Black PVC - opinions sought

[VK3QI]

Hi,

Although this question relates to a non-ham situation, I would be interested in your expert opinions.

I have a Cisco WAP that has to mounted on the underside of the eaves on a school building, at a height of 3.0 metres above ground. Naturally it is a target for students to try and hit the various antennas on it.

I had intended mounting a white plastic alarm siren cover over the WAP, but the dimensions are a little too large to fit in the conventional plastic alarm covers.

I am looking at using a 300mm square black PVC drainage pit (the type that has a removable metal lid) as a cover instead. I can drill a series of holes in the sides of the plastic, but I was wondering what effect the "black" in the PVC will have on the WAP signal - think 2.4 GHz.

I have searched high and low for a larger white plastic alarm cover, but it seems that since they went from the old metal covers to the plastic ones, they reduced dimensions.

Any other suggestions for a source or some other alternative would be welcomed.

Cheers

Peter VK3QI

[VK3MIX]

Best way to test this is place it in the microwave if it fits or cut a piece off and see if it melts. Not sure if a cup of water needs to be placed with it or not.

[VK2JDS]

definately put in a cup of water
microwave ovens need some sort of load, if you run them empty they arc and carry on. hence water solves the problem.
but, the microwave test is good for checking plastics of all types for rf absorbtion

just dont put in anything metal...
73 Dave

[VK3QI]

Thanks Dave,

I will be doing the microwave oven test.

Interestingly I have a couple of 2.4 metre C band mesh dishes that had a circular cone made of the same plastic over the illuminator, as a weather shield (radome), so my initial thoughts are that it will be OK. However I have seen mention on several web sites that some PVC's that have been treated for ultraviolet protection can have a detuning effect, especially if close to the actual antenna elements.

Fortunately this particular Cisco WAP has 6 antennas which can be arranged so that they are clear of the sides of the intended cover.

Cheers

Peter VK3QI

[VK6SIX]

Some black and even white PVC tubing at one time contained carbon.

ZS6AVM built new beam insulators from carbon impregnated PVC pipe stuff and could not tune the beam!

Took him a fair while to find out about that HI.

I think there may be a compostion chart etc of various PVC pipes.

It may be called a "schedule"

Maybe check that first?

E&OE

[VK3XL]

I work in polymer science in a uni in Melbourne and can tell you that almost all polymers that are black contain carbon black, as it is the cheapest way to make polymers black and provides a fair degree of UV stabilization to boot.
So it won't matter if the plastic material is PVC, HDPE or GRP (fibreglass) or any other plastic, if it is black it is most likely to contain carbon black. Grey PVC also has carbon black but only a very small amount. Other additives in PVC include plasticizers and talc (calcium carbonate etc) which may or may not be affected by RF so the microwave test is a good idea to make sure your plastic will not cause any problems with your project.

Not sure about white PVC having carbon black in it,the smallest amount of carbon black in a polymer soon turns things grey. I think the culprit here is Titanium dioxide which is used to make it white. Older PVC may have lead oxide as the whitening agent, but has not been used in PVC manufacture for many years.

[VK4TI]

Hi,

Although this question relates to a non-ham situation, I would be interested in your expert opinions.

I have a Cisco WAP that has to mounted on the underside of the eaves on a school building, at a height of 3.0 metres above ground. Naturally it is a target for students to try and hit the various antennas on it.

I had intended mounting a white plastic alarm siren cover over the WAP, but the dimensions are a little too large to fit in the conventional plastic alarm covers.

I am looking at using a 300mm square black PVC drainage pit (the type that has a removable metal lid) as a cover instead. I can drill a series of holes in the sides of the plastic, but I was wondering what effect the "black" in the PVC will have on the WAP signal - think 2.4 GHz.

I have searched high and low for a larger white plastic alarm cover, but it seems that since they went from the old metal covers to the plastic ones, they reduced dimensions.

Any other suggestions for a source or some other alternative would be welcomed.

Cheers

Peter VK3QI

Go visit a cctv wholesaler and buy a medium perspex dome cover , doesn't interfere with the rf appreciably and are very strong and impact resistant , or you could catch and hurt a teen damaging the system , the abuse would stop

[VK2OMD]

I work in polymer science in a uni in Melbourne and can tell you that almost all polymers that are black contain carbon black, as it is the cheapest way to make polymers black and provides a fair degree of UV stabilization to boot.
So it won't matter if the plastic material is PVC, HDPE or GRP (fibreglass) or any other plastic, if it is black it is most likely to contain carbon black. Grey PVC also has carbon black but only a very small amount. Other additives in PVC include plasticizers and talc (calcium carbonate etc) which may or may not be affected by RF so the microwave test is a good idea to make sure your plastic will not cause any problems with your project.

Not sure about white PVC having carbon black in it,the smallest amount of carbon black in a polymer soon turns things grey. I think the culprit here is Titanium dioxide which is used to make it white. Older PVC may have lead oxide as the whitening agent, but has not been used in PVC manufacture for many years.

It has been my observation by measurement of loss on two wire transmission lines using PVC insulation (eg 'speaker wire') that the dielectric loss of flexible PVC can be much higher than predictions such as K&L for rigid PVC. I had come to a view that it is likely that there are additives for colouring, UV resistance, and flexibility to name a few that make a significant difference to the loss tangent of PVC.

One interesting example was a very flexible clear speaker cable sample that had significantly higher loss than a light grey one with thinner conductors... so the explanation was not carbon black, nor is avoiding grey or black colour a reliable avoidance strategy. BTW, BOTH of these lines were higher loss at HF than RG58.

I suspect that other polar plastics will be similarly sensitive to these agents.

Non-polar plastics are about two orders of magnitude better (loss tangent), and should be the choice for applications where E field is significant. Sometimes, think a little about basic electric field distribution gives some idea whether the application will do the max to heat the plastic.

Mike, your comments offer possible reason for this.

Nevertheless, many hams like to use materials for insulation that are unsuitable... mostly justified on availability or cost, and the mediocrity driver than "any antenna is better than no antenna". I expect that this discussion will be an annoyance to them.

Recently here, it was suggested that the ordinary kitchen cutting boards sold for kitchen use are Teflon. In my limited experience, I have not seen Teflon used for low cost cutting boards, it would be a great bargain if it was. I am confident that the ones I have used were not Teflon, but a lower cost polymer, very suited to the antenna insulator application, but not Teflon. Teflon cutting boards are probably ham folk lore.

The microwave oven test is often proposed. One question is how to interpret any warming observed, is it significant enough to discard the material. Another is the relevance at say HF, where it is likely that the dielectric loss is a hundredth of that at the frequency used in the microwave oven. Is a material that is ok as a base insulator for a vertical or a centre insulator for a dipole necessarily good for a transmission line dielectric?

Sometimes there is an element of 'hard to get' that drives people, eg people who import the magic black braided Dacron cord from the USA because it is held to be the ONLY synthetic rope suited to outdoors use. Of course, it is available here directly and cheaper at ships chandlers and IIRC, Bunnings, as black venetian cord. Some of us know that plastic does not need to be black for UV resistance, I have been using white 'SIlver rope' for decades and it has been as UV resistat as any other rope I have tried... while well suited to the application, it is just a lot cheaper than most.

For Peter, the composition of the box is a bit of an unknown. I suspect it is a polar plastic, so in the poorer family but that does not condemn it. If you could obtain one, you could do a test with an AP using something that indicates field strength (eg INSSIDER). A cruder test would be a Smartphone with RSSI available, though better if you can get it on the 2GHz band than say 850/900. Yeah, I know it is a measurement based approach... but that is my approach, show me the numbers! As you noted, keeping the antennas as far as you can from the plastic box might minimise dielectric loss in the box.

Owen

[VK3XL]

Hi Owen, I agree with your observations. Flexible PVC contains plasticizers (usually oils) to make the normally rigid PVC flexible. These oils are probably the culprit in your loss experiments.

There are many newer (read more expensive) UV stabilizers used in plastics now, but carbon black remains a very cheap source of UV stabilizer in plastics as long as you dont mind it being black.

The cheap chopping boards you get in the $2 shop etc are not teflon. they are either HDPE or a blend of HDPE and LDPE, most likely the latter for cheapness.

Polar plastics may present a problem for microwave applications as they tend to be hydroscopic (absorb moisture from the atmosphere) and thus would offer some loss of signal if put in the rf path. Examples of Polar plastics include Polyamide, Polycarbonate, PMMA (acrylic sheet or Perspex) ABS. Non polar plastics include Polypropylene, Polyethylene, Polystyrene PTFE (teflon).

Peter many drainage pits are now made from HDPE rather than PVC. Does the pit you are proposing to use have a recycling number in a triangle on it? http://www.zerowaste.sa.gov.au/at-home/ ... stics-mean
hope this helps you to make a decision on a cover for the WAP to keep the little darlings and their tennis balls at bay.

[VK2OMD]

...
The cheap chopping boards you get in the $2 shop etc are not teflon. they are either HDPE or a blend of HDPE and LDPE, most likely the latter for cheapness.

And still quite good dielectric for a lot of applications, just not Teflon, and so different permittivity and dielectric strength.

Fibreglass (GRP) is often used for radomes. I suspect it is usually made with polyester resin which I think is a polar plastic... and so not a stunning dielectric, but the electric field gradient across a radome would usually be quite small and so, losses small. Mike, can you confirm the polyester resin is polar?

Owen

[VK3XL]

Polyester resin is polar. The polyester resin used in fibreglass is a co-polymer of polyester and styrene monomer.

[VK3QI]

Sorry for the delay in responding, but other work got in the way.

Thanks for all those who responded with various opinions.

I have done a few tests.

(1) Firstly, surprise surprise, what I thought was PVC for the stormwater pit is in fact polypropylene.

Model NameEverhard Series 300 Stormwater Pit DeepModel Number84816
MaterialPolypropylene ColourBlack
Product Dimensions (mm)W:357 H:450 L:357Package Dimensions (mm)W:357 H:450 L:357W

Apparently to give it more strength but to remain sufficiently flexible to take soil sinking of drains etc.

(2) I tried the material in a 750watt inverter microwave for 3 minutes. Result water boiled, material read approx. 30 degrees on IR temperature measurer, from ambient of 24 degrees. This slight increase would be due to the ambient air temperature in the oven rising over the 3 minutes (difficult to measure) - so conclusion little if any heating taking place.

(3) With the aid of our IT bod we set up a control situation with a laptop measuring the strength of a similar WAP at a distance of 80 metres and noted the signal strength (no other WAPs in the vicinity). Then replicating the situation but this time with the polypropylene cover over the WAP, no noticeable change in signal strength at 80 metres - so conclusion little if any absorption of the signal.


(4) The only thing we can't be sure of is whether the proximity of the cover to any of the 6 antennas on the WAP might be detuning some of them to a limited extent.

The WAPs are Cisco Aironet devices

(5) I compared the pit material with one of the black plastic radomes that were used extensively on the various C band mesh dishes that used to grace our skyline. As far as I can tell, they are identical materials - conclusion - good enough for C band satellites - good enough for a WAP cover.


Overall an interesting exercise.

Cheers

Peter VK3QI

[VK2OMD]

...

Overall an interesting exercise.

Cheers

Peter VK3QI

All good Peter.

A long time ago, I had an assignment to asses radome material for C band earth stations as birds were a problem with the Mylar-W that was used. It was shiny and they attacked their reflection, then the stuff failed with long tears and the birds built nests inside the feed unit. Of course, there were instant alarms due to depressurisation of the feed / waveguide system. This cost lots of money and downtime to replace the radome membrane as the dish had to be taken out of service to point it to the zenith to enable riggers access... so all vital services restored by alternative bearers.

The alternative was ordinary woven polypropylene truck tarp material. It was sufficiently transparent in the colours tested, and proved a much better solution. It wasn't reflective, and even if the birds pecked a hole in it, it did not rip clean across and admit them for nest building. The receive system noise figure approached 0.1dB, so attenuation was critically important.

Some quite ordinary materials can be well suited to the task as you have found with some intelligent analysis.

On the other hand, some materials held to be suitable in ham folk lore are rather poor (eg Figure-8 flex as feed line).

Owen