PVC as radome
blanker
I am attempting to use a schedule 40, grey PVC pipe as an antenna radome,
and am concerned about RF losses.
My application is a UHF transmitter with an output power of 40 Watts (46
dBm) into an omnidirectional antenna with 3 dB gain, or total estimated
EIRP of 49 dBm (80 Watts). RF is Right Hand Circularly Polarized (RHCP)
emissions at an operating frequency of 401 MHz.
I cannot tolerate more than about a half dB loss through the PVC pipe. Are
there any significant RF losses in radiating such a signal through Schedule
40, grey PVC pipe?
---------------------------------------
Posted through http://www.Electronics-Related.com
mpm
wrote:
Research the dielectric constant of PVC.
I expect it will be low, and reasonably close to air. (?)
Fiberglass and (dry) balsa wood are also reasonable candidates.
Cellular companies often use specialized structural composites (I
think it's called "FRX", or something like that.
Perhaps you can locate a source for these?
The materials used for microwave radomes (which I used to know the
name, but it escapes me at the moment), is another avenue to try.
IIRC, Maxrad antennas had some land-mobile antennas enclosed in PVC,
so you're probably fine.
Those would have been little UHF or 800 MHz antennas (5/8th over a 1/2
wave, or similar).
The PVC was thin-wall however, as you would expect in a mobile.
GregS
>Hello,
>
>I am attempting to use a schedule 40, grey PVC pipe as an antenna radome,
>and am concerned about RF losses.
>
>My application is a UHF transmitter with an output power of 40 Watts (46
>dBm) into an omnidirectional antenna with 3 dB gain, or total estimated
>EIRP of 49 dBm (80 Watts). RF is Right Hand Circularly Polarized (RHCP)
>emissions at an operating frequency of 401 MHz.
>
>I cannot tolerate more than about a half dB loss through the PVC pipe. Are
>there any significant RF losses in radiating such a signal through Schedule
>40, grey PVC pipe?
>
What if you put it in a microwave oven and see if it heats up ?
Along with a load.
greg
Tim Wescott
> Hello,
>
> I am attempting to use a schedule 40, grey PVC pipe as an antenna radome,
> and am concerned about RF losses.
>
> My application is a UHF transmitter with an output power of 40 Watts (46
> dBm) into an omnidirectional antenna with 3 dB gain, or total estimated
> EIRP of 49 dBm (80 Watts). RF is Right Hand Circularly Polarized (RHCP)
> emissions at an operating frequency of 401 MHz.
>
> I cannot tolerate more than about a half dB loss through the PVC pipe. Are
> there any significant RF losses in radiating such a signal through Schedule
> 40, grey PVC pipe?
I know that it's a fairly popular material for amateur radio antennas at
HF, but I'm not sure about its properties at UHF.
Try your antenna with and without the PVC. Pay attention to de-tuning,
if any -- that would muck up calculations.
To get a quick estimate of the attenuation, you could wind a small coil
around some PVC and resonate it with a cap, then measure the Q. If I
did a measurement with and a measurement without the PVC core I would
expect a small but possibly significant change in resonant frequency,
and I would expect that after I finished measuring the Q change I'd have
a better notion of how well the pipe would work.
If you're going to do this for production, you'd be smart to set up a
test range and check every pipe, or at least every lot of pipe, before
you use it. Stuff like that can vary in how it's manufactured as long
as it does the job for which it's intended and marketed -- and radomes
isn't one of those jobs.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
Vladimir Vassilevsky
blanker wrote:
> Hello,
>
> I am attempting to use a schedule 40, grey PVC pipe as an antenna radome,
> and am concerned about RF losses.
>
> My application is a UHF transmitter with an output power of 40 Watts (46
> dBm) into an omnidirectional antenna with 3 dB gain, or total estimated
> EIRP of 49 dBm (80 Watts). RF is Right Hand Circularly Polarized (RHCP)
> emissions at an operating frequency of 401 MHz.
>
> I cannot tolerate more than about a half dB loss through the PVC pipe. Are
> there any significant RF losses in radiating such a signal through Schedule
> 40, grey PVC pipe?
There is no quick answer to this question. A lot depends on the geometry
of your antenna and the particular spots with the high intensity of EM
field. The practical way would be try and see for yourself.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Ian Iveson
>Research the dielectric constant of PVC.
>I expect it will be low, and reasonably close to air. (?)
RF is used for welding PVC, which has a dielectric constant
of about 4.5. Other common thermoplastics are downwards of a
half of that, so they can't be welded in the same way.
Ian
Tim Wescott
Dielectric constant and absorption constant are two different things,
though. In principal there's no reason that a high dielectric material
can't have low absorption.
And again, there are so many modifiers that you can put into plastics
that you can't say "will plastic XYZ work as a radome". You have to ask
"can plastic XYZ _ever_ work in a radome, and how do I make sure to get
a flavor of it that will?".
Tim Wescott
Perhaps a quick "no" if the stuff is really badly absorptive.
But a "yes" certainly requires the experiment (and IMHO, constant repeat
testing if you're buying pipe meant for plumbing).
mpm
That's an interesting thought....!!
I would make sure the microwave oven has a turntable, or at least an
RF stirrer, as I'm sure most do nowadays?
I think you could also shred an amount of the PVC material and place
it into a tuned cavitity filter (like a big VHF or VHF bandpass cavity
filter) and check the response both before and after insertion of the
material.
If the tuning doesn't change too much, I would think the material
would be acceptable as a radome.
That said, I would not fill the cavity completely, or even allow the
PVC material to come into contact with the center plunger of the
cavity.
I think that may skew the results of the tests, but I would have to
think hard about the exact reason why....
It seems to me I did this test once before in my life... ????
-mpm
Tim Wescott
> On Aug 24, 11:39 am, zekfr...@zekfrivolous.com (GregS) wrote:
>> In article<zbOdneA13P8odu7R4p2d...@giganews.com>, "blanker"<shamale@n_o_s_p_a_m.hotmail.com> wrote:
>>> Hello,
>>
>>> I am attempting to use a schedule 40, grey PVC pipe as an antenna radome,
>>> and am concerned about RF losses.
>>
>>> My application is a UHF transmitter with an output power of 40 Watts (46
>>> dBm) into an omnidirectional antenna with 3 dB gain, or total estimated
>>> EIRP of 49 dBm (80 Watts). RF is Right Hand Circularly Polarized (RHCP)
>>> emissions at an operating frequency of 401 MHz.
>>
>>> I cannot tolerate more than about a half dB loss through the PVC pipe. Are
>>> there any significant RF losses in radiating such a signal through Schedule
>>> 40, grey PVC pipe?
>>
>> What if you put it in a microwave oven and see if it heats up ?
>> Along with a load.
>>
>> greg
>
> That's an interesting thought....!!
> I would make sure the microwave oven has a turntable, or at least an
> RF stirrer, as I'm sure most do nowadays?
Not necessary, although helpful.
> I think you could also shred an amount of the PVC material and place
> it into a tuned cavitity filter (like a big VHF or VHF bandpass cavity
> filter) and check the response both before and after insertion of the
> material.
> If the tuning doesn't change too much, I would think the material
> would be acceptable as a radome.
Given the dielectric constant it _would_ change the tuning. That's not
the issue -- the issue is whether it absorbs the waves as well as
diffracting them.
Dave Platt
>and am concerned about RF losses.
>
>My application is a UHF transmitter with an output power of 40 Watts (46
>dBm) into an omnidirectional antenna with 3 dB gain, or total estimated
>EIRP of 49 dBm (80 Watts). RF is Right Hand Circularly Polarized (RHCP)
>emissions at an operating frequency of 401 MHz.
>
>I cannot tolerate more than about a half dB loss through the PVC pipe. Are
>there any significant RF losses in radiating such a signal through Schedule
>40, grey PVC pipe?
PVC does introduce some loss and some de-tuning. A lot of hams use it
(e.g. for radomes on simple twin-lead J-pole antennas), but it's
difficult to predict whether any specific batch will have a low enough
loss to meet your requirements. Since it isn't being manufactured with
RF characteristics in mind, different batches or brands may be
significantly different in their RF behavior.
The suggestion to stick a few inches in a microwave (with a cup of
water some distance away), zap it for a few seconds, and see if it
heats up is a good one. It's not a certain test, of course, but if
the tube does warm up appreciably when exposed to microwaves, it's
probably not a good choice at UHF.
How large a diameter to you require? If it's anywhere up to 2.5" OD,
you might want to get some fiberglass tubing from Max-Gain Systems...
fiberglass-and-resin is a traditional RF radome material and it may have
lower losses than PVC.
If this is a one-off project, you might want to consider making
your own radome, using fiberglass cloth and resin... you could
probably fabricate a radome which has a thinner wall than most
commercial fiberglass or PVC pipe.
--
Dave Platt <dpl...@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
GregS
>On Aug 24, 11:39=A0am, zekfr...@zekfrivolous.com (GregS) wrote:
>> In article <zbOdneA13P8odu7R4p2d...@giganews.com>, "blanker" <shamale@n_o=
>_s_p_a_m.hotmail.com> wrote:
>> >Hello,
>>
>> >I am attempting to use a schedule 40, grey PVC pipe as an antenna radome=
>,
>> >and am concerned about RF losses.
>>
>> >My application is a UHF transmitter with an output power of 40 Watts (46
>> >dBm) into an omnidirectional antenna with 3 dB gain, or total estimated
>P)
>> >emissions at an operating frequency of 401 MHz.
>>
>=A0Are
>> >there any significant RF losses in radiating such a signal through Sched=
>ule
>> >40, grey PVC pipe?
>>
>> What if you put it in a microwave oven and see if it heats up ?
>> Along with a load.
>>
>> greg
>
>That's an interesting thought....!!
>I would make sure the microwave oven has a turntable, or at least an
>RF stirrer, as I'm sure most do nowadays?
I just thought of a grid dip meter !!
Vladimir Vassilevsky
GregS wrote:
Rather useless experiment. Whatever the result would be, it is largely
irrelevant if the material could be used for OP's antenna or not.
FWIW, once long ago I did a mistake: isolated a junction of coax cables
with PVC electric tape. The power level was ~1kW at ~60MHz. The tape
burned away in few hours. Then I used polyethylene for isolation and it
worked fine.
Wimpie
Hello,
It depends strongly on how close the radome is to the actual antenna.
When it is significantly within 0.25lambda of the antenna structure,
it will experience the (strong) reactive field. So you need to guess
(measure, simulate) the field strength experienced by the PVC.
With 40W, and about 80mm from the antenna, you may get about 1kV/m,
the worst case is when this field is parallel to a thin sheet of PVC
as the field inside the PVC equals the air field strength. Assuming Er
= 3 and tandelta = 0.015 gives about 1kW/m^3 material loss. With a
radome material thickness of 3mm, this would result in 3 W/m^2 loss
(guess surface area exposed by the field to find the "actual" loss).
When the unobstructed field is perpendicular to the material, loss
drops significantly (as the E-field inside the PVC drops to about 33 %
of the value in air).
0.5 dB from 40W is about 4W. When you increase the distance between
the antenna and the radome, E drops and loss/m^3 is proportional to
E^2. However if the material is very close to the antenna, over a
large area, overall loss may be above 0.5 dB. Considering PVC is not
that bad. Other issue to check: UV resistance and plastic deformation
(creep) under mechanical loading in combination with high
temperature.
I hope this gives you some guidance in whether it is worth to try
PVC.
Best regards,
Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me very likely
whit3rd
> > I am attempting to use a schedule 40, grey PVC pipe as an antenna radome,
> > and am concerned about RF losses.
> If you're going to do this for production, you'd be smart to set up a
> test range and check every pipe, or at least every lot of pipe, before
> you use it. Stuff like that can vary in how it's manufactured as long
> as it does the job for which it's intended and marketed -- and radomes
> isn't one of those jobs.
Exactly.
Pure PVC is white, not gray; that color is a pigment of some sort, and
the manufacturer of 'schedule 40' pipe is making his tests on burst
strength, not RF absorption. It wouldn't be unlikely for the
labeling
ink on the pipe to be active in this case, either (hematite is a
common
pigment).
Dennis
"Dave Platt" <dpl...@radagast.org> wrote in message
news:jjgdk7-...@radagast.org...
Perhaps look at model rocketry suppliers. They use f/g tubes for rocket
fuselages - from an inch or so up to 6" diameter.
GregS
>On Aug 24, 9:59=A0am, Tim Wescott <t...@seemywebsite.com> wrote:
>
>> > I am attempting to use a schedule 40, grey PVC pipe as an antenna radom=
>e,
>> > and am concerned about RF losses.
>
>> If you're going to do this for production, you'd be smart to set up a
>> test range and check every pipe, or at least every lot of pipe, before
>> as it does the job for which it's intended and marketed -- and radomes
>> isn't one of those jobs.
>
>Exactly.
>Pure PVC is white, not gray; that color is a pigment of some sort, and
>the manufacturer of 'schedule 40' pipe is making his tests on burst
>strength, not RF absorption. It wouldn't be unlikely for the
>labeling
>ink on the pipe to be active in this case, either (hematite is a
>common
>pigment).
I once painted black the piece covering a coil on a CB or other
type of antenna. That screwed that up pretty good.
I found different black paints have different characteristics.
Some make pretty good ED shielding or grounding.
Allright, I want some PVC to stick in the microwave, anyone ?
I'm thinking of other stuff at The Home Depot !!
greg
Wimpie
Hello Greg,
Using the microwave oven test is nice to compare two or more
materials, but you still don't know whether the best (or worse)
material matches your requirements. When the shape of the materials
differs, you may get strange (unreliable) results.
Properties of paint amaze me also (huge difference in reflectivity and
transmissivity). Some (spray) paints with metallic look do virtually
not block the (UHF) signal.
I ones used black thick polycarbonate sheets as an antenna cover
(mechanically heavy loaded). It did detune the antenna somewhat, but
after retuning I couldn't measure the difference in produced field
(w.r.t. uncovered situation, mid UHF situation). With black material
I am always afraid to encounter too high carbon black levels.
Best regards,
Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me.
Ian Iveson
Using the microwave oven test is nice to compare two or more
materials, but you still don't know whether the best (or
worse)
material matches your requirements. When the shape of the
materials
differs, you may get strange (unreliable) results.
Properties of paint amaze me also (huge difference in
reflectivity and
transmissivity). Some (spray) paints with metallic look do
virtually
not block the (UHF) signal.
**They use flakes of stuff with optical properties like
mica, or "fools gold".
I ones used black thick polycarbonate sheets as an antenna
cover
(mechanically heavy loaded). It did detune the antenna
somewhat, but
after retuning I couldn't measure the difference in produced
field
(w.r.t. uncovered situation, mid UHF situation). With black
material
I am always afraid to encounter too high carbon black
levels.
**Black is the riskiest colour for RF welding of PVC. Clear
(pure PVC is clear pale blue) is the most predictable. We
assumed that the problem was the use of carbon black, but
never found out for sure. Another problem is metallic
inclusions, which resulted in spectacular fireworks and blew
holes in the brass tooling.
**Handle also made a big difference. Rigid is hard to weld,
and very soft is too easy and heat build-up in the tooling
can melt the surface before the inside. Different fillers,
such as chalk, make a difference, especially if they hold
water.
**The chlorine is polar so the chains wriggle in an AC
field. Shorter chains, especially when the structure is
loosened by the addition of plasticiser, mean more
wriggling. Presumably energy is lost in the breaking and
remaking of whatever bonds hold the chains together.
**Dunno what this has to do with radio waves. It does
illustrate how much variation there is in the material, as
others have pointed out.
**The chlorine inhibits fire, incidentally. PVC in
electrical installations can be fire-prone because of its
electrical properties, but the risk of a serious fire is
considered acceptably small.
Ian
Paul Keinanen
<shamale@n_o_s_p_a_m.hotmail.com> wrote:
>Hello,
>
>I am attempting to use a schedule 40, grey PVC pipe as an antenna radome,
>and am concerned about RF losses.
>
>My application is a UHF transmitter with an output power of 40 Watts (46
>dBm) into an omnidirectional antenna with 3 dB gain, or total estimated
>EIRP of 49 dBm (80 Watts). RF is Right Hand Circularly Polarized (RHCP)
>emissions at an operating frequency of 401 MHz.
>
>I cannot tolerate more than about a half dB loss through the PVC pipe. Are
>there any significant RF losses in radiating such a signal through Schedule
>40, grey PVC pipe?
How are you going to accurately measure a 0.5 dB drop of the RF field?
Anyone (at least on Earth) are going to ask for trouble by designing a
radio link with only 0.5 dB fade margin :-).
mpm
A 0.5dB loss at the transmitter is not the same thing as a 0.5dB fade
margin.
RST Engineering
wrote:
And the ink comes off quite easily with lacquer thinner or acetone.
Exactly what "uhf" frequencie(s) are you using?
Jim
mi...@sushi.com
401MHz. Are you a fed?
This topic has been covered often on the group. You can put the PVC in
a microwave oven, but you need to put some water in the oven as well.
I don't recall the details, but the topic can been covered before.
PVC detunes antennas, at least in wifi land. You can roll your own
fiberglass radome material. Not cheap. Tap Plastics has all the
materials. Since you used a funny list (Electronics-Related.com), I
think I'll save myself the hassle of explaining how to make composites
in your garage since I suspect you are not going to get my reply.
RST Engineering
>On Aug 24, 8:56 am, "blanker" <shamale@n_o_s_p_a_m.hotmail.com> wrote:
>401MHz. Are you a fed?
>
>This topic has been covered often on the group. You can put the PVC in
>a microwave oven, but you need to put some water in the oven as well.
>I don't recall the details, but the topic can been covered before.
>
A microwave oven at 2.4 GHz. will tell you little, if anything about
something at 400 MHz. I'd probably experiment with lots of different
off the shelf plastics. TAP, as you noted, has a pretty good
selection of plastic tubing to experiment with.
Jim
Paul Keinanen
If the material has some kind of resonance around 400 or 2400 MHz, the
microwave oven test does not tell much.
However, most materials are either lossy on both 400 and 2400 MHz or
have a very low loss at those two frequencies, so the microwave oven
test will give some useful information.
Wimpie
You may be right, there is zero reply from Blanker...
Wim
mi...@sushi.com
Yeah, I keep forgetting not to answer posts from those spambots.
However, radome design sure is an interesting area of interest. These
UAVs have huge radomes as part of the airframe itself to support the
satellite links. The same for links to troops on the ground. I suspect
radome design is a good guru discipline. The market probably doesn't
support many with such knowledge, but those that know the biz probably
do alright. Note these UAV radomes have structural problems as well as
RF properties.