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Scanner / Receiver Antennas For discussion of any type of receiving antenna used by a scanner or receiver base, mobile or handheld.

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Old 11-08-2012, 6:37 PM
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Question Will PVC Pipe Block Signal?

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?
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Old 11-08-2012, 7:07 PM
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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.

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Old 11-09-2012, 12:34 AM
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Quote:
Originally Posted by n0nhp View Post
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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Old 11-09-2012, 4:35 AM
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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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Old 11-09-2012, 8:30 AM
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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.
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Old 11-09-2012, 2:28 PM
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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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Old 11-09-2012, 4:35 PM
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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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Old 11-09-2012, 4:43 PM
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Default Pvc pipe

Quote:
Originally Posted by big420dog94550 View Post
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.
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Old 11-13-2012, 3:49 PM
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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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