Microwave RF hitting a metal surface

[soniferrous]

Greetings,

I've got some questions about microwave radiation hitting a metal surface. The radiation is at 14 GHz and the metal surface is about 0.7 mm thick and is steel. Let's say 14 GHz at 20-30 dB. I've done calculations to determine the absorption loss in dB and it's clear that all of the signal is being attenuated. The metal surface is large, irregular, and not grounded. Both the top side and bottom side of the surface is surrounded by air.

My question is what happens to the absorbed RF energy? I understand that a current can be generated through the conductor...

"At high frequencies the skin effect is so strong that the current flows only through a very thin surface layer. As the frequency increases, this layer becomes thinner making its resistance higher. As a result, metals are not good conductors at very high frequencies."

I've calculated the skin depth for steel at this frequency and it's 0.16 µm. So I can see that the current flow is very limited and perhaps a bit of heat is generated.

Is that the end of the story, or can any of the energy be re-emitted either above or below the surface, either as RF or a magnetic field?

thanks!

 

[davidgcet]

some will be absorbed but most will be reflected. and i assume you mean the power is 20-30 db either radiated at the antenna or out of the transmitter, to still be 20-30 db when it hits the metal would mean either extremenly high radiate power from your antenna or a very close metal object. so to answer how much current it could put on the metal would require a lot more data and math. 14ghz stuff is all going to be used with some type of directional antenna, and should be designed NOT to point at a large attenuator.

for the record i have used a high rise building to "bank shot" around another building for temp use on a non-critical 1.9ghz setup one time. it was a fixed wireless system where i did not have LOS to the tower we were working on and i wanted internet in the hotel. at that time, over 20 years ago, hotels did not offer free internet so i made it work using a spare subscriber unit. would i use something like that for a long term critical service, absolutely not, but for personal needs for a short time it worked great.

 

[prcguy]

The RF will reflect off the metal with not enough absorbed to measure. There should be little to no heat as no power should be absorbed and turned into heat. If the RF currents induced onto the metal surface are high enough there can be heat generated from I/R losses in the metal surface. This happens on the inside surfaces of waveguide at high power. Even 1mil thick aluminum foil will provide 100% shielding straight through the material not counting RF currents that will flow on the surface facing the source and refracting off all edges possibly leaking around the back side.

When you say 14GGz at 20-30dB do you mean dBm or dBw? Those numbers with just a dB suffix doesn’t make sense.

BTW I have a long career transmitting high power at 14GHz and in one case we had to generate 10,000watts into a Ku antenna feed for final acceptance testing, 5,000 watts per polarity with both polarities driven. Stuff got hot and the system was built with lots of finned heat sinks on the entire waveguide run to the antenna feed. The feedhorn pointed right into a sub reflector just a few feet away and I don’t believe there was any heating in the sub reflector to notice.

 

[mmckenna]

Greetings,

I've got some questions about microwave radiation hitting a metal surface. The radiation is at 14 GHz and the metal surface is about 0.7 mm thick and is steel. Let's say 14 GHz at 20-30 dB. I've done calculations to determine the absorption loss in dB and it's clear that all of the signal is being attenuated. The metal surface is large, irregular, and not grounded. Both the top side and bottom side of the surface is surrounded by air.

My question is what happens to the absorbed RF energy? I understand that a current can be generated through the conductor...

Yeah, it's not absorbed, it's reflected. With the uneven surface, it's just splattered all over the place.

Using passive microwave reflectors like this is/was very popular in the telecom world. In fact, one of the central offices one of my sites is connected to is still fed by a microwave antenna on a mountain top, off a passive reflector on another mountain top, and down into this small remote central office.

The passive reflectors are just a large metal surface that looks like a billboard carefully aligned to reflect the microwave path down to where it's needed. Beauty of it is there's no power needed and they rarely need maintenance.

"At high frequencies the skin effect is so strong that the current flows only through a very thin surface layer. As the frequency increases, this layer becomes thinner making its resistance higher. As a result, metals are not good conductors at very high frequencies."

That's feedline related.



Here's a photo from a site in Rangely, Colorado. This was a compressor station. You can see the microwave antenna pointed back at the hilltop where you see the billboard like passive reflector:

 

[RFI-EMI-GUY]

Greetings,
(snip)



Is that the end of the story, or can any of the energy be re-emitted either above or below the surface, either as RF or a magnetic field?

thanks!

If that surface is irregular, the signal will be scattered in various directions. Generally. not very useful. If a flat surface, it will be reflected at the angle of incidence.

In either case, it will be reflected as an electromagnetic field at radio frequencies.

 

[soniferrous]

Thanks guys for all the great responses. I'll try to answer each. FYI I'm a total newb to this realm but have a very strong background in IT, general science know-how, and a bit of math. Pardon me if anything I say down below sounds like it's from a crazy person.

some will be absorbed but most will be reflected. and i assume you mean the power is 20-30 db either radiated at the antenna or out of the transmitter, to still be 20-30 db when it hits the metal would mean either extremenly high radiate power from your antenna or a very close metal object. so to answer how much current it could put on the metal would require a lot more data and math. 14ghz stuff is all going to be used with some type of directional antenna, and should be designed NOT to point at a large attenuator.

Sorry, I misspoke on the dB. It's a satellite dish on a metal roof... a phased array directional antenna. The main lobe is putting out about 32-35 dB. I calculated power density in the far field (all lobes) at 0.04-0.13 mW/cm2 or 40,000-1,300,000 uW/m2. The main lobe has max power level of 4.06 watts or 6.09 dBW. Gain is 32.2 dB. Side lobe levels I'm guessing are 12 dB, and the main back lobe maybe about 2 dB. Apparently the main lobe is narrow and pulsed, and somehow the power is ramped up a great deal through some magic that I don't understand. Needless to say, I don't have access to a tester that could provide readings at these frequencies. I do have access to a German unit that works up to 3.3 GHz.

I was looking into the back lobe radiation to understand if there was any concern about this radiation going anywhere unwanted. I did a power density calculation for the back lobe at 0.5 meter and came up with 226.25 mW/m2 (226,250 uW/m2). At 6 meters, 18.55 mW/m2 (18,550 uW/m2). If unshielded, I calculated the 6 meter range power density would be about 4 times that of a typical WiFi router (0.2 w) at 2 meters away.

So then I wondered if this unwanted radiation is able to penetrate the roof. That's when I did some absorption loss calculations, but according to the responses here, steel isn't an absorber of RF signals. Maybe I'm all wet on this, but the research I did is saying that steel is a very good absorber, while copper and aluminum are very good reflectors. The reason being is that steel has magnetic properties.

I did a bit more research and realized that a very thin layer of copper could reflect all of this radiation. The better the conductivity, the greater the reflection ability (Cu near the top, Fe at the bottom). BUT, apparently steel is going to do the same? Thanks.

 

[RFI-EMI-GUY]

Dish network uses steel dishes as reflectors. I have no idea if they are plated. But they are going for cheap which is why they are left on roofs and curbs after the subscription is cancelled. Is your end game to not fry your brain while using Starlink ??

 

[prcguy]

Dish network uses steel dishes as reflectors. I have no idea if they are plated. But they are going for cheap which is why they are left on roofs and curbs after the subscription is cancelled. Is your end game to not fry your brain while using Starlink ??

Dish network, DirecTV and many small dishes are painted or powder coated steel. I’ve tested bare aluminum machined prototype DirecTV dishes and they are no different than steel production dishes in performance.

 

[soniferrous]

Dish network uses steel dishes as reflectors. I have no idea if they are plated. But they are going for cheap which is why they are left on roofs and curbs after the subscription is cancelled. Is your end game to not fry your brain while using Starlink ??

I'm just trying to understand the physics of what's happening with that back lobe hitting the metal roof. At this point it looks like the physics is both absorption and reflection. Steel absorbs RF radiation. All the metals absorb, but steel absorbs about 3.4x as much, due to its magnetic permeability. They both reflect, and copper reflects more. This paper from a building engineering journal is where I found the absorption loss formula...

Electromagnetic skin depth of cement paste and its thickness dependence

The skin depth δ describes the ability of electromagnetic radiation (particularly microwave) to penetrate a material. This ability is relevant to rada…

www.sciencedirect.com

I did that math and found that the absorption loss (dB) is a lot greater than the back lobe gain (2 dB).

The Starlink antenna isn't a reflective dish. It's just a PCB with an array of chips that are doing both transmitting and receiving. The original Starlink dish had an aluminum back plate. The newer ones don't have any back shielding.

I am just still wondering about the physics of the energy that gets absorbed into the steel. We're talking just 0.2W/m2 here, so it just gets attenuated and I suppose that no current is formed that goes anywhere. I was thinking the roof at that point becomes a big antenna, and then I guess slightly "glowing" with RF off either side of the metal.

Thinking a bit more, the steel is both reflecting and absorbing, so we are talking sub-2 dB being absorbed, which probably means close to zero!

Well, thanks for listening. Have a great day.

 

[prcguy]

I'm just trying to understand the physics of what's happening with that back lobe hitting the metal roof. At this point it looks like the physics is both absorption and reflection. Steel absorbs RF radiation. All the metals absorb, but steel absorbs about 3.4x as much, due to its magnetic permeability. They both reflect, and copper reflects more. This paper from a building engineering journal is where I found the absorption loss formula...

Electromagnetic skin depth of cement paste and its thickness dependence

The skin depth δ describes the ability of electromagnetic radiation (particularly microwave) to penetrate a material. This ability is relevant to rada…

www.sciencedirect.com

I did that math and found that the absorption loss (dB) is a lot greater than the back lobe gain (2 dB).

The Starlink antenna isn't a reflective dish. It's just a PCB with an array of chips that are doing both transmitting and receiving. The original Starlink dish had an aluminum back plate. The newer ones don't have any back shielding.

I am just still wondering about the physics of the energy that gets absorbed into the steel. We're talking just 0.2W/m2 here, so it just gets attenuated and I suppose that no current is formed that goes anywhere. I was thinking the roof at that point becomes a big antenna, and then I guess slightly "glowing" with RF off either side of the metal.

Thinking a bit more, the steel is both reflecting and absorbing, so we are talking sub-2 dB being absorbed, which probably means close to zero!

Well, thanks for listening. Have a great day.

In practical use there is no perceivable difference in absorption between a reflector made of steel, copper or aluminum. It just doesn't show up, otherwise there would be a big push to use copper or whatever to decrease the loss and increase the gain of a microwave reflector type antenna. When you get to millimeter wave at 30GHz or higher most reflector surfaces are gold plated to provide a low loss reflecting surface with no paint or lossy coatings and it will not corrode very easily possibly causing higher I/R loses on the surface with high RF currents.

I've worked on many mm wave projects with small gold plated horns and dishes but have also never seen any degradation compared to aluminum or other metals. I also had some projects that used a very fine mesh screen reflector embedded in basically Bondo resin filler used for repairing car dents and the mfrs of these mm wave dish antennas strive for the least amount of loss and most gain they can come up with.

I believe this is an E band 38-53GHz mm wave horn antenna that is gold plated. Aluminum versions seem to have about the same gain meaning no excessive loss due to materials.



This 4 1/2" dia mm wave parabolic reflector was left over from a three letter agency project which I can't discuss frequency or details. These were stamped out of copper sheet then gold plated and fitted with a button hook feed. The gold plating was mainly to avoid corrosion as this bare copper version has sustained over the years just sitting on my shelf.

 

[soniferrous]

I've worked on many mm wave projects with small gold plated horns and dishes but have also never seen any degradation compared to aluminum or other metals. I also had some projects that used a very fine mesh screen reflector embedded in basically Bondo resin filler used for repairing car dents and the mfrs of these mm wave dish antennas strive for the least amount of loss and most gain they can come up with.

I found this aluminum horn antenna on the 'Zon... quite spendy

 

[soniferrous]

(sorry... above got posted... don't think I clicked on Post reply... no way to edit it.)

https://www.amazon.com/Antenna-1GHz-18GHz-Microwave-Telescope-Measurement/dp/B0BTH34W45

was thinking about using it for a project to make a low-cost microwave tester up to 18 GHz.