Topband: Antenna terminations
w8ji at w8ji.com
Tue Mar 12 14:01:51 EDT 2013
> 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
More information about the Topband