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
A dirty dish can also reduce performance. I used to use a 2.8m dish in Cyprus to watch UK tv. I found that if I got up on the roof with a bucket of warm soapy water and a sponge and cleaned the dish every few months there was a noticeable improvement in the reception of some marginal channels once the dish was visibly clean again. Grime, bird droppings and Sarhara sand built up quite quickly in the winter.
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.
So you used a strawman and a joke to defeat me! Amazing!
Bad Aiblingen and ECHELON prove visual concealment is the goal. I never talked about aerodynamic designs as on AWACS.
Radomes on almost every single outdoor antenna of every type, from radio telescopes to cell towers, proves you should lighten up on the conspiratorial thinking.
Right, and that’s clearly why like every last meteorological weather radar on the planet sits inside a radome – so that sneaky, sneaky clouds don’t hide their precipitation when the antenna is looking their way.
As an Air Force satellite tech…no not really. Our radomes had a couple more important purposes.
They protect the antennas from gusty wind loads which are significant for large parabolic antennas and the jittering signal levels you get from moving antennas. For very large antennas the load on azimuth and elevation drives is a real concern. A lot of military sats are not geostationary so they are constantly being tracked.
They help even out solar heating of the antenna which tends to distort its shape and put additional load on LNA cooling systems.
Prevents icing. It is much easier to deice the radome than the antenna itself. We had one radome that was flexible enough to deice similar to a deicing boot on an aircraft. You just increased positive pressure inside the radome using ventilation and it would pop ice right off.
Most likely the enemy knows where your antenna is pointed and it would not be hard to figure out what it is used for. The satellite themselves are easy to track.
NASA does not use them on the deep space antennas because….the signals are extremely weak from deep space (like radio telescopes) and the attenuation of the radome is not acceptable and because the deep space antennas are purposely located in mostly desert areas where the protection is not really needed.
A very popular quick fix for icing microwave antennas was PAM cooking spray.
Excellent voice of reason. Thanks Steven.
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
1960s military paint probably contains every heavy metal known to mankind with a sprinkling of asbestos, that’s just how they rolled.
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?
I need to do some more experiments, but no longer have a “free” wall to look at.
I think it was seeing its own shadow on the wall. There’s RF from all directions from the walls. The dish itself blocks some of the radiation coming from behind it, casting a shadow.
I need to figure out a way to verify it, and find a place in the house where I can move stuff out of the way to have a clear view of the wall.
There was an advancement in materials science back in the 60’s applicable to aircraft radomes. You will note that prior to then, aircraft had black or grey “noses” – this was naked fiberglass necessary for the transparency to radio frequencies; the paints at the time contained metals which interfered. It wasn’t until advances in paint technology that materials that could withstand the rigors of flight and be transparent to radio could be employed in this application.
Aircraft still largely use fiberglass for this – carbon-fiber attenuates RF, as is taken advantage of in “stealth” technology by constructing the airframe out of this material.
Naked fiberglass is not typically dark. Early radomes were coated in black rubber for weatherproofing.
Look at the radomes they use on maritime stabilized VSAT antennas such as a Seatel 9711. Those things put additional 1 or 2 tenths of a dB of attenuation. Virtually nothing.
They make buildings with the top floor external structure that are completely made from fiberglass (with minimal structural metal) for enclosing master antenna arrays without having any visible antennas that decrease the overall visual aesthetics (and revenue). Heritage Plaza 111 Bagby Houston TX.
Some buildings like Williams Tower (also Houston) do the same thing with huge panels of Raydel for a radome to shield the antennas from weather and to shield them from view.
Really important for buildings that get in the range of a million dollars per year from rooftop rental without the antennas changing the perceived aesthetics.
Architects call these radome structures “hats”.