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Re: [TowerTalk] Plastic Cutting Boards for Antenna Stuff ?

Subject: Re: [TowerTalk] Plastic Cutting Boards for Antenna Stuff ?
From: John Lyles <>
Date: Sun, 22 Apr 2012 19:46:57 -0600
List-post: <">>
Exactly, 2450 MHz microwave fields are probably not as important as 
mechanical strength, HV breakdown, water absorption and mechanical 
properties for antenna insulators.

Microwave ovens generate a relative low field strength, compared to
those at the ends of dipole antennas and resonant circuits with kW 
levels at HF. I use a commercial dielectric heater at 90 MHz, that I 
bought at a junkyard, to test plastics and hoses for RF amplifier work. 
It subjects the material, typically about an inch thick or less, to 
fields such as 15 kV peak. If a material is to be trusted in a high 
field environment, such as supporting components in a tank circuit or 
hoses to the plate of a water cooled tube, this test is much more valid 
than a microwave oven. Materials that just are barely warm in the 
Radarrange sometimes erupt in smoke and melt in the dielectric 
preheater. The white cutting board UHMW PE is pretty good, not as good 
as Teflon but acceptable.

But for center insulators where the rf voltage is readily calculated 
from P=E^2/Z, the electric field won't eat UHMW PE or the like. In this 
case, you want to prevent tracking when the material is wet, or 
contaminated with dust, smoke residue, or after aging in the sun.


 >Message: 6
 >Date: Sun, 22 Apr 2012 07:54:01 -0700
 >From: Jim Lux <>
 >Subject: Re: [TowerTalk] Plastic Cutting Boards for Antenna Stuff ?
On 4/22/12 7:14 AM, Bill Harris wrote:
 >> Zapping a piece of material in the microwave for a minute or so, 
 >>should give you some idea what the insulating properties are.  If it 
 >>doesn't get hot, (how hot?) it should be okay for an insulator at HF 
 >>frequencies, right?

 >the microwave test is sort of so, so.. it's at 2.5 GHz and it more
 >measures RF absorption than things that are actually more important 
 >like HV breakdown and water absorption.


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