Jim Higgins wrote:
On Wed, 27 Apr 2005 17:23:33 -0700, in
, "John Smith"
wrote:

Thanks Cecil/Atec... I have expanded my list of antenna materials! And, the
microwave will be my authority...

You might want to remember to put about a cup of water - or half
a cup at least - into the microwave along with whatever you test
to avoid possible problems with pumping power into an unloaded
cavity. Every microwave instruction manual I've ever seen warns
about running them empty, and a lot of unaffected material like
PVC tubing is the same as empty.

Because the inside of the oven tends to get warm and steamy, everything
tends to warm up a little. It's best to compare the material you're
testing against samples of known good materials like PTFE, polyethylene
(the insulator from RG214) or a hot-melt glue stick.

The regular translucent hot-melt glue material (amorphous polypropylene)
is also an excellent RF insulator. It's useful for all kinds of antenna
building and weather-sealing.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Actually, I have a set of "dummy loads" for microwave ovens, don't remember
where I picked 'em up at (gov't auction?)... they seem to be silicone
rubber??? impregnated with some kind of conducting material (carbon? Metal
oxide?) You surely must be able to get 'em from a "microwave tech?" You
can even use a thermometer in them, "nuke" for a certain time and arrive at
different measurements--mainly power output....
I am sure some here have seen these things and can describe them much better
than I....
But, the smaller ones work perfect for my "microwave experiments" (ever
chuck a fly in a bottle and see how long it lives in microwave environment?)
(chuck a BIG one of these dummy loads in, and the oven won't even warm a
glass of water, or seemingly, harm a fly!!!)

Regards,
John

John Smith wrote:
Thanks Cecil/Atec... I have expanded my list of antenna materials!
And, the
microwave will be my authority...

You might wind up throwing the baby out with the washwater. Somebody
somewhere once pointed out to me that many materials have different
resistivities at different frequencies, the higher the frequency the
lower the bulk resistance. One implication being that a candidate
spreader material which gets hot in a 2 Ghz microwave oven does not
necessarily mean it will be at all lossy at 7 Mhz. The FCC human RF
exposure limits rules are based in this principle. Being a few feet
away from an antenna radiating 100W of 40M RF is not a problem, 100W
of 2 Ghz RF into the same antenna is deadly or close to it, etc.

About a year ago I picked up some 2"-3" scrap lengths of carbon fiber
composite hunting arrow shafts from a local sporting goods store and
put them to the microwave test. This material has an extremely high
strength/weight ratio which makes it attractive for use as spreaders.

They got pretty warm after nuking them for five or so minutes. Then I
checked several of them with my DMM set to it's 200 megohm range. In
all cases the DMM indicated completely open circuits. From this
experiment I've concluded that this material is plenty "good enough" to
use for HF feedline spreaders. Unfortunately it's a bit difficult to
drill & cut.


Warmest regards,
John

w3rv

Cecil Moore wrote:
Brian Kelly wrote:
Cecil, who sells that line??

http://www.w7fg.com

I've wondered what he's using. Tnx Cecil.

73, Cecil http://www.qsl.net/w5dxp

w3rv


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On 28 Apr 2005 10:23:55 -0700, "Brian Kelly" wrote:


Cecil Moore wrote:
Brian Kelly wrote:
Cecil, who sells that line??

http://www.w7fg.com

I've wondered what he's using.

It's 1/2" pipe, rated for 100 PSI in its original use; looks sturdy...

bob
k5qwg

Tnx Cecil.

73, Cecil http://www.qsl.net/w5dxp

w3rv


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any pnumatics supplyer will sell you 6mm 8 mm 10mm diameter tubing in both
pvc and poly prop in long lenghts and you can cut it down i paid about £10
or $20 or so for 30 mts of the stuff today chop it with a pipe slice into
the lenght you need
hope this helps
dave