On 6 Jul 2020 17:50:43 GMT, Allodoxaphobia
wrote:
On Sun, 05 Jul 2020 10:18:00 -0700, Jeff Liebermann wrote:
Take a small sample of the tinted plastic and put it in a microwave
oven. Cook it for maybe 15 seconds. If it's warm, it's absorbing RF
and probably unsuitable for covering an antenna (radome).
You should always do this with a companion glass of water.
Microwave ovens do not like running without something "in the box"
to act as a dummy load in to which to dump the power.
Jonesy
Agreed. It's safer to give it something to cook or boil. However, I
don't think it's necessary for this test. 15 seconds of cooking
plastic isn't going to kill the magnetron from over heating due to
reflected power or over voltage due reinforcement of voltage peaks.
Also, see:
https://physics.stackexchange.com/questions/309154/what-happens-in-an-empty-microwave-oven
All the microwave ovens I have ever come across,
dismantled or repaired have always included a "stirrer"
which is in the waveguide path between the magnetron
and the oven cavity. Usually it is in the form of a
metal rotating shape like a fan. This chops up the
otherwise nicely formed e-m microwaves into a jumble
and hence there can be virtually no standing waves
inside the oven.
"Microwave stirrer for microwave oven"
https://patents.google.com/patent/US4833286A/en
I guess I should mention that this test should be performed in a
microwave oven that is not used for cooking food. Plasticizers,
VoC's, fireproofing, UV proofing, nasal desensitizers, dyes, paint,
clear coatings, and other noxious volatiles are not considered edible.
Please remember that you have only one life to give to your hobby.
Also, for antennas that can be crammed into a tubular radome (also
known as a plastic or fiberglass pipe), a VNA is quite handy to view
the detuning effects of the plastic radome. The VNA produces a
frequency sweep across the operating range on the X axis, and a
corresponding VSWR or return loss on the Y axis. For example, on VHF:
"nanoVNA-H – standalone VHF antenna VSWR sweep"
https://owenduffy.net/blog/?p=16052
It's really discouraging watching my calculations turn to trash as the
antenna resonant frequency moves when a radome is placed over the
antenna.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558