plastics and behavior w/RF

[stevez]

I am looking for a table or list of what I'll call RF dissipation factors for various common plastics.

I am wondering if the choice I made for a gamma capacitor enclosure on my VHF antenna could be affecting the performance. It's behaving strangely. The enclosure is an ABS box from Radio Shack.

I thought I'd find something in my amateur radio references but it's either not there or I am missing it. Lots of stuff turns up on google but only real data applies to very special materials.

 

[Nigel Goodwin]

I am wondering if the choice I made for a gamma capacitor enclosure on my VHF antenna could be affecting the performance. It's behaving strangely. The enclosure is an ABS box from Radio Shack.

I've never heard of any problems like that, you should be able to use an ABS box without any side effects at all.

 

[spuffock]

Put the plastic part in the microwave oven along with a cup of water as a load, and blast it at full power for 3 mins or so. If the plastic is still stone cold then it's suitable for RF applications. Otherwise forget it.

 

[Dean Huster]

Many, maybe most, plastics absorb water, something I did not know until just a few months ago. One of our local injection molding companies told me that if the moisture content of the plastic parts they ship out is too low, it becomes brittle and breaks easily. They bag the parts (hundreds to thousands of parts per bag depending upon the size of the part and toss in a small cupful of water before sealing the bag.

Therefore, not all plastics will work equally depending upon the various applications. Some may change their characteristics with temperature, time, humidity or applied voltage. Some types of plastics that are wonderful for high puncture voltage per mil of thickness may have a lousy dielectric constant or undesireable dielectric absorption ("soakage" as Bob Pease of NSC calls it). It's difficult to get all the best characteristics in any one plastic. That's why the good RF caps are usually mica or ceramic types. And, of course, ceramics have their own detailed set of problems, not the least of which is awful tempco except for the NP0 types.

Maybe try a sheet of PTFE (Teflon), although I truly don't know about its characteristics either.

Dean

 

[Oznog]

The plastic can potentially add capacitance to nearby wires & components, detuning an RF circuit. That's a separate property to being lossy.

 

[stevez]

Plastics or polymers are quite interesting materials. The part that creates some difficulty is that it's so easy to include additives that can profoundly change the material characteristics. The implication is that using scraps of material in a critical application may be quite risky. Even purchasing something like the box from RS has some risk - who knows what they add to the raw material as part of manufacturing.

After my intial post I did find some information that indicated that ABS was likely to be a good choice of material. In my VHF setup I can remove the ABS box from the antenna leaving the wiring in open air - then evaluate. If I get it working but it fails in the presence of ABS I'll have my answer though as Nigel suggests, it's unlikely that the box is the problem.

I have some experience with polymer extrusion. Some of them have to be stored carefully or dried because of their affinity for moisture. Too much moisture can create some interesting results in an extruder (high temp, high pressure).

 

[Roff]

My suspicion that carbon black is the pigment in black ABS has been confirmed by a little Googling. The effect of carbon black on the conductivity is less clear. I did find that it is added to increase the conductivity of antistatic ABS, but I found one reference that implied that standard black ABS is a good insulator. I would be suspicious of any black plastic when considering its use as a dielectric.