Does metallic paint block RF?

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DarthMuffin

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This seems to be a hard one to search for, can't find anything here or on google. It's easy to test that metallic or metal flake paint doesn't completely block RF, but does it attune it some? I don't have a way to test for that.

I realize it may depend largely on what paint, some may use something non-conductive for the sparkles (mica?). Looking for guidelines to keep my trackers broadcasting as best as possible.
 

tfish

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This seems to be a hard one to search for, can't find anything here or on google. It's easy to test that metallic or metal flake paint doesn't completely block RF, but does it attune it some? I don't have a way to test for that.

I realize it may depend largely on what paint, some may use something non-conductive for the sparkles (mica?). Looking for guidelines to keep my trackers broadcasting as best as possible.

Tony
 

Voyager1

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It would depend upon the metallic content of the paint and the proximity of the antenna. The level of metal in the paint will likely attenuate the RF signal accordingly. The monopole antennas will detune in proximity to metallic objects due to impedance changes, causing their resonant frequency to change slightly.

The best test is to coat a cardboard tube with the paint with at least one of the ends sealed and painted as if it was the nosecone, or avbay, depending on where you place the tracker. Do a field test to see if your signal level is affected with the tracker in or out of the enclosure. Preferably perform this a few metres off the ground to improve propagation.

Some tracker receivers will provide RF signal level, or RSSI, to evaluate the comms link.
 
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RocketDestroyer

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As long as the gaps between the metal flakes are smaller than a small percentage of the wavelength of the frequency used, it will appear as a solid metal shield the the RF. The paint forms a Faraday Cage. This is why you can have a mesh screen in the door of a microwave oven and still be safe from radiation.
 

jderimig

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I'd love to see measurement data on this. This would be a relatively easy to measure with a transmitter in both a painted and unpainted cylinder with field strength measurements. I am skeptical on the degree of attentuation.. Lets see some real data and not (thoughtful) handwaving.

Personally I haven't seen ANY GPS satellite S/N reduction in a heavily painted HOK Silver flake (very high aluminum flake content) nosecone versus open air. That is the only measurement I have made.
 

Grog6

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Some silver paint is aluminum powder in clear paint; it will act as a broadband attenuator, but if connected to transmitter or reciever ground, it will act as a shield to anything inside.
 

RocketDestroyer

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Personally I haven't seen ANY GPS satellite S/N reduction in a heavily painted HOK Silver flake (very high aluminum flake content) nosecone versus open air. That is the only measurement I have made.
I haven't seen a huge drop in the signal levels on the GPS receiver. However, I helped a friend install a tracker on 144 MHz in his rocket and the signal loss was pronounced. We had signal on the pad but lost it as soon as the rocket left the pad. We then removed all the metal flake paint from the nosecone and had good signals for the next flight. That's my anecdotal evidence. I'm retired now and no longer have access to the equipment required to do a substantive test. Probably the best most of us could do would be to measure the received signal strength with the painted body in place and then removed as suggest by Voyager1 above.
 

Voyager1

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Yes, I agree that without actual data on specific installations, it is just speculation. If you’re concerned, you do the tests. My gut feeling is that you shouldn’t just take it for granted that it won’t be an issue. Even if it only attenuates the RF by a dB or so, or less, that might mean the difference between losing your rocket or keeping track of it - all for the sake of a pretty paint job.

Faraday shielding will probably only be significant if the ‘mesh’ dimension is less than roughly 1/10 of the wavelength of interest; the shielding is completely surrounding the electronics; and the shield is at the same potential as the electronics ground.

At 1 GHz the wavelength is about 30 cm, so 3 cm represents the maximum ‘mesh’ dimension here. Any metallic particles in the paint are likely to be significantly closer than that. So, the potential for shielding to occur could be real. However, given that the metallic paint coating will most likely be electrically ‘floating’ with respect to the electronics inside the nosecone or body, then there is the possibility that it might act as a passive re-radiator, rather than a Faraday cage.

Also, given that the metallic paint coating does not completely surround the electronics, then there is plenty of opportunity for RF to get around the ‘shield’. It will completely depend upon the particular installation.

I haven’t performed any tests on RF shielding in a rocket, but I have performed bench tests on the effects of metallic components adjacent monopole antennas inside avbays. The presence of metal conductors near the antennas does indeed effect their impedance and resonant frequency. I posted some results of these tests in another thread on TRF perhaps a year or so ago. I will endeavour to repeat them with a metallic painted enclosure and include some attenuation measurements.
 

Grog6

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I've got copper 'gauze' and tape that shuts down radio, but that's what it's sold for.
I also have sheets of mu-metal. :)
 

jderimig

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Yes, I agree that without actual data on specific installations, it is just speculation. If you’re concerned, you do the tests. My gut feeling is that you shouldn’t just take it for granted that it won’t be an issue. Even if it only attenuates the RF by a dB or so, or less, that might mean the difference between losing your rocket or keeping track of it - all for the sake of a pretty paint job.

Faraday shielding will probably only be significant if the ‘mesh’ dimension is less than roughly 1/10 of the wavelength of interest; the shielding is completely surrounding the electronics; and the shield is at the same potential as the electronics ground.

At 1 GHz the wavelength is about 30 cm, so 3 cm represents the maximum ‘mesh’ dimension here. Any metallic particles in the paint are likely to be significantly closer than that. So, the potential for shielding to occur could be real. However, given that the metallic paint coating will most likely be electrically ‘floating’ with respect to the electronics inside the nosecone or body, then there is the possibility that it might act as a passive re-radiator, rather than a Faraday cage.

Also, given that the metallic paint coating does not completely surround the electronics, then there is plenty of opportunity for RF to get around the ‘shield’. It will completely depend upon the particular installation.

I haven’t performed any tests on RF shielding in a rocket, but I have performed bench tests on the effects of metallic components adjacent monopole antennas inside avbays. The presence of metal conductors near the antennas does indeed effect their impedance and resonant frequency. I posted some results of these tests in another thread on TRF perhaps a year or so ago. I will endeavour to repeat them with a metallic painted enclosure and include some attenuation measurements.
I will violate my post and do some handwaving of my own ;-). I think any attenuation is not due to a Faraday cage effect because I do not think the conduction in a metallic paint surface (with common automotive flakes) forms a continuous conduction layer. I think different areas of the surface can support different levels of charge so the surface might not be a equipotential surface which is required for a Faraday cage.

Possibly attentuation might totally be due to antenna detuning.

Interestingly EMF paint specifically sold for shielding does not use metal but instead uses high loading of carbon.

Love to see some well designed tests on this. I don't have the time right now but if others can do it it would make a great contribution to the hobby if they can perform such measurements at commonly used transmitting frequencies (433 and 900 mhz).
 

Johnly

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Before I retired I had access to an EMC test facility and could have collected the data needed to answer this question with 20 minutes of chamber time.
Just about anyone I knew there has retired as well, except for the facility director. Might be time to call in a favor when Covid lets up.
 

OverTheTop

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I saw about 1dB reduction of signal on some silver painted 3D nosecones. Almost in the noise. This was with a TeleMega at 435MHz.

I should jump into the chamber for some serious tests one day. No time currently. We have two EMC chambers.
 

Johnly

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I saw about 1dB reduction of signal on some silver painted 3D nosecones. Almost in the noise. This was with a TeleMega at 435MHz.

I should jump into the chamber for some serious tests one day. No time currently. We have two EMC chambers.
We would do a chamber calibration every morning from 40MHz to 2 GHz using a RF generating source with a vertical antenna.
I was thinking of placing pieces of body tube around the antenna and collecting the results.
 

OverTheTop

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We would do a chamber calibration every morning from 40MHz to 2 GHz using a RF generating source with a vertical antenna.
I was thinking of placing pieces of body tube around the antenna and collecting the results.
That is what I was thinking too. We have a similar comb generator for calibration. Getting the test samples will take time too.
 

Adrian A

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The Featherweight GPS tracker shows you signal strength data on both the GPS satellites (1.5 GHz) and the LoRa telemetry link (915 MHz). It's straightforward to run the tracker and measure the attenuation with and without the airframe. I did this with a 29mm carbon airframe and posted about it a few years ago. Anyone with the Featherweight GPS system can evaluate their own particular setup to make sure they won't have a problem.

There is so much link margin with the LoRa telemetry link for flights under 100,000 feet, that the GPS signal attenuation is really the one to pay attention to. GPS receivers need a certain signal strength (32 dB of C/N0) in order to receive the data about the satellite's orbit, which is transmitted every 30 seconds. Without that ephermeris data, the receiver can hear that there is a satellite, but won't have enough information to calculate its own location from the signals it hears. Once it has the data, however, it can continue to track using that satellite for lower signal strengths, down to 20-24 dB of C/N0.

)

The Featherweight GPS app GPS page has a display of how many satellites it's tracking and their signal strength, using color bins corresponding to 40 dB (blue), 32-40 dB (green), 24-32 dB (yellow), and under 24 dB (orange). If attenuation from the airframe is a concern, an easy test is to let the system get a lock outside outside of the airframe, and see how many satellites shift into the lower-strength categories once you put it inside the airframe. As long as at least 4 satellites are green or blue (32 dB C/N0 or higher), then the system has at least the minimum RF performance to get a lock from a cold start.

If putting the tracker inside the airframe reduces the signal strength for all the satellites into the yellow or orange categories, it could still work it's allowed to get a lock on lots of satellites in view before buttoning up the airframe for launch, and then launch before too many of those satellites go below the horizon. GPS satellites that pass nearly overhead are in view for about 4-5 hours, and ones that have their highest elevation closer to the horizon are in view for shorter time. So on these timescales, if you have a lock on 8 satellites and then close up the airframe and can't pick up any new satellites, then the ones you already have ephemeris data for should be enough to keep a GPS lock if you launch within an hour or two.
 
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