[Antennas] Lampblack filled polyethylene

[Barry L. Ornitz]

Avi Esterson, 4X6UA, asked:

> I've wondered about the effect of the lampblack  (carbon) 
> used in the formulation of many of these plastics to give 
> them UV resistance. Anybody know whether this is an issue at 
> DC or at RF?

The amount of lampblack needed to achieve a dark black in 
polyethylene or polybutylene is typically less than a few 
percent.  If blended properly, the lampblack will be 
thoroughly coated by the resin, and the resultant plastic will 
not be conductive at DC.  It may make a difference at high 
microwave frequencies, but you would not use ladder line here 
anyway.  Even this small amount of lampblack will give 
significant ultraviolet protection - but I still would count 
on replacing it every five years or so.  Basically 
polyethylene has no ultraviolet resistance without this.

I once had a number of samples of a "color concentrate" 
extruded in the lab for use as a microwave absorber.  This was 
about 40 percent lampblack in polyethylene.  The resistivity 
at DC was still too high to be measured with a simple DVM (but 
the Keithley electrometer meter could).  It was only a fair 
microwave absorber at 10 GHz.

Speaking of laboratory extrusions, I still have a large bag of 
"dogbone-shaped" tensile test bars extruded from glass filled 
thermoplastic PET polyester.  The size is perfect for 600 ohm 
lines using #12 enameled wire, and they seem to hold up 
forever in sunlight.  I have also used this stuff for end 
insulators.  Some of it has been up 15 years and it has 
carried a fairly heavy ice load during many a storm.  The 
worst thing about it is that it ruins drill bits.

        73,  Barry     WA4VZQ     ornitz@tricon.net