Tuesday, January 29, 2013

Short Note on RF Absorption and Coil Forms

Recently, on the "Glowbugs" google group, there was a discussion about whether a pill or vitamin bottle was suitable for use as a coil form for a tank curcuit, or other more general RF use (loading coil form, etc).
According to that fine bunch of OMs and YLs, the test is temperature after being microwaved for a bit. Here's how the test works:
1. Take a microwave safe dish, and put a cup of water in it. Make sure dish is shaped so that both dish and test bottle can fit in the microwave oven without one being on top of the other.
2. Clean and dry the container you want to test. I tested a bottle that held fiber, that's what's in the picture. Trickiest thing about the prep work on this baby is that you want to make sure all the metal from the freshness seal is removed from the rim.
3. Place both the test bottle and the cup of water in the microwave. Take note of the temperature of each, "room temperature" is as precise as this measurement needs to be.
4. Nuke it for a minute. (see below for further advice)
5. Compare temperatures: the test bottle should still be at room temperature, and the water should be noticably hotter (scalding hot on some microwaves, so be aware!)

Them what was tested.
Above is my test bottle, a repurposed fiber containment vessel. Errr, it's not mine man... I'm to young to take a fiber supplement! (maybe...)

K5KVH had mentioned the RF microwave test, and this is what hed had to say way back in 2011 on the Glowbugs list:

PVC is easy to verify if it has additives to increase loss. You can

check your PVC easily with the microwave test. Put a sample of the pvc
on a paper towel in the microwave oven. Put a cup of water in there
next to it. (no metal trim on cup, plain china).

Set the microwave to 3 or 4 minutes. When you see the water is boiling,
stop the oven. Carefully test the pvc with a wet finger tip, to see if
it got warm, ie warmer than the hot air around the cup.

If not, it is good for HF for sure. This is based on a formula for
dielectric loss that shows such loss is directly proportional to
frequency, the higher the test frequency, the higher the loss would be
for a given material. Thus, no heating at microwaves means it is
a good insulator at HF.

Ok, I will buy that.
My test results indicate that this material is good for winding tank circuits, or whatever, at HF frequencies.
It is made of "Recycle Material Number 2", ie High Density Polyethylene (HDPE).
I recently acquired some more tubes, so I'm probably going to be using this soon.
I also have pillbottles.

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