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Will PVC Pipe Block Signal?

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#1
I've built this vhf ground plane antenna and I was planning on mounting it outside in the elements. I wanted to know if I was to cut a piece of PVC pipe and put it over the main/ center element, if it would block the reception of any radio waves?
 

n0nhp

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Most PVC is not RF resistive. The definitive test is to take a small piece and place it in a microwave oven next to (Not in) a cup of water. Blast it with microwaves for 30 seconds or so and very carefully test it to see if it has warmed up. Do not use PVC that warms up around a radiating or receiving element. This test will also work to see if paint is RF conductive. Paint a small sample on paper, let dry and place in Microwave for a few seconds, then test. The water is used to provide a load for the microwave emitter. The new ovens are not as sensitive as the older ones, but it is better safe than sorry.

Bruce
 
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#3
Most PVC is not RF resistive. The definitive test is to take a small piece and place it in a microwave oven next to (Not in) a cup of water.
Just a comment...

That's a decent general test, but materials can have different properties at different frequencies. Also, one thing that's not detected in this test is the dielectric constant, which will have some effect on the tuning of an antenna, even if it doesn't cause a noticable amount of absorption.

I've made a number of antennas over the years, and PVC makes a pretty decent radome below a few GHz, but in every case, I've had to account for the detuning that takes place when the antenna is inserted into the pipe.
 
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#4
To add to what zz0468 correctly pointed out regards relative permittivity and dielectric constant: if you raise the dielectric constant of the near field surrounding an antenna you raise the resonant frequency of the antenna.

By how much the antennas' resonant freq changes will be a function of the exact PVC chemical composition. There is huge variation in PVC dielectric constants - from as little as just over 1 to over 5 is possible with PVC. That, in combination with the frequency to start with, could translate to as much as a 30% raise in resonant frequency in extreme cases!!

As a rule: the higher the frequency to start with, the greater the impact a change in near field dielectric constant will have on resonance.
 
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#5
Another little hint about PVC and antennas. In general, meaning that it isn't always the case, the white stuff tends to affect antennas less than the grey stuff. That's just from experience with the PVC available around here. Also, depending on how your antenna is constructed, the 'elements'/weather, seldom destroys an antenna's usefulness. That's time dependent naturally. And since this antenna will be used for receiving (not transmitting?) it shouldn't make any practical difference.
Have fun.
- 'Doc
 

kb2vxa

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#6
The microwave test is excellent for determining relative conductivity of PVC but like the man said using it as a "radome" adds distributed capacitance like using insulated wire. The net effect is it somewhat lengthens the electrical properties of the element so it has to be cut slightly shorter than a bare conductor to resonate at the same frequency. If you're using it to transmit an SWR meter and a bit of trimming will get it there but for receiving it really doesn't matter. A ground plane is fairly broad band so I doubt you'll run into difficulty if you cut it properly for the band to begin with.

More important is weatherproofing, I have no idea of its construction so I can't tell you how to go about that. You really don't need to shield it from the elements but keeping water out of the assembly and coax is paramount.

As an aside, I really don't know why they use the word radome when the true meaning indicates a radar in a bubble protecting the dish from ice and snow that would interfere with its operation. THIS is a radome.
 
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n0nhp

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I see your radome and raise you :)
Radome over 13M C-Band dish pointed at 7.5 degrees above the horizon serving McMurdo Station Antarctica.
 

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rfking123

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#8
Pvc pipe

I've built this vhf ground plane antenna and I was planning on mounting it outside in the elements. I wanted to know if I was to cut a piece of PVC pipe and put it over the main/ center element, if it would block the reception of any radio waves?
If you use PVC piping, use "schedule 40" pipe because it has no metal fillers in it, and works great.
 

chief21

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Actually, as long as you don't need to rely on the PVC for structural reasons, 3/4" PVC pipe rated at 200 psi works best as a radome. Both schedule 40 and schedule 80 PVC are stronger but are also much thicker and will affect the signal more than the thinner 200 psi pipe.

John
 
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#12
To add something about PVC conduit - if we run co-ax thru PVC conduit will it help against electrical interference ?
No, it's transparent to RF. PVC conduit is a mechanical protection.

Putting it in steel conduit might help with interference, but if you are using good coax with 100% shield, I doubt you'd see a difference.
 
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#14
Just another air gap and physical protection. WHY ? A question put out for comment.I am a unlimited master election ship & shore + Electrical engineer. Not a smart *** but do like discussions of different matters.

Thank you.
 

JamesO

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Actually, as long as you don't need to rely on the PVC for structural reasons, 3/4" PVC pipe rated at 200 psi works best as a radome. Both schedule 40 and schedule 80 PVC are stronger but are also much thicker and will affect the signal more than the thinner 200 psi pipe.

John
I assume this is considered Schedule 20 PCV pipe? But the wall thickness of the PVC can and will impact the antenna performance and as mentioned above you want the thinner walled PVC pipe.

See comments in Ed Fong published papers for his J-Pole antenna in this link - http://edsantennas.weebly.com/about.html
 
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#16
When you get to the point where the material thickness gets to be fractions of a wavelength its an issue for PVC pipe or any plastic. When choosing radome material for say 14GHz or 28GHz a 1/6" or 1/8" thick can be a problem but for HF through at least 1GHz schedule 80 PVC pipe as a radome has no noticeable impact.

If you cover the actual antenna conductor with PVC where is coating the metal then you have a dielectric insulation that will change the resonant frequency some and that will happen at all frequencies.
prcguy


I assume this is considered Schedule 20 PCV pipe? But the wall thickness of the PVC can and will impact the antenna performance and as mentioned above you want the thinner walled PVC pipe.

See comments in Ed Fong published papers for his J-Pole antenna in this link - About - Ed's Antennas
 
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#17
When you get to the point where the material thickness gets to be fractions of a wavelength its an issue for PVC pipe or any plastic. When choosing radome material for say 14GHz or 28GHz a 1/6" or 1/8" thick can be a problem but for HF through at least 1GHz schedule 80 PVC pipe as a radome has no noticeable impact.
I've successfully used schedule 40 PVC pipe as a radome for 10 GHz antennas. The impact to antenna gain was negligible, but it certainly affected the return loss a great deal. A 3 screw tuner was drilled and tapped into the waveguide to restore the RL at the frequency of interest, and the whole assembly worked just fine.

I have no firm way of knowing what it might have done to the pattern, but it feels like it smoothed out the ripple and scalloping that exists in the bare antenna. I ended up being quite pleased with the end result.
 
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#18
A lot of good info. guys. All I used pvc conduit for was to protect my co-ax for about 2'. From roof mount to side of cab. Rg 58 inside a 3/4" ID pipe is real loose and plenty of space.Good discussion anyway - Thanks for all the input.
 

KG7A

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#19
Question on using PVC for structure on Magnetic Loop.

I recently purchased a Active Magnetic Loop Antenna by DX Engineering, RF-PRO-1B. Now that DX Engineering also owns the rights to Clifton Laboratories designs, we can share this amazing fact about the RF-PRO-1B Active Magnetic Loop preamplifier. There is only concern that questions me, and that is the 2 bolts that hold the loop up on the angled loop bracket. My Idea was to use a 1.25 in. PVC as a mast that bolts in the rotor but extends up on the vertical middle tube of the loop and fasten the top part to the PVC for extra strength for high winds. Will that effect the incoming receiving frequencies 100 KHz to 30 MHz? Please advise...
 
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#20
I am using schedule 40, PVC as my boom, 1" inside 1&1/4" the inner is for added strength, the boom is only 10 foot long.????? . From what I am reading here, PVC to separate the 2 elements on a direct connect beam.is not good. I am hearing fiberglass is best ? I could use a flat plastic plate for separation not something inside the element - by the way all elements are copper.You folks are very hepful.
 
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