Topics

Using PU foam in feedhorns?

Neil Smith G4DBN
 

Does anyone have the tan-delta and relative permittivity data for ordinary (B&Q style) polyurethane building foam (in expanded form) at microwave frequencies?  Just thinking of how to combat condensation inside the feedhorn. I know there are special foams which perform really well at RF, but they seem to come in giant barrels and cost $$$$$$.

Neil G4DBN

Steve G4HTZ
 

Interesting Neil ....I was just wondering how to waterproof the 24ghz feedhorn....just starting to mount the new DB6NT 24ghz transverter into some sort of box .
steve 
G4htz

John Lemay
 

I'll be interested to see some answers too .......... contemplating filling dipole boxes with foam on the basis that it can't be worse than water !

John G4ZTR

-----Original Message-----
From: UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of Neil Smith G4DBN
Sent: 22 November 2018 12:57
To: UKMicrowaves@groups.io
Subject: [UKMicrowaves] Using PU foam in feedhorns?

Does anyone have the tan-delta and relative permittivity data for
ordinary (B&Q style) polyurethane building foam (in expanded form) at
microwave frequencies? Just thinking of how to combat condensation
inside the feedhorn. I know there are special foams which perform really
well at RF, but they seem to come in giant barrels and cost $$$$$$.

Neil G4DBN

F6DRO
 

Hi Neil ,

tested on sun noise and heavilly used here , this foam has absolutely no influence up to at least 47Ghz
73
Dom/F6DRO

Le 22/11/2018 13:56, Neil Smith G4DBN a écrit :
Does anyone have the tan-delta and relative permittivity data for ordinary (B&Q style) polyurethane building foam (in expanded form) at microwave frequencies? Just thinking of how to combat condensation inside the feedhorn. I know there are special foams which perform really well at RF, but they seem to come in giant barrels and cost $$$$$$.

Neil G4DBN


Neil Smith G4DBN
 

At present, I just have a PE cap about 1mm thick over the front of the horn, sealed to the outside of the horn with self-amal rubber and a PVC over-wrap.  I use O ring seals on the two square flange joints, and the coupling rings on the round flanges at the ends of the flexi guide are encased in self-amal rubber tape, so in theory, the entire waveguide system is sealed, but there are multiple ally to ally machined face joints in the feedhorn, and I suspect that there is a small amount of air leakage there, which just might  result in diurnal pumping and a buildup of water in the lowest part of the horn.

I could dismantle the horn and fit O rings (or silicone sealant?) at the face joints, but I thought just filling it with foam might be as good.

If the dielectric constant of the foam is well defined and more than say 1.05, it ought to be easy to recalculate the dimensions to scale the horn by the few percent required.  If it is less than that, it won't move the RL sweet spot far enough to matter.

If the numbers are not published, I guess I'll just foam the thing up, slice it flat at the ends, pop the cap on and sweep the RL to compare with the no-foam version and try to work out the effective dielectric constant and iterate until the machine shop is full of aluminium swarf.

Ah, just seen the message from Dom.  That gives me some confidence to try a test!

Thanks Dom.
Neil G4DBN

On 22/11/2018 13:03, Steve G4HTZ via Groups.Io wrote:
Interesting Neil ....I was just wondering how to waterproof the 24ghz feedhorn....just starting to mount the new DB6NT 24ghz transverter into some sort of box .
steve 
G4htz

Barry Chambers
 

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

Richard GD8EXI
 

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" <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
>
>
>
>

Richard <richard@...>
 

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" <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@...

 HTTP  :  http://www.g8jvm.com
 
 ######################################################################

 Ham Call: G8JVM . QRV: 50-432 MHz + Microwave 
 Maidenhead QRA: IO82SP38, LAT. 52 39.720' N LONG. 2 28.171 W
 QRV 6mtrs 200W, 4mtrs 150W, 2mtrs 300W, 70cms 200W, 
Microwave 1296MHz 140W, 2320MHz 100W & 10368MHz 5W
 OS: Linux Mint 19 x86_64 Tara, on a Dell Inspiron N5030 laptop
 ######################################################################

Neil Smith G4DBN
 

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" <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@...

 HTTP  :  http://www.g8jvm.com
 
 ######################################################################

 Ham Call: G8JVM . QRV: 50-432 MHz + Microwave 
 Maidenhead QRA: IO82SP38, LAT. 52 39.720' N LONG. 2 28.171 W
 QRV 6mtrs 200W, 4mtrs 150W, 2mtrs 300W, 70cms 200W, 
Microwave 1296MHz 140W, 2320MHz 100W & 10368MHz 5W
 OS: Linux Mint 19 x86_64 Tara, on a Dell Inspiron N5030 laptop
 ######################################################################


Alan Melia
 


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 -----
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" <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@...

 HTTP  :  http://www.g8jvm.com
 
 ######################################################################

 Ham Call: G8JVM . QRV: 50-432 MHz + Microwave 
 Maidenhead QRA: IO82SP38, LAT. 52 39.720' N LONG. 2 28.171 W
 QRV 6mtrs 200W, 4mtrs 150W, 2mtrs 300W, 70cms 200W, 
Microwave 1296MHz 140W, 2320MHz 100W & 10368MHz 5W
 OS: Linux Mint 19 x86_64 Tara, on a Dell Inspiron N5030 laptop
 ######################################################################


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@...
 
 
######################################################################

Ham Call: G8JVM . QRV: 50-432 MHz + Microwave
Maidenhead QRA: IO82SP38, LAT. 52 39.720' N LONG. 2 28.171 W
QRV 6mtrs 200W, 4mtrs 150W, 2mtrs 300W, 70cms 200W,
Microwave 1296MHz 140W, 2320MHz 100W & 10368MHz 5W
OS: Linux Mint 19 x86_64 Tara, on a Dell Inspiron N5030 laptop
######################################################################
 

F6DRO
 

Hi All ,

I am using this , I have it pasted on my 3cm horn for something like 10 years and never got the sligthest trace of water.
73
Dom

Le 22/11/2018 18:16, Barry Chambers a écrit :
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



Alan Melia
 


Hi Simon, yes I remember Hydrogen......I received a valve at DH from the shore end of CANTAT where we could not get the cathode  to emit. When we cracked the valve in the mass spectrometer it was full of hydrogen. This had reduced the metal-oxide-glass seal round the pins allowing the valve to suck in hydrogen from the repeater body. It was produced by electrochemical reaction between the repeater casing and the sacrificial anode, meant to stop the steel case rusting.
 
Alan
G3NYK
 

----- Original Message -----
Sent: Friday, November 23, 2018 12:42 AM
Subject: Re: [UKMicrowaves] Using PU foam in feedhorns?

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@...
 
 
######################################################################

Ham Call: G8JVM . QRV: 50-432 MHz + Microwave
Maidenhead QRA: IO82SP38, LAT. 52 39.720' N LONG. 2 28.171 W
QRV 6mtrs 200W, 4mtrs 150W, 2mtrs 300W, 70cms 200W,
Microwave 1296MHz 140W, 2320MHz 100W & 10368MHz 5W
OS: Linux Mint 19 x86_64 Tara, on a Dell Inspiron N5030 laptop
######################################################################