Re: Using PU foam in feedhorns?

simonchettle
 

Back in the 70s we realised how Ions can penetrate “solid plastic” – working on submarine cables with PC insulation the amount of hydrogen that appeared in the centre conductor was surprising and interesting – to repair a joint you could (but did not) use the Hydrogen flame to melt the solder for a joint repair.  All right the cables were very deep so pressure is a consideration however the ingress of H atoms was observed in cables laid in shallow waters.
 
One or two cable failures were more of a puzzle as lengths of  cable were missing – something to do with foreign trawlers I understand.
 
Simon
G8ATB
 
 

From: Alan Melia via Groups.Io
Sent: Thursday, November 22, 2018 10:34 PM
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] Using PU foam in feedhorns?
 
Neil even if the foam is ''closed'', even ''solid'' plastic is really a load of holes tied together with chains of carbon, water vapour or more specifically OH ions are quite small and will diffuse in driven by the concentration gradient. Eventually every hole in the foam will be filled with water vapour or liquid water depending in the temperature. This will not do much for RF transparency I suspect. (I was involved in plastic encapsulated IC reliability and conformal coatings at BTRL.)  Dunking in water for 24 hours wont tell you much about the long term.
 
When feed horns are out of reach this is a pain but overall it is probably best to just let it drain out occasionally.
 
Alan
G3NYK.
----- Original Message -----
From: Neil Smith G4DBN
To: UKMicrowaves@groups.io
Sent: Thursday, November 22, 2018 8:50 PM
Subject: Re: [UKMicrowaves] Using PU foam in feedhorns?
 

I am confident that the only feasible ingress path is water vapour in damp air through the joints.  The open end of the horn is sealed with a PE cap, and the interior of the waveguide/horn is connected to the steel equipment box with some serious seals.  The box has a semi-permeable breather, but the waveguid is the lowest point, so any damp air might gather there.  The potential for diurnal pumping *should* be very low, but I've seen surprisingly large volumes of water inside "sealed" compartments after a year or two of temperature cycling in damp air.   As (hopefully) only water vapour (and any other fluids in the vapour-phase) will be able to get in between the joints in the aluminium parts, I think I can discount much by way of salts and other rubbish getting inside.

One simple option is just to TIG the seams on the outside of the horn, but that seems a bit extreme compared with a squirt of magic foam.

I know where there is some excess foam which has been exposed to the outside air for 5 years, so I'll carve a bit of that off and try the microwave test on it to see if there is water inside the foam.  I tried the foam in tghe microwave with no obvious heating seen, and I've got a lump of it weighted down in a bucket so I'll test that in a few days.

I need to do some work on the antenna system in a week or two anyway, so I'll have a careful check for any water droplets inside the waveguide and horn.  Ultimately, a dry air overpressure system is the solution, but I don't want to go to those lengths.

Neil G4DBN


On 22/11/2018 19:08, Richard wrote:
I'd be inclined to use your local rain water for the second part.
Tap water is pretty pure , unlike what falls out of the sky , salt ,bits of carbon etc, which could contaminate the surface.
It would avoid any misleading results
Unless of course if you are in the London area where they reckon the water has been through many kidneys before it come out of the tap.
 
On Thu, 2018-11-22 at 18:56 +0000, Richard GD8EXI wrote:
Can I suggest an experiment with B and Q polyurethane foam. Put a freshly made lump of it in a microwave oven along with a dummy load (cup of water) and see how hot it gets after say 3 minutes at full power. Then take the same the lump of foam, soak it in water for 24 hours or more, dry its surface and repeat the experiment in the microwave.

PU foam is used for microwave application as you already know for example.

https://www.generalplastics.com/wp-content/uploads/2016/12/White-Paper-PU-Foam-Dielectric-Materials-for-Use-in-Radomes-and-Other_Applications.pdf

However some of the additives in B and Q foam may make it less suitable. It only takes 1% or less of something with a high dielectric loss or something in there that make it hydroscopic to ruin the performance in a horn.


Richard
GD8EXI



On 22/11/2018, 17:16, "Barry Chambers" <wlmailhtml:b.chambers@...> wrote:

> Neil
>
> I suspect the Er will be around 1.05 at worst but you will need to think
> about whether the pores in the foam are open or closed. I'm thinking
> about water getting into the foam.
>
> 73
>
> Barry, G8AGN
>
>
>
>
-- 
Best wishes /73
Richard Bown
 
Email : richard@...
 
 
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