PVC as radome

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=A0Are

le

Research the dielectric constant of PVC. I expect it will be low, and reasonably close to air. (?)

Fiberglass and (dry) balsa wood are also reasonable candidates. Cellular companies often use specialized structural composites (I think it's called "FRX", or something like that. Perhaps you can locate a source for these? The materials used for microwave radomes (which I used to know the name, but it escapes me at the moment), is another avenue to try.

IIRC, Maxrad antennas had some land-mobile antennas enclosed in PVC, so you're probably fine. Those would have been little UHF or 800 MHz antennas (5/8th over a 1/2 wave, or similar). The PVC was thin-wall however, as you would expect in a mobile.

What if you put it in a microwave oven and see if it heats up ? Along with a load.

greg

I know that it's a fairly popular material for amateur radio antennas at HF, but I'm not sure about its properties at UHF.

Try your antenna with and without the PVC. Pay attention to de-tuning, if any -- that would muck up calculations.

To get a quick estimate of the attenuation, you could wind a small coil around some PVC and resonate it with a cap, then measure the Q. If I did a measurement with and a measurement without the PVC core I would expect a small but possibly significant change in resonant frequency, and I would expect that after I finished measuring the Q change I'd have a better notion of how well the pipe would work.

If you're going to do this for production, you'd be smart to set up a test range and check every pipe, or at least every lot of pipe, before you use it. Stuff like that can vary in how it's manufactured as long as it does the job for which it's intended and marketed -- and radomes isn't one of those jobs.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

There is no quick answer to this question. A lot depends on the geometry of your antenna and the particular spots with the high intensity of EM field. The practical way would be try and see for yourself.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

RF is used for welding PVC, which has a dielectric constant of about 4.5. Other common thermoplastics are downwards of a half of that, so they can't be welded in the same way.

Ian

Dielectric constant and absorption constant are two different things, though. In principal there's no reason that a high dielectric material can't have low absorption.

And again, there are so many modifiers that you can put into plastics that you can't say "will plastic XYZ work as a radome". You have to ask "can plastic XYZ _ever_ work in a radome, and how do I make sure to get a flavor of it that will?".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Perhaps a quick "no" if the stuff is really badly absorptive.

But a "yes" certainly requires the experiment (and IMHO, constant repeat testing if you're buying pipe meant for plumbing).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

,

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=A0Are

ule

That's an interesting thought....!! I would make sure the microwave oven has a turntable, or at least an RF stirrer, as I'm sure most do nowadays?

I think you could also shred an amount of the PVC material and place it into a tuned cavitity filter (like a big VHF or VHF bandpass cavity filter) and check the response both before and after insertion of the material. If the tuning doesn't change too much, I would think the material would be acceptable as a radome.

That said, I would not fill the cavity completely, or even allow the PVC material to come into contact with the center plunger of the cavity. I think that may skew the results of the tests, but I would have to think hard about the exact reason why.... It seems to me I did this test once before in my life... ????

-mpm

PVC does introduce some loss and some de-tuning. A lot of hams use it (e.g. for radomes on simple twin-lead J-pole antennas), but it's difficult to predict whether any specific batch will have a low enough loss to meet your requirements. Since it isn't being manufactured with RF characteristics in mind, different batches or brands may be significantly different in their RF behavior.

The suggestion to stick a few inches in a microwave (with a cup of water some distance away), zap it for a few seconds, and see if it heats up is a good one. It's not a certain test, of course, but if the tube does warm up appreciably when exposed to microwaves, it's probably not a good choice at UHF.

How large a diameter to you require? If it's anywhere up to 2.5" OD, you might want to get some fiberglass tubing from Max-Gain Systems... fiberglass-and-resin is a traditional RF radome material and it may have lower losses than PVC.

If this is a one-off project, you might want to consider making your own radome, using fiberglass cloth and resin... you could probably fabricate a radome which has a thinner wall than most commercial fiberglass or PVC pipe.

--
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Not necessary, although helpful.

Given the dielectric constant it _would_ change the tuning. That's not the issue -- the issue is whether it absorbs the waves as well as diffracting them.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

I just thought of a grid dip meter !!

Rather useless experiment. Whatever the result would be, it is largely irrelevant if the material could be used for OP's antenna or not.

FWIW, once long ago I did a mistake: isolated a junction of coax cables with PVC electric tape. The power level was ~1kW at ~60MHz. The tape burned away in few hours. Then I used polyethylene for isolation and it worked fine.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

)

=A0Are

le

Hello,

It depends strongly on how close the radome is to the actual antenna. When it is significantly within 0.25lambda of the antenna structure, it will experience the (strong) reactive field. So you need to guess (measure, simulate) the field strength experienced by the PVC.

With 40W, and about 80mm from the antenna, you may get about 1kV/m, the worst case is when this field is parallel to a thin sheet of PVC as the field inside the PVC equals the air field strength. Assuming Er =3D 3 and tandelta =3D 0.015 gives about 1kW/m^3 material loss. With a radome material thickness of 3mm, this would result in 3 W/m^2 loss (guess surface area exposed by the field to find the "actual" loss). When the unobstructed field is perpendicular to the material, loss drops significantly (as the E-field inside the PVC drops to about 33 % of the value in air).

0.5 dB from 40W is about 4W. When you increase the distance between the antenna and the radome, E drops and loss/m^3 is proportional to E^2. However if the material is very close to the antenna, over a large area, overall loss may be above 0.5 dB. Considering PVC is not that bad. Other issue to check: UV resistance and plastic deformation (creep) under mechanical loading in combination with high temperature.

I hope this gives you some guidance in whether it is worth to try PVC.

Best regards,

Wim PA3DJS

without abc, PM will reach me very likely

e,

Exactly. Pure PVC is white, not gray; that color is a pigment of some sort, and the manufacturer of 'schedule 40' pipe is making his tests on burst strength, not RF absorption. It wouldn't be unlikely for the labeling ink on the pipe to be active in this case, either (hematite is a common pigment).

Perhaps look at model rocketry suppliers. They use f/g tubes for rocket fuselages - from an inch or so up to 6" diameter.

I once painted black the piece covering a coil on a CB or other type of antenna. That screwed that up pretty good. I found different black paints have different characteristics. Some make pretty good ED shielding or grounding.

Allright, I want some PVC to stick in the microwave, anyone ? I'm thinking of other stuff at The Home Depot !!

greg

dom=3D

long

Hello Greg,

Using the microwave oven test is nice to compare two or more materials, but you still don't know whether the best (or worse) material matches your requirements. When the shape of the materials differs, you may get strange (unreliable) results.

Properties of paint amaze me also (huge difference in reflectivity and transmissivity). Some (spray) paints with metallic look do virtually not block the (UHF) signal.

I ones used black thick polycarbonate sheets as an antenna cover (mechanically heavy loaded). It did detune the antenna somewhat, but after retuning I couldn't measure the difference in produced field (w.r.t. uncovered situation, mid UHF situation). With black material I am always afraid to encounter too high carbon black levels.

Best regards,

Wim PA3DJS

without abc, PM will reach me.

Using the microwave oven test is nice to compare two or more materials, but you still don't know whether the best (or worse) material matches your requirements. When the shape of the materials differs, you may get strange (unreliable) results.

Properties of paint amaze me also (huge difference in reflectivity and transmissivity). Some (spray) paints with metallic look do virtually not block the (UHF) signal.

**They use flakes of stuff with optical properties like mica, or "fools gold".

I ones used black thick polycarbonate sheets as an antenna cover (mechanically heavy loaded). It did detune the antenna somewhat, but after retuning I couldn't measure the difference in produced field (w.r.t. uncovered situation, mid UHF situation). With black material I am always afraid to encounter too high carbon black levels.

**Black is the riskiest colour for RF welding of PVC. Clear (pure PVC is clear pale blue) is the most predictable. We assumed that the problem was the use of carbon black, but never found out for sure. Another problem is metallic inclusions, which resulted in spectacular fireworks and blew holes in the brass tooling.

**Handle also made a big difference. Rigid is hard to weld, and very soft is too easy and heat build-up in the tooling can melt the surface before the inside. Different fillers, such as chalk, make a difference, especially if they hold water.

**The chlorine is polar so the chains wriggle in an AC field. Shorter chains, especially when the structure is loosened by the addition of plasticiser, mean more wriggling. Presumably energy is lost in the breaking and remaking of whatever bonds hold the chains together.

**Dunno what this has to do with radio waves. It does illustrate how much variation there is in the material, as others have pointed out.

**The chlorine inhibits fire, incidentally. PVC in electrical installations can be fire-prone because of its electrical properties, but the risk of a serious fire is considered acceptably small.

Ian

How are you going to accurately measure a 0.5 dB drop of the RF field?

Anyone (at least on Earth) are going to ask for trouble by designing a radio link with only 0.5 dB fade margin :-).