol PL-259 on  
something like LMR400 or Commscope 3227 (similar shield 
construction).  The connector is a tight screw-on fit to the 
jacket, the braid and foil  continue all the way to the point where 
they butt into the hole for the  center, and I solder all four 
holes carefully, with the foil and braid  (hopefully) in contact 
under the solder. Would that make an adequate RF  connection? I'd 
guess that the answer depends on the frequency of  interest. Or 
would only a crimp-type connector be capable of an adequate  

73, Jim  K9YC


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