[TowerTalk] Open Wire Line

Eric Gustafson n7cl@mmsi.com
Sun, 25 Oct 1998 09:32:33 -0700

Hi Mike,

Around here, we build a few very specialized products for the
benefit of a really amazing group of people normally referred to
in industry as "users".  Our "users" are relatively unique in
that not one of them has less than a BS degree in some branch of
engineering.  I can tell you unequivocally that if we were
building microwave ovens, even for _this_ group of users, we
would be designing them so that they didn't do themselves any
harm regardless of the contents or lack thereof of the user
accessable area of the cavity. ;-)

I just checked the user manual for the three microwave ovens here
at work.  The oldest one is about 5 years old.  None of them warn
about running with the cavity empty.  Possibly, this was a problem
with the earliest ovens.

The cup of water won't hurt anything.  But it will reduce the
power available for testing the dielectric - probably not a big
deal for the purpose though.

I have to agree with Tom that the dielectric characteristics
(beyond simply not being a conductor) of open wire spreaders for
use at HF are probably a lot less important than their
environment survival characteristics.

Normally when we do an "oven" test on dielectric material here it
is to find a suitable material for protecting antennas that
operate in the 0.9 to 10 GHz range.  We have to keep the flying
rocks off the antennas and still let the RF out.

73, Eric  N7CL

>Date: Sat, 24 Oct 1998 21:20:50 -0600
>From: Mike Lamb <n7ml@imt.net>
>Eric, I heard this from another Amateur many years back.  He
>said it was to provide a load for the Magnetron so that you
>don't blow it.  I don't know if he is right or not, but I always
>did it any way.  I felt if you boiled the water and nothing
>happened to heat the sample, it was probably a fair dielectric.
>Eric Gustafson wrote:
>> Mike,
>> We frequently use this crude but relatively effective method to
>> identify dielectric material that is potentially suitable for RF
>> work.  But we never use any water along with the sample.
>> Why are you including a cup of water?  And what is the "it" that
>> shouldn't warm up?  The cup of water or the plastic?  The water
>> will almost certainly get warm regardless of how good the plastic
>> is.
>> Is the water to use to determine the length of time?  For
>> example; "Water is warm so the time was long enough to heat the
>> plastic if it was going to heat"?
>> Just curious.
>> 73, Eric  N7CL

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