Painting an antenna...

So the comm/vor antenna from '66 on the Bo went bad and the used one I bought was showing its age too. Before filling in some dings with micro, priming w Akzo, and painting w PPG Concept, a friend tested the wattage output and reflectivity at 118, 125, 135 mHz then compared to after paint...end result is the same. I've heard not to paint comm/vor antennas...but I think it may be a old wives tale related to different types of paint (lead based/metallic/etc).

I plan on installing it tomorrow and ops checking...don't expect anything but success.

Has any one else ever painted a comm/vor antenna? A little late for me to use in my decision...but interesting to know for sure.

Fly safe,

Jim

Hello Jim,

I have painted the antennas I produce to test the effects on performance using both spray can and auto two part paint and found the following:

The VHF antennas are the least sensitive to the painting process and could measure no performance degradation.

The Transponder and ADS-B are more sensitive, painting slightly shifted the resonate frequency but they still meet the
design parameters.

Based on the testing I completed you will detect no performance difference.

I believe the commercial makers of TSO'd antennas do not approve of painting because once it is it is out of their control
they can not insure the antenna will meet the TSO standards

dpansier said:

Hello Jim,

I have painted the antennas I produce to test the effects on performance using both spray can and auto two part paint and found the following:

The VHF antennas are the least sensitive to the painting process and could measure no performance degradation.

The Transponder and ADS-B are more sensitive, painting slightly shifted the resonate frequency but they still meet the
design parameters.

Based on the testing I completed you will detect no performance difference.

I believe the commercial makers of TSO'd antennas do not approve of painting because once it is it is out of their control
they can not insure the antenna will meet the TSO standards

Click to expand...

Your results may be diferent if it's metallic paint with actual metal in it...

Let's say you have a thin dielectric sheet in front of an antenna. When the antenna radiates, there will be an in-phase reflection from the nearest surface back toward the antenna, and then there will be an out-of-phase reflection from the far surface of the dielectric after the radio wave passes through it. The reason for the difference in reflection phase is that the wave is going faster in air when it meets the slower propagation rate of the dielectric and it sort of bounces off in-phase, but then after traveling through the dielectric with its slower propagation rate, it speeds up when it reaches the interface between the dielectric and the outside air. Because there is a slight phase shift in the arrival of the two wavefronts because of the slower wave in the dielectric, they will not be exactly 180? out of phase and a slight signal loss will be felt. As the frequency is increased and/or the dielectric thickness is increased, the two waves ariving back at the radiator will have more of a signal cancellation. It's possible that if we move the dielectric closer and closer to the antenna until it is actually touching the surface the same phenomonenon will occur. As has been noted, more loss of or change of electrical length has been observed at higher frequencies which will give frequency pulling to some output stages.

Thanks Paul, that makes perfect sense. Okay, not really, but I'll take your word for it. BTW, my buddy said it had a SWR of about 3:1...not sure if that's good or not compared to other newer comm antennas, but it is a lot better than the 'off the scale' of the old one. I'm happy.

-Jim

PS, flew it Saturday and it worked just fine.

Also, thanks to everyone for the comments...what a great group! -Jim

elippse said:

Let's say you have a thin dielectric sheet in front of an antenna. When the antenna radiates, there will be an in-phase reflection from the nearest surface back toward the antenna, and then there will be an out-of-phase reflection from the far surface of the dielectric after the radio wave passes through it. The reason for the difference in reflection phase is that the wave is going faster in air when it meets the slower propagation rate of the dielectric and it sort of bounces off in-phase, but then after traveling through the dielectric with its slower propagation rate, it speeds up when it reaches the interface between the dielectric and the outside air. Because there is a slight phase shift in the arrival of the two wavefronts because of the slower wave in the dielectric, they will not be exactly 180? out of phase and a slight signal loss will be felt. As the frequency is increased and/or the dielectric thickness is increased, the two waves ariving back at the radiator will have more of a signal cancellation. It's possible that if we move the dielectric closer and closer to the antenna until it is actually touching the surface the same phenomonenon will occur. As has been noted, more loss of or change of electrical length has been observed at higher frequencies which will give frequency pulling to some output stages.

Click to expand...

Lets save the theory for the textbooks, journals, and glasses of beer, please? These effects are only important when the electrical thickness is an appreciable portion of the wavelength. The really thin layer of paint here won't have much effect on the antennas we use in aircraft.

Jim, as you've seen from others and your own experience, paint on and live a long life!

Loboflyer said:

Lets save the theory for the textbooks, journals, and glasses of beer, please? These effects are only important when the electrical thickness is an appreciable portion of the wavelength. The really thin layer of paint here won't have much effect on the antennas we use in aircraft.

Jim, as you've seen from others and your own experience, paint on and live a long life!

Click to expand...

...and to add to the above, the antenna even comes from the factory with a layer of dielectric on it - in the form of white paint or a plastic coating.

Loboflyer said:

Lets save the theory for the textbooks, journals, and glasses of beer, please? These effects are only important when the electrical thickness is an appreciable portion of the wavelength. The really thin layer of paint here won't have much effect on the antennas we use in aircraft.

Jim, as you've seen from others and your own experience, paint on and live a long life!

Click to expand...

I painted my APRS antenna (144 mHz 1/4 wave whip) and it works fine.

Now as far as metallic paint is concerned, there is no metallic paint that has the surface resistivity of aluninum, copper, silver, or gold. That being the case, any paint with lower conductivity will dissipate power. That is why steel is never used for an antenna or reflector as power would be consumed in its less than ideal conductivity.
So you don't like the dielectric theory discussion of why paint might cause a loss in an antenna and the change in its impedance and electrical length, huh? I agree it's related to the thickness of the dielectric relative to a wavelength, but as some of the antenna makers related, it's definitely a factor. So can any of you give a better explanation, huh?
I actually had to use this theory with the radome we had on our x-band Atlas Guidance Sytem, with the fabric window we TX-RX through. It was made of teflon over fiberglass, about 0.016" thick, and when it was dirty we would get a boresight shift whenever the California mist would cause moisture to adhere to the window. Not good for precision trajectory guidance. We eventually found a company in New Jersey that put out a product that was hydrophilic; water rolled up and beaded on its surface. We had to have special spraying equipment that would blast a layer of this on the window, but it didn't stick like paint, so we couldn't touch it as it would rub off. I wonder if they're the same outfit that makes RAIN-X?

We used to call it PFM.... Pure F*****G Magic