Not too far away from where this is being written is one of Uncle Sam’s NATO outposts, a satellite earth station for their comms system. Its most prominent feature is a radome, a huge golf-ball-like structure visible for miles, that protects a large parabolic antenna from the British weather. It makes sense not just for a superpower to protect its antennas from the elements, and [saveitforparts] is doing the same with a geodesic dome for his radio telescope experiments. But what effect does it have on the received signal? He’s made a video to investigate.
The US military radome is likely constructed of special RF-transparent materials, but this smaller version has a fibreglass skin and an aluminium frame. When he compares internal and external sky scans made with a small motorised satellite TV antenna he finds that the TV satellites are just as strong, but that the noise floor is higher and the frame is visible in the scan. It’s particularly obvious with such small dish, and his planned larger array should improve matters.
We would be curious to know whether an offset-fed dish constructed to minimise ground noise reaching the LNB, would improve matters further. It’s no surprise that the frame doesn’t impede the TV satellites though, as it is many wavelengths wide at that frequency. The video is below the break, and meanwhile, we featured the antenna he’s using here in 2023.
The now defunct Waihopai spy dishes in New Zealand were covered in inflatable radomes, which we know because some subversive types deflated them with scythes in 2008.
As I say, not needed now. The GCSB snoops on the Southern Cross Cable data instead.
Yes, a radome can effect the radio signal. No material is entirely RF-transparent, they are instead in various ways attenuating, refracting, reflective. Lenses are sometimes effective at microwave, paraffin is a material often used to make them.
For non-metallic materials like plastic, the issue is paramagnetism. This is due to unpaired electrons in the molecule, so most materials with unfilled atomic orbits are paramagnetic.
One of the consequences of this is that PVC isn’t really a great material for coil forms, it has some attenuation. Nylon is about as good as you can get.
Dielectric loss tangent is also a concern for many plastics, probably much more so than paramagnetism (or diamagnetism for some plastics), which isn’t even necessarily a loss mechanism.
Just don’t try making one out of carbon fiber. It’s conductive enough to make a really RF good attenuator.
Radomes are often coated with a water repelling (hydroscopic) coating, so any water will sheet off of the radome. Water that remains on the radome tends to depolarize the signals that pass thru it. Depending on the frequency, the water can contribute additional signal loss and increase the background noise level.
Their whole purpose is to conceal where your dishes are point. So they do an excellent job. If they would protect from anything else we’d see them on NASA deep space dishes. So apparently it’s only a spy thing.
Is that the purpose? I thought it had something to do with bird poop or weather, but I never actually thought much about radomes
Ah, that’s why all the radomes on cruise ships are there! They are actually secret listening posts for three-letter agencies!
And the weather radar installations too! They all have domes, so they must be up to no good too.
And airport radars! Of course, good citizens have no need to see that dish turning!
So what about all those big military radar dishes we see whirling around? Are they just decoration then? A confuse-the-enemy thing?
Yeesh.
As a field engineer in the 1960’s working on ships missile systems, I had one experience where one of the antenna radomes was coated in gray paint because “the captain didn’t like the color of the fiberglas radomes”. That seriously attenuated the already weak signals. I wrote it up and went on to my next assignment.
You mean he went and painted the thing that said DO NOT PAINT in giant stencil block letters on every side? Yep sounds about right. Was it lead paint? I have a little hunch it might have been
The Aluminum ladders can affect things. Also there looks like there are some metal structures outside near by the dome. These can affect things. A ground reference reflector made of a metal screen around the doam angled at an angle to the ground such that reflections are sent over and past the dish can help with noise. This angled reflector must be very strongly grounded to Earth ground (BIG GROUND WIRES !). Any nearby metal objects need to be heavily grounded.
Interestingly, there is significant thermal radiation from room temperature objects, even at 12 GHz. I’m guessing the radio receiver is seeing the thermal signature of the dome structure. It would be instructive to try the same measurement on a hot summer day, or even summer day and clear summer night: a 30 C warmer dome will give 50% more thermal signal.
Yep. There’s enough that you can make images with a small satellite TV dish and LNB.
https://josepheoff.github.io/posts/3-rfcamera-toc
Nifty. DId you ever figure out what the cold spot on the wall was?