info
Google Groups no longer supports new Usenet posts or subscriptions. Historical content remains viewable.
Dismiss
Does polystyrene foam block rf?
793 views
Skip to first unread message
oak
Mar 21, 2016, 6:53:20 AM3/21/16
to
If possible I would like to bury an antenna in a piece of polystyrene
foam. The frequency is circa 2.4 gHz. The foam thickness will be about
1".
foam. The frequency is circa 2.4 gHz. The foam thickness will be about
1".
Ian Jackson
Mar 21, 2016, 7:30:03 AM3/21/16
to
In message <ncojqu$11hv$1...@gioia.aioe.org>, oak <m...@privacy.net> writes
'enclose'. [Literally 'burying' it - even if enclosed in polystyrene
foam - would certainly affect 2.4GHz!]
The answer is almost certainly 'yes'. The foam would probably be
reasonably low-loss, but its dielectric constant is higher than air, and
this will lower the resonant frequency of the antenna. It's probably
best to try it to see how much the antenna's performance actually
suffers.
--
Ian
>If possible I would like to bury an antenna in a piece of polystyrene
>foam. The frequency is circa 2.4 gHz. The foam thickness will be about
>1".
It took a second reading for me to realise that by 'bury', you mean
>foam. The frequency is circa 2.4 gHz. The foam thickness will be about
>1".
'enclose'. [Literally 'burying' it - even if enclosed in polystyrene
foam - would certainly affect 2.4GHz!]
The answer is almost certainly 'yes'. The foam would probably be
reasonably low-loss, but its dielectric constant is higher than air, and
this will lower the resonant frequency of the antenna. It's probably
best to try it to see how much the antenna's performance actually
suffers.
--
Ian
rickman
Mar 21, 2016, 9:15:55 AM3/21/16
to
On 3/21/2016 7:42 AM, Brian Morrison wrote:
> is fully surrounded by the foam.
>
> That suggests an antenna length of 62% of the free space size. If the
> antenna is already encased in some sort of radome then the effect of
> polystyrene foam will be much less due to the reduced proximity.
You two are forgetting that Styrofoam is not the same as polystyrene in
terms of the electrical properties. The DC of Styrofoam is low because
it is mostly trapped air. I found values from 1.03 to 1.11. It varies
because the density of Styrofoam varies.
More important than the electrical properties of the Styrofoam is the
fact that it tends to absorb moisture and does not withstand
environmental conditions very well. So outdoor use is not indicated
unless protected from the elements by a coating.
--
Rick
> On Mon, 21 Mar 2016 11:29:54 +0000
> Ian Jackson wrote:
>
>> but its dielectric constant is higher than air
>
> About 2.6 in fact, so you need the square root of that if the antenna
> Ian Jackson wrote:
>
>> but its dielectric constant is higher than air
>
> is fully surrounded by the foam.
>
> That suggests an antenna length of 62% of the free space size. If the
> antenna is already encased in some sort of radome then the effect of
> polystyrene foam will be much less due to the reduced proximity.
You two are forgetting that Styrofoam is not the same as polystyrene in
terms of the electrical properties. The DC of Styrofoam is low because
it is mostly trapped air. I found values from 1.03 to 1.11. It varies
because the density of Styrofoam varies.
More important than the electrical properties of the Styrofoam is the
fact that it tends to absorb moisture and does not withstand
environmental conditions very well. So outdoor use is not indicated
unless protected from the elements by a coating.
--
Rick
rickman
Mar 21, 2016, 11:17:40 AM3/21/16
to
On 3/21/2016 10:38 AM, Brian Morrison wrote:
Styrofoam is a trade name. Polystyrene is not expanded in the general
case. Styrofoam is expanded polystyrene.
> Are they really that different? Essentially it's density that matters,
> if the air/polystyrene ration is fixed then the Er should be similar.
That's right. When you say polystyrene you are referring to a solid
plastic material with no air. *That* has an er of about 2.5. Being
mostly air Styrofoam-like materials have a much lower er usually very
close to 1.0.
> At my work we use some stuff called Rohrcell which has a dielectric
> constant of about 1.01. It's brittle but again mostly air. It's rather
> expensive.
>
--
Rick
> On Mon, 21 Mar 2016 09:15:57 -0400
> rickman wrote:
>
>> You two are forgetting that Styrofoam is not the same as polystyrene
>> in terms of the electrical properties.
>
> And there I was thinking that Styrofoam was just a trade name...
> rickman wrote:
>
>> You two are forgetting that Styrofoam is not the same as polystyrene
>> in terms of the electrical properties.
>
Styrofoam is a trade name. Polystyrene is not expanded in the general
case. Styrofoam is expanded polystyrene.
> Are they really that different? Essentially it's density that matters,
> if the air/polystyrene ration is fixed then the Er should be similar.
That's right. When you say polystyrene you are referring to a solid
plastic material with no air. *That* has an er of about 2.5. Being
mostly air Styrofoam-like materials have a much lower er usually very
close to 1.0.
> At my work we use some stuff called Rohrcell which has a dielectric
> constant of about 1.01. It's brittle but again mostly air. It's rather
> expensive.
>
--
Rick
Brian Reay
Mar 21, 2016, 12:18:02 PM3/21/16
to
On 21/03/16 14:38, Brian Morrison wrote:
> On Mon, 21 Mar 2016 09:15:57 -0400
> rickman wrote:
>
> On Mon, 21 Mar 2016 09:15:57 -0400
> rickman wrote:
>
>> You two are forgetting that Styrofoam is not the same as polystyrene
>> in terms of the electrical properties.
>
>> in terms of the electrical properties.
>
> And there I was thinking that Styrofoam was just a trade name...
>
>
> Are they really that different? Essentially it's density that matters,
> if the air/polystyrene ration is fixed then the Er should be similar.
>
> if the air/polystyrene ration is fixed then the Er should be similar.
>
> At my work we use some stuff called Rohrcell which has a dielectric
> constant of about 1.01. It's brittle but again mostly air. It's rather
> expensive.
>
The other thing to consider is colour. If the material is 'loaded' with
> constant of about 1.01. It's brittle but again mostly air. It's rather
> expensive.
>
a pigment, especially to make it black, then it is probably high in
carbon. That will cause loss, in addition to the other issues mentioned
thus far.
Wimpie
Mar 21, 2016, 12:30:20 PM3/21/16
to
El 21-Mar-16 a las 11:53, oak escribió:
the white relatively soft material consisting of small foam grains that
is normally used for insulation.
There will be a slight reduction in center frequency for lowest SWR, but
that will be < 1%. As the relative useful bandwidth is >> 1%, you will
not notice this in practice.
Wim, PA3DJS
> If possible I would like to bury an antenna in a piece of polystyrene
> foam. The frequency is circa 2.4 gHz. The foam thickness will be about
> 1".
>
Yes you can do that, as long as the foam is dry. I assume that you mean
> foam. The frequency is circa 2.4 gHz. The foam thickness will be about
> 1".
>
the white relatively soft material consisting of small foam grains that
is normally used for insulation.
There will be a slight reduction in center frequency for lowest SWR, but
that will be < 1%. As the relative useful bandwidth is >> 1%, you will
not notice this in practice.
Wim, PA3DJS
Spike
Mar 21, 2016, 1:15:30 PM3/21/16
to
On 21/03/2016 16:17, Brian Morrison wrote:
> the Er for polystyrene and didn't consider the foaminess of the final
> stuff.
> To the OP, in the very worst case you could see the effective antenna
> length increase due to dielectric loading with a limit of about 1.67x.
> With polystyrene foam then this factor will be much less, probably only
> a few per cent. You may find the effect negligible but it's sensible to
> investigate any potential for detuning a little more.
One thing to bear in mind is that although expanded polystyrene is a
closed-cell material, nevertheless passages and voids can exist within
it, trapping water-vapour if not liquid water. This has the potential to
alter considerably the electromagnetic properties of the antenna
covering. Even extruded polystyrene cannot be considered a vapour barrier.
--
Spike
"They thought that because they had power, they had wisdom"
- with apologies to Stephen Vincent Benet
> On Mon, 21 Mar 2016 11:17:40 -0400
> rickman wrote:
>>> Are they really that different? Essentially it's density that
>>> matters, if the air/polystyrene ration is fixed then the Er should
>>> be similar.
>> That's right. When you say polystyrene you are referring to a solid
>> plastic material with no air. *That* has an er of about 2.5. Being
>> mostly air Styrofoam-like materials have a much lower er usually very
>> close to 1.0.
> OK, I think we're in agreement now. You are correct that I looked at
> rickman wrote:
>>> Are they really that different? Essentially it's density that
>>> matters, if the air/polystyrene ration is fixed then the Er should
>>> be similar.
>> That's right. When you say polystyrene you are referring to a solid
>> plastic material with no air. *That* has an er of about 2.5. Being
>> mostly air Styrofoam-like materials have a much lower er usually very
>> close to 1.0.
> the Er for polystyrene and didn't consider the foaminess of the final
> stuff.
> To the OP, in the very worst case you could see the effective antenna
> length increase due to dielectric loading with a limit of about 1.67x.
> With polystyrene foam then this factor will be much less, probably only
> a few per cent. You may find the effect negligible but it's sensible to
> investigate any potential for detuning a little more.
One thing to bear in mind is that although expanded polystyrene is a
closed-cell material, nevertheless passages and voids can exist within
it, trapping water-vapour if not liquid water. This has the potential to
alter considerably the electromagnetic properties of the antenna
covering. Even extruded polystyrene cannot be considered a vapour barrier.
--
Spike
"They thought that because they had power, they had wisdom"
- with apologies to Stephen Vincent Benet
Stephen Thomas Cole
Mar 21, 2016, 2:05:08 PM3/21/16
to
FranK Turner-Smith G3VKI
Mar 22, 2016, 10:06:01 AM3/22/16
to
"Stephen Thomas Cole" <use...@stephenthomascole.com> wrote in message
news:ncpcul$csm$1...@stc.eternal-september.org...
Is that anything like Steam Radio?
--
;-)
.
73 de Frank Turner-Smith G3VKI - mine's a pint.
.
http://turner-smith.uk
news:ncpcul$csm$1...@stc.eternal-september.org...
--
;-)
.
73 de Frank Turner-Smith G3VKI - mine's a pint.
.
http://turner-smith.uk
0 new messages