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Old 28th Feb 2019, 6:50 pm   #1
Biggles
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Default Which is the best material for an aerial sleeve?

I am at present putting together a VHF/UHF antenna for outside use and was going to use a length of white plastic waste water pipe of about 40mm diameter as the weather cover. I had heard a few years ago that some plastic pipes can attenuate the signal reaching the antenna, depending on the material they are made from. One suggestion was to put a test piece in the microwave for a few seconds and if it becomes warm then the material is unsuitable for the antenna cover as it absorbs RF energy. I have some doubts over this theory and was wondering if anyone has any experience of this and which material to avoid or suggestions for alternative forms of construction?

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Old 28th Feb 2019, 7:25 pm   #2
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Default Re: Which is the best material for an aerial sleeve?

The issue can become a bit of a minefield - it's often quite difficult to identify the precise chemistry of a piece of plastic just by looking at it (which is one of the difficulties when trying to recycle the stuff) - similarly a kind of plastic that is 'good' at RF in its basic form may become lossy when colourants (like carbon-black...) are added to it.

In the past I've used Polythene for RF coil/choke formers in transmitters where a few hundred Watts are flying around, and it's worked just fine.

I believe the outer covering of the 'Comet'-branded white-stick VHF/UHF collinears is actually glass-reinforced-plastic (GRP): you may be able to source a hollow windsurf-mast! Though be aware that some of these are now Kevlar-reinforced and some Kevlar is conductive...
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Old 28th Feb 2019, 9:30 pm   #3
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Default Re: Which is the best material for an aerial sleeve?

I think you'll be OK with white. As G6.. says it when it is coloured, especially black, that problems may arise.
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Old 28th Feb 2019, 9:55 pm   #4
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Default Re: Which is the best material for an aerial sleeve?

GRP is a bit complicated for me. The whole project as usual is to be built at near zero cost, usually from what I already have lying around in the garage. I was considering radiating a small signal from a genny and connecting the bare antenna to a receiver to provide a reference level, then putting the pipe over the antenna and seeing if it made a difference, but that will take a lot of setting up, which is why I thought I would see if anybody who is into antenna building had done any experimenting already. The pipe I have is standard white "plastic" waste pipe so hasn't any added colourant to my knowledge. The actual antenna is an active one I found at a rally in unmade kit form, and uses a MAR-X type device, so has to be weatherproof if used outside. I think it is a Howes AA-1 or something, from some time ago.
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Old 28th Feb 2019, 10:21 pm   #5
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Default Re: Which is the best material for an aerial sleeve?

Quote:
Originally Posted by Biggles View Post
I had heard a few years ago that some plastic pipes can attenuate the signal reaching the antenna, depending on the material they are made from. One suggestion was to put a test piece in the microwave for a few seconds and if it becomes warm then the material is unsuitable for the antenna cover as it absorbs RF energy.
There is some logic in that....if there is any metal content. Try it for a minute or so but put a cup of water in the microwave as well to act as a 'load'. The cup of water should get warm but the test plastic should stay cold.
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Old 28th Feb 2019, 11:35 pm   #6
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Default Re: Which is the best material for an aerial sleeve?

I will try that out of sheer curiosity and report back, just as soon as my other half has gone out. I still remember the consequences of her unexpected return as I was "boiling" my motorbike chain in lube on the cooker hob. I will be extra careful to avoid having to remove a mess of melted plastic from inside the microwave.
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Old 1st Mar 2019, 12:49 am   #7
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Default Re: Which is the best material for an aerial sleeve?

I seem to recall that polystyrene has some nice dielectric properties (see the HRO HF coil-packs), but is not the toughest material in the world. I used to work for a company where one of the senior technical guys had a big thing about using PEEK (polyether ether ketone) for all things RF. This got a big thumbs up from the guy who had to machine it, who said it was a dream to work with.

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Old 1st Mar 2019, 9:57 am   #8
Steve G4WCS
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Default Re: Which is the best material for an aerial sleeve?

I built one of the maplin type active antennas into a length of the grey waste pipe and it worked fine
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Old 1st Mar 2019, 6:56 pm   #9
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Default Re: Which is the best material for an aerial sleeve?

I would think that the plastic will absorb very little energy. However, I have had white waste pipe disintegrate when used outside and the local water company installed some large plastic pipes which had been stored outside for some time. They burst and hundreds of tons of soil were washed away.
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Old 1st Mar 2019, 7:38 pm   #10
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Default Re: Which is the best material for an aerial sleeve?

As TrevorG3VLF observes, if your antenna is to be erected outdoors you need to consider the possible effects of UV light on your chosen plastic as well as its RF-characteristics!

Personally, I'd just go with a length of _white_ waste-pipe (so avoiding the risk of RF-lossy 'fillers' or colourants). Another thing though - if it's to go outside at any sort of elevation make sure you put some sort of vibration-damping between the tube and whatever metal element you fit inside. A few foam-rubber discs threaded onto the element before stuffing it into the tube..

Why? Well, at a previous QTH I put a 'white stick'-type 144/220/430MHz collinear antenna on the chimney and the first time we had a decent wind it excited a vibration between the outer tube and the inner element - with the rattle conducted down the stub-mast, the chimney, and into our bedroom which was immediately below. Definitely not conducive to sleep! or domestic harmony!
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Old 1st Mar 2019, 8:12 pm   #11
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Default Re: Which is the best material for an aerial sleeve?

The active antenna (checked out now as the Howes AA-4) is actually formed on a PCB so the antenna elements are print. The PCB forms a blade which fits quite snugly inside the intended plastic pipe, so spacers shouldn't be necessary. The bottom section which is going to be clamped to a bracket is a piece of ali tube which again fits snugly inside the plastic pipe. Two end caps (a waste product from the day job) effectively seal the unit from the weather. All built from scrap materials. I think the kit cost me about a quid at a rally a few years back. I think I will just use the plastic pipe I have and risk it. The general opinion seems to be that it won't be a problem. The neighbours won't notice another antenna among the dozen or so I already have on the shack. They are quite used to see me clambering about on the roof. I always wave and say hello.
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Old 4th Mar 2019, 11:16 am   #12
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Default Re: Which is the best material for an aerial sleeve?

My ancient Panasonic microwave oven has the usual 3 wheeled bearing assemly under the glass plate.
The moulded shaft for one of the wheels snapped off, and I did some modding and ended up with a white number-plate fixing bolt and nut as a replacement axle. Very satisfactory.
A similar accident happened to another wheel assembly about a year later, and I repaired that one with a black version of the other nut and bolt. It immediately started to melt when the oven was switched on.
I guess from that experience the filler/colourant is important in the choice of material. Tony.
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Old 4th Mar 2019, 1:26 pm   #13
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Default Re: Which is the best material for an aerial sleeve?

Quote:
Originally Posted by G6Tanuki View Post
...you need to consider the possible effects of UV light on your chosen plastic...
From my observations of various bits of white waste pipe either lying around the garden or actively channeling water somewhere, only the horizontal bits seem to be affected by the UV (actually looking burnt in colour).

The vertical pieces don't seem to be affected at all.
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Old 4th Mar 2019, 2:21 pm   #14
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Default Re: Which is the best material for an aerial sleeve?

I guess that would be down to the vertical pipe getting less overall exposure to the UV as the sun direction moves round than a horizontal one which would have one side exposed all day. When the sun's high in the sky, the energy/area exposed for vertical would be less, too.

Whether this would just slow down the degradation or there's a "threshold below which" effect, I've no idea.
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Old 4th Mar 2019, 2:30 pm   #15
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Default Re: Which is the best material for an aerial sleeve?

When I started with Post Office Telephones back in the 1960's I was taught that white or clear polythene sheathed underground cable should never be used above ground as it would be affected by the uV rays in sunlight.

Black polythene sheathed cable could be used above ground as it contained carbon which mitigated the uV affect.

PVC sheathed designed for internal use could be used out of doors so long as it wasn't exposed to direct sunlight.
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Old 4th Mar 2019, 6:53 pm   #16
Biggles
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Default Re: Which is the best material for an aerial sleeve?

The same theory applies to cable ties apparently. Always use the black ones for outdoors. From my experience the white ones quickly go brittle but the black ones seem to have a fairly long life when exposed to UV and the elements. I am not sure of the actual composition but they may use similar plastics to U/G cable sheaths. Most of the U/G cable we use has a black outer sheath, with some armoured types having a grey or green sheath. None seem to be affected by exposure to daylight.
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Old 4th Mar 2019, 7:27 pm   #17
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Default Re: Which is the best material for an aerial sleeve?

A drink bottle slipped over the top? No need to waterproof the bottom and easily replaceable. A glass one would be perfect but I have yet to see a degraded PET https://en.wikipedia.org/wiki/Polyet..._terephthalate bottle on the ground.
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