protecting yagi with PVC or other material

[dvp]

I am in the process of building a homemade yagi (for 2.4GHz). Once done I would like to be able to protect it from the elements and thought I could place it inside a PVC pipe. I cannot find any definitive answers on the web regarding the effects of PVC and 2.4GHz. Would PVC have any effect on the RF? Additionally, is there a material that would 'help' the directionality (narrow it) and/or the gain of the yagi?

Thanks

 

[prcguy]

PVC is fairly transparent at 2.4GHz and you will not notice any degradation from placing it inside a PVC pipe. A lot of microwave ovens operate at 2.45GHz and you can test your PVC or other plastic pipe by placing it in a microwave oven for a minute then see if it gets warm. White PVC is generally cold after this kind of test.

Certain plastics are used as RF lenses placed in front of microwave antennas and they operate similar to glass lenses with light. They can increase the apeture size of an antenna and focus the RF presented across the lense to a smaller concintrated area.

RF lense geometry is beyond most home experimentors but a large solid round plastic ball can make a crude lense for RF just like a clear glass marble can be used to make a microscope but with a lot of distortion. I would think you could get some useable gain from a 1ft diameter solid plastic ball in front of a small 2.4GHz antenna if focused properly.
prcguy

 

[dvp]

Thanks prcguy - I had heard about the microwave test but do not think the wife would appreciate my attempt at 'scientific' experimentation. Would paint have any effect if I were to paint the pvc pipe afterwards?

 

[prcguy]

Some paints will be lossy at 2.4GHz and you would would have to test it. When i was testing some satellite LNBFs one of our guys made a caliberated weak signal unit by layering coats of spray paint over the feed cover until he got the desired amount of attenuation.

If you have a stable test signal at 2.4GHz and something that will detect small changes in level you can take a reading with a thin peice of plastic in front of your antenna then spray a few coats of paint over the plastic and see if there is any attenuation. If there is none add a few more coats to see what happens. If its barely detectable with several coats then you are probably safe with a single coat and maybe more.
prcguy

Thanks prcguy - I had heard about the microwave test but do not think the wife would appreciate my attempt at 'scientific' experimentation. Would paint have any effect if I were to paint the pvc pipe afterwards?

 

[KK4VRE]

radome paint

To 'dvp' -

In reference to painting the exterior of a PVC pipe used as a radome for an antenna element...

You must make sure the paint has no metal in it what-so-ever. In the commercial commuications world (in NYC) we used a certain line of Sherman Williams paint made specifically for antennas. The color would always center around a light blue/ soft gray color so as to make the antenna disappear against the background of the sky. The variations came in the form of different mixes to work best in different locals, such as cities with smog, or cities with constant clear skies. The shading was selected to best suite your specific location. To this day I still own a few antennas with this paint on them.

 

[zz0468]

While PVC is relatively transparent to RF at 2.4 GHz, I would not be surprised if the antenna gets detuned by enclosing it in PVC. The issue is nopt necessarily one of absorption, it's that the velocity factor is quite different from air, and within the near field of the antenna, a dielectric like PVC will impact how the antenna behaves.

Now, if the antenna is designed from the ground up to live inside a PVC radome, it'll work just fine.

I once did an experiment with a simple dipole on VHF. The dipole was made with #8 solid wire, and it's resonant frequency was measured. It was then encased in PVC and the resonant frequency was remeasured, and found to have moved over a MHz. That was at VHF. I expect the effect at 2.4 GHz would be worse.

I have some microwave transitions where the probe is covered in teflon. The teflon changes the resonant frequency of the probe, and damps out ripple in the return loss.

So, try it but don't be surprised to see some interaction, something besides just loss.

 

[KK4VRE]

zz0468 -

Your single-sentence second paragraph says it all.

ANY material between the origin of the radiator and free-space will have -some kind- of impact on the radiating system parameters. There is isotropic and ultimately real-world; there is theory and then there is in practice. This is why all antenna systems that radiate must be adjusted ("tuned"). RF engineers will dictate what type of transmission line (ladder-line, coaxial cable, hard-line cable, or waveguide) should be used for a plethora of factors under general pupose conditions, but it is the installer who will then tweek the system for the exact circumstances. Just like when a simplistic vertical CB antenna is created, such as a 102" whip, which technically should be roughly 108", but due to using stainless steel (for ridgity and durability) vs copper as this alters the "actual" length used (due to the velocity factor difference of the radiator), or using 18' of coax when using a -phased- CB antenna array, or the minimum horizontal/vertical distance between 2 omni-radiating antennas on the same frequency, or the miniumum AGL height at which some antennas should be deployed, etc. There are so many factors in deploying a radiating antenna system it can get crazy. The radome material is one of least concern or impact, unless you get down deep into the depths of QRP.

Fiberglass will be less impacted by higher levels (kW range) of RF than PVC (to the point of 'melting' or 'sag').
Fiberglass is physically much more rigid than PVC, even if the PVC has a much thicker wall ("schedule").
Fiberglass is lighter than PVC.
Fiberglass sheds water better than PVC; but will absorb water if not maintained (fully 'sealed').
Fiberglass is more resistant to atmospheric pollutants (if maintained) than PVC.
Fiberglass is less pervious (if maintained) to UV than PVC.

Yes, fiberglass is clearly a better radome material than PVC, but primarily for reasons other than radiation propagation.

Since this is a home brew antenna for the 13cm band, I'm sure 'dvp' will be making plenty of adjustments to get it just right... even to the point of not painting it with metallic paint, or allowing water to get inside the radome.

 

[KK4VRE]

As to furthering beam gain even more (pin-point directivity), appropriatly increase the length of the element support arm and the number of directors per design parameters (which will increasingly drop off in effectiveness). It will also likely require a RF transparent support arm so it does not sag in the least.

The problem with this concept is... it is going to start to get very difficult at 2.4 GHz to 'hit' the exact point on the other end (a 5" target) past a certain level of gain in a single antenna.

A better idea would be to use a phased-array.

 

[rja1]

Here's a quick & dirty way to find out.......put a piece of PVC in the microwave for one minute. If it doesn't get hot, you're good to go.

Bob
N2OAM

 

[KK4VRE]

Yep, that would be a down-and-dirty way for the 2.4 GHz arena at 1,000 watts (which I truly believe he will be nowhere near)... just don't forget the duty cycle of on and off to make it a fair test.

...and don't forget he is firing directionally (yagi), not omni.