[TowerTalk] Help with antenna

[Ian White G/GM3SEK]

Jim Lux wrote:
>
>Cutting boards (the white ones) are usually High Density PolyEthylene 
>(HDPE) or UHMW (Ultra High Molecular Weight) PE. You can look up the 
>material properties at somewhere like http://www.matweb.com/ or any of 
>a zillion plastics suppliers.
>
>Real low friction (aka poor man's teflon), fairly good dielectric 
>properties (somewhat lossy at HF and higher, compared to some other 
>plastics) (It's great for HV DC insulators)
>
Old cutting boards are also good for dipole center pieces, custom end 
insulators, gamma arm supports and many similar applications.

In these islands we can't speak much about long-term effects of 
sunlight, but HDPE/UHMW does have the advantage of being a continuous 
solid material, so it doesn't flake and de-laminate like fibreglass and 
other composites.

>The real problem with using it is that the melting point is quite low, 
>which makes for trouble machining, and if it's going to sit outside 
>with a static load because it will ooze.
>
The ooze or "creep" problem can be minimized by making most of the 
bearing out of metal, and using the plastic only as a facing for the 
bearing surfaces; and also by taking care to spread the total load over 
the largest possible area.

However, as W0UN pointed out a few weeks ago, if you make the whole 
bearing out of a big solid chunk of UHMW, that is enough to spread the 
load. For most TowerTalk-type applications, you aren't worried about 
precision pointing - even if a small amount of play does develop over 
the years, who cares?

Moonbounce is where precision pointing really does matter... and even 
there, plastic bearing faces have been used very successfully. W5UN used 
teflon very successfully for the bearing surfaces of his "Mighty Big 
Array" - the one that was rotated by a motorized truck chassis on a 
circular track. Teflon is notorious for creeping under load, but W5UN 
took care to spread the loads over very large bearing faces. Inspired by 
Dave's description in QST, I used pipe flanges faced with 1/8-in teflon 
sheet for my own much smaller array. It had extremely unbalanced 
sideways loading, but it worked fine with no evidence of creep or play 
developing over 10+ years.



-- 
73 from Ian G/GM3SEK