PVC Pipe for RF

of RF

"Any" is the wrong word to ask. "How much" is the right word to ask. If you are pumping many tens of kilowatts through the PVC, then 1% attenuation will result in the PVC bursting into flames pretty much immediately. But 1% probably won't matter a bit for a low-power or receiving application.

Different PVC formulations and even batches have been to known to exhibit different RF characteristics, so it is important not to ask "is PVC generically OK" but "is this piece of PVC I bought at Home Depot OK". After it's out in the elements and sunlight its characteristics will change. Even at much lower HF frequencies, the dirt and dust and grime that accumulates on insulators exposed to the elements is almost always a bigger RF and even DC factor than the actual material of the insulator.

One very very rough test is to stick the plastic piece in the microwave oven with a few cups of water. (The water is so that you don't damage the oven.) If the plastic heats up, then it is absorbing a certain uncalibrated chunk of the 2.45 GHz and it may also absorb at lower frequencies. If the plastic chars quickly, then it may well have some contaminant in it that would make it inappropriate for RF use.

Tim N3QE

of RF

Hello Joe,

As Tim also mentions, whether or not PVC depends on the application itself and how you apply the PVC.

When looking to your link, it seems to me you want to use the PVC pipe as a radome. While losses of PVC as a material are very high with respect to PE or PTFE, it is not useless in a radome application (from an RF standpoint).

When Wall thickness

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A cheap and dirty test is to put a piece of the pipe in your microwave oven, nuke it, and see if it gets hots.

I agree with the microwave oven test ! The black pvc tends to fare worse in adsorbing RF. I've used white UV stabilised PVC at 23cms without problems !

--
Best Regards:
                     Baron.

on

Hello Baron,

I think that using the microwave test is nice to test an unknown material against another (known) material. After the test you know what material is better, but it does not give info on whether the material is good for a certain application. For a high Q resonator, I would not use PA or PVC as a mechanical support (from experience, with smoke). For a radome PVC can mostly be used (as you mentioned for the

23 cm case).

Regarding black material. I did attenuation tests on 8mm green and black (carbon black) Polycarbonate sheets at 868 MHz (radome function). Difference in loss was to small to measure in an on-site situation.

Best regards,

Wim PA3DJS

Yes I remember the smoke ! Nasty acrid stuff with cyanide in it...

But it works quite nicely for weather protection as in the radomes.

I found that the material temperature was quite a bit higher for the same exposure time in an oven for the black PVC material and it visibly degraded on the surface.

I've never tried the gray stuff. White was always preferred because of the appearance and public perception of the finished product.

Its a neat idea for making wind turbine blades though. I wonder what the endurance is before the blade rips off or the mountings fail. I might just make up a set and stick them on an old alternator to see how they fare.

73's From the old Tiger. :-)

--
Best Regards:
                     Baron.

n of RF

So what happens if you don't put water in the oven?

on

Both black and gray PVC often have bits of iron oxide and tiny bits of metal hidden in it. I would avoid it for RF work for that reason. The white stuff is generally far cleaner. If no light is getting to the part, clear PVC may be an even better way to go.

Even the pure PVC is a very lossy dielectric. Once I did a mistake of using the PVC tape to isolate the connection of the coax cables feeding

1kW at VHF low band. Although it was no obvious heating initially, the PVC burned out within a day. Replaced it with polyethylene tape - no problem.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

This subject comes up periodically on alt.internet.wireless. If you know how to make your own fiberglass tubing, that is the way to go. It's not difficult, but a bit messy. Obviously, don't use any carbon fiber, just S-2 fiberglass and matting. S-2 is the kind of fiber glass used in aircraft.

for one you loose all calibration for your absorbancy test.

the wave intensity in there rises until losses match the input, the energy leaks out, or goes into the internal fittings of the oven.

I've read reports of the glass platter melting etc.

tion of RF

OK, I didn't know a microwave oven has feedback. Still, if the water heats up, some heat will be transferred to the material under test.

there's no feedback, just the laws of thermodynamics.

Then I don't understand your post:

----- the wave intensity in there rises until losses match the input, the energy leaks out, or goes into the internal fittings of the oven.

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The oven is a Faraday shield right?

Yeah to the waves the walls look like mirrors.

Think of it this way:

the power absorbed increases at higher signal levels.

the power input is fixed.

the power absorbed is equal to the power input.

n.

Look at it this way. You design a product, you design it for idiots. I can't imagine someone designing a microwave oven where they didn't account for an empty oven because it will happen. Thus, I just have a hard time believing your explanation.

I have found threads on running empty microwaves, but never from a designer of the ovens. There is all sorts of protect put in devices that you never mention in the manual.

057.html is an interesting thread.