Topband: Antenna terminations

donovanf at donovanf at
Wed Mar 13 01:54:53 EDT 2013

Like Tom, I install my carbon composition Beverage termination resistors fully exposed to the weather.  I measure them annually and over a 20 year period just a few have changed resistance outside of their marked tolerance band.

I'm sure there are better ways to protect them, but open air has been very reliable for me.


---- Original message ----
>Date: Tue, 12 Mar 2013 14:01:51 -0400
>From: "Tom W8JI" <w8ji at>  
>Subject: Re: Topband: Antenna terminations  
>To: <topband at>
>> Depends on the method used to provide the conductivity.  If it's carbon 
>> black filled, the shrinking is goin g to push the molecules of the carbon 
>> black closer together and that results in a net decrease in volume 
>> resistivity.  It may not matter however, as carbon filled stuff is going 
>> to be in the K-ohms range in the lowest case.  Other fillers may have 
>> lower values.
>This is of interest to me, because I use carbon resistors and heat shrink so 
>I just measured several carbon composition and film resistors, and none 
>changed value with pressure (in a table vice) until they fractured.
>I can't find any conductive heatshrink, either. This potential issue really 
>should concern all of us, because it is a serious hazard if we heatshrink 
>power line leads or HV cables.
>I'm not doubting the results, but the only resistors that I've seen change 
>value outside of age or abuse are large carbon bar resistors when submerged 
>in a liquid. For dummy loads, like a Cantenna, the resistor has to be 
>specially treated before spending any time submerged.
>I routinely mount Allen Bradley carbon composition and Ohmite metal 
>composition termination resistors right out in the open air, directly 
>exposed to weather, and they stay good for years! They can look really ugly 
>and still test good. This isn't saying that is a good practice, just that I 
>haven't ever seen an issue outside of direct lightning hits. I have some 
>pretty ugly resistors retrieved when redoing antenna end connections (the 
>fence wire rusts with age), and they still are good!
>If there is a certain brand of materials that is sensitive, it would be 
>helpful to learn what it is.
>I do not use metal or carbon films outside, or in high overload 
>applications. BTW, some carbon films look **exactly** like carbon 
>composition resistors!!
>73 Tom 
>Topband Reflector

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