Gentleman:
I suspect that in the example Rich gave below, the PVC insulator
and nylon failed due dielectric breakdown caused by the high potential
present at the end of the dipole. This is not the same phenomenon as
energy absorption which will cause a steady rise in temperature. Think
about how much heat energy you would have to deliver to a PVC insulator
to cause it to burst into flames. In the case of the dipole, I think the
key factor was poor dielectric strength as opposed to dissipation factor,
Admittedly, the materials in question could still have a poor dissipation
factor, but the fact that the materials failed in this situation doesn't
prove it.
This is a lot of intuition talking though, perhaps the displacement current
flowing at the end of dipole is significant enough to cause I^2R heating,
but I doubt it.
Flame suit prepared!
Mike, W4EF......
----------
From: Rich Measures[SMTP:measures@vc.net]
Sent: Wednesday, August 12, 1998 6:44 PM
Cc: amps@contesting.com
Subject: Re: [AMPS] Propeties of plastics at 10 GHz
>
>> >Sorry this is not stictly about amps, but I guess someone here might
>> >know the answer.
>> >
>> >We have a student working on using a 10 GHz transmitter (burgler alarm
>> >type, using a 10GHz Gunn diode) and detector (Schottky diode) to measure
>> >attenuation caused by cloathing soaked with water (to determine the
>> >amount of water in the cloathing). We need to find a plastic (or other
>> >material that we can machine things from) that *absorbs* microwave
>> >energy at 10 GHz. I don't want a good reflector (metals), but something
>> >that will attenuate by absorption (ie it would get hot if we used enough
>> >power).
>> >
>> Nylon. Vinyl. Delrin.
>>
>> Rich...
>>
>> R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
>------------
>Howdy,
>Nylon, vinyl and delrin are poor absorbers at 10 GHz.
This issue came up during the "grate vhf parasitics debate", so I went to
the local plastics distributor to get some info. on dissipation factor -
a.k.a. D-factor Nylon has a high D-factor at 1MHz and an even higher
D-factor at 10MHz. The various vinyls and Delrin have a lower D-factor,
however, not nearly as low as the stryrenes, polyethelynes or Teflon.. .
> I had PVC water pipe catch fire at the end of a 40m dipole. I had a
> 40m dipole that was supported with nylon line reliably decend to the
> ground when the linear amplifier was switched on. . Does it seem likely
> that nylon would have high absorption at 7MHz and poor absorption at
> 10GHz.? - - - To check it out, put a styrene cup half full of water,
> along with a sample of the plastic you want test, in a microwave oven and
> blast away for a dozen or so seconds. .
- later
> They do work well if you want to change the phase of a signal @ 10 GHz.
>
>Cuming Corporation, 230 Bodwell St., Avon, Ma 02322 manufactures
>machinable, absorptive materials that will do what you want.
>
>Good luck,
>Bob, W5AH
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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